Black Widow Pulsars: The Vengeful Corpses of Stars

you good evening everyone my name is andrew frak noise i'm the astronomy instructor here at Foothill College in Los Altos Hills California and it's a pleasure for me to welcome everyone here in the Smithwick auditorium and everyone watching at home on the web to this lecture in the 14th annual Silicon Valley astronomy lecture series these lectures are made possible by NASA's Ames Research Center the Foothill College astronomy program the Astronomical Society of the Pacific and the SETI Institute all of which do a great deal of work in outreach in astronomy and endorse and support the spirit of these lectures which is to share astronomy with a wider public tonight's speaker is dr. Roger Romani of Stanford University dr. Ramani is a professor of physics and a member of the Kavli Institute at Stanford his research focuses on neutron stars and black holes the corpses of stars he enjoys finding new strange phenomena in the sky and then building theoretical models to explain them past recognition for his work includes the sloan Foundation and Cottrell scholars fellowships and the prestigious Rossi prize of the American Astronomical Society dr. Ramani is also a well known public speaker and he wanted me to mention that he has diverse credentials in astronomy his work ranges across the entire spectrum of possible photons from the radio spectrum all the way to the high-energy gamma ray spectrum and he says he's never met a photon he didn't like so it's a for Mia both a pleasure and an honor to be able to introduce to you discussing black widow pulsars the vengeful corpses of stars dr. Roger Romani thank you Andy and welcome all now I adhere to the Peripatetic school of lecturing and so I my tendency is to wander a bit I will try to cleave to the microphone to make sure that I stay on tape let me go ahead and start our discussion tonight I hope we don't have too many arachnophobes in the audience but I promise I won't show too many spider pictures the story I want to tell you tonight is one that starts off as a story of a quest for discovery a real survey of the sky but then it turns into a missing persons mystery followed by a story of murder and cannibalism at least in a stellar sense but it ends happily with an opportunity to use the objects that we study to do fundamental physics to really learn some deep truths about nature so we're going to be ranging quite widely across the fields of astrophysics and physics tonight and I hope that these objects gamma-ray Black Widows will inspire you to think a little more deeply about the extremes of nature okay to start we started a program about ten years ago of trying to study the highest energy photons in the sky gamma rays and it's a program NASA sponsored but also the Department of Energy and this project involved putting an instrument into space is this pointer bright enough for you all if it is let's continue this instrument would have to be strapped on top of a rocket and so it's a bit it's compelling but a bit disturbing when you put five to ten years of your life into building a thing and you put it on top of a bomb and light the fuse but of course it ended boiling 0 has liftoff of the Delta rocket carrying blasts a gamma-ray telescope searching for unseen physics in the stars of the galaxy let's not let the NASA announcer go on too long but it is an exciting period about five years ago when the mission finally got up into space and we put the thing onto orbit and started our survey of the gamma-ray sky now I want to start by telling you the sort of objects that we find and then will continue on to the sort of objects that we saw but didn't understand so here's a picture of our instrument it's called Fermi now was originally called blast at launch but as NASA tends to do once they're successful its renamed after somebody fame the Fermi mission here is in orbit and it's been observing the sky now for about five years and during those observations could we have the lights down even a bit further on the screen is that possible if it is you might be able to see the background sky as in many astronomical talks the background is dark and the objects are light so not the audience but the screen would be ideal thank you that looks great so now you get a picture of what we could see if we had gamma-ray eyes now this is the whole sky presented to you here unfolded out into a single projection this is the plane of the Milky Way and there's various dots of gamma-ray emission the emission here has been color-coded by energy red green and blue corresponding to low energy four damn arrays medium energy and a really high energy gamma rays and this is what Fermi sees when it looks at the sky at least in a time average sense we'll come back to that in a minute now I would like to tell you what this gamma-ray sky survey is uncovered the first thing you notice is this diffuse kind of glow well that's the plane of the Milky Way with the center of our galaxy right about here and that diffuse glow comes from basically high-energy particle collisions relativistic high-energy protons are accelerated in space in a variety of ways and when they go wandering through the galaxy and hint' the hit the interstellar medium the gas between the stars those collisions create high-energy particles including the famous PI ons and those decay into gamma rays which we see so that's a diffuse glow but in addition most of you probably notice these little dots of light those dots of light are individual sources pumping out gamma rays at ferocious energies and it turns out that they are dominated by two classes of objects it turns out that one class of objects we see is made up of powered by black holes now you see this little UPI that stands for unipolar inductor I'm going to teach you a little bit of physics as we go through today it turns out a spinning sphere seems to be a very important and dominant way of producing high-energy gamma rays but that sphere can be one of two kinds of objects can be a black hole in which case the object is known as a blazar I'll explain those a bit more in a minute but it can also be what's called a neutron star and if that kind of object is spinning the the power that comes out is in the form of pulsations and we see it as an object we call a pulsar so in fact some of these objects that we see in the gamma-ray sky are spin powered black holes like this one right here others are spin powered neutron stars like this one right there now the black holes are enormous their masses of millions to billions of times the mass of our Sun located in the nuclei of galaxies external galaxies the pulsars in contrast are the dead corpses of relatively normal heavier than the Sun but otherwise normal ish stars masses of aha that's going to be the punchline of the talk so we'll hold off on that so here we go first let me tell you a little bit about what that first class of objects is the blaze ARS as I said blazed ours are located deep in the cores of external galaxies there's supermassive black holes that have collapsed out of the cores and accretion flow of matter onto the surface of those black holes is spinning them up so if you look at this little animation look right down at the center is the accretion just swirls material around and it falls on the black hole you'll get the impression from this artist rendition that the center of object is spinning the spinning very rapidly indeed now it turns out that that spin is crucial because the spin the energy that's put by accretion into the black hole makes a flywheel and that flywheel holds lots of power and as it bleeds it out over cosmic periods of time in this case many millions to billions of years that power is converted into gamma rays now a spinning sphere making power is also the subject of the second class of objects the pulsars in this case as I said it's a it's a corpse of a normal star a heaviest star about 10 times the mass of our Sun when the core of that star collapses down it forms what's called a neutron star about ten kilometers across about city-sized the spin periods of these objects vary from the fastest being about a thousandth of a second to the ones that we see that are relatively slow or five or ten seconds and the basic mechanism behind the pulsations is of course very well known you've got some sort of magnetic field structure and in some completely almost mysterious way that I and my colleagues have been working on for 40 years or so now it accelerates particles to an enormous energy and those particles in turn generate beams of radiation that is complicated but the basic thing that makes them pulse is simple those beams are tied to that magnetic field and as the magnetic field rotates they sweep across the earth line-of-sight giving rise to pulses of radiation so the magnetosphere is mysterious and we enjoy discovering it but the mechanism behind pulsations is pretty straightforward now what you see in the middle here is an artist's rendition of a spinning neutron star and what you see over on the right is an image I made from data from the family the Fermi gamma-ray Space Telescope I took data from a piece of the galaxy which you can see over here is this faint glow and I folded it at the period of a particular pulse are the famous Vela pulsar which flashes about ten times the second rotates about ten times a second and I deconvolve with energy smoothing the gamma rays that are coming from that object and you can see it there pulsing off and on if you look carefully you may notice that it's not a single burst per period it kind of gives you a double flash and then a double flash so what's going on there is a little more complicated than just a single beacon coming out of the pole of the star but the basic story is correct a spinning neutron star stores energy as a flywheel and then bleeds it out over millions of years in the form of high-energy particles and gamma rays okay those those two classes of objects are the main things we see in the gamma-ray sky so here's a little bit of physics I don't want to put many equations in this talk so don't worry but I do want to give you the basic idea of what we're studying you see when you have a sphere that's conducting and you embed it in a magnetic field and spin it it makes a kind of a motor in fact Michael Faraday was the first to back in the early 19th century was the first to discuss this at Faraday disk or unipolar inductor or homopolar generator they're all names for this basic phenomenon a spinning conducting disc in the magnetic field generates voltage and that voltage can accelerate particles so remember there are two classes of these things the first is the blazar if you have a spinning black hole well there's something important about black holes that we all know that is that they're black they're black because nothing can come out of them well yes you can have the gravitational field of the object and the external metric is affected if the object is spinning but you black holes have no hair as the famous theory in the 1960s says there's no other information that comes out of the surface of them they're bald in that sense so when I speak of a magnetic field coupled to a black hole it must be a magnetic field that is externally supported in this case remember I refer to this accretion disk in the center of the galaxy where material is flowing onto the black hole well in this case that disk is the font and support for the magnetic field now it holds a magnetic field that can be pushed into the black hole as this little diagram suggests here but the important thing is that can't be locked to the black hole itself what's happening here is that the black hole is spinning in the presence of a field which ends up aligning itself with the black hole now the beauty of this is that it's actually symmetric as it rotates it looks the same from all directions well all directions about a single axis so these are 2d structures and what you tend to get is a disc spinning around the black hole and a jet of radiation power coming out of the north and south direction now if that jet of radiation points at you then it's very bright it's a blazar in contrast if you have a neutron star it's not a black hole it's on the edge of becoming a black hole it's very compact very compressed but it still has a surface neutron stars are not black and because of that you could have a magnetic field locked in that surface and that magnetic field can therefore be internally supported that means in turn that in general it's going to take some angle with respect to the spin axis so this is the spin axis the magnetic field will point off at some angle which we traditionally call alpha in this business but what's more important is now you've got to imagine us here at earth looking at this thing in some distant place we look at a another angle let's call that Zeta here's the line of sight denoted by little eyeballs sweeping across and as you look at the system from different directions you expect to see different pulses different formation of the Sahra mission and so this is a more complicated problem the black hole problem is 2d this is truly 3d and well it's not the subject of today's lecture that's one of the main things that my research group has occupied itself with the results of this parameter day's lecture is going to focus on a very peculiar class of these gamma ray producing objects it's a very peculiar class of spin powered pulsars now you might ask yourself how can we tell these two things apart I showed you a picture of the gamma-ray sky it was just a bunch of dots how can you tell which is which well we have our ways and here's one way in which you can do it if you look at this little animation you may have noticed these two objects circled here which are in fact the two I pointed to you before these are images that are going well as when the animation was running there were images spaced about a month apart and if you recall when you were looking at that picture you saw that this object seemed to flash off and on to vary month-to-month whereas this object was relatively steady at least averaged over a time period of several days okay what that means is that variable objects in the sky can be distinguished from steady ones and it turns out the blazers the things with the jets they flare and vary on the week to month timescale pulsars in contrast once you average over that spin the pulsations they're very very steady so that's one way to distinguish these two classes of objects another way goes back to that original kind of image of the sky showed you that was color-coded by gamma ray energy it turns out if you take the gamma ray spectrum as a function of energy and look at how much light's coming out we call that a spectrum I'm going to just subdivide it into red green and blue low medium and high energy gamma rays it turns out for reasons that needn't occupy us too much here that the blazar is the black hole systems are I guess white well really they're either sort of pink ie a little more red or the blue pale blue they have a little more blue than red they're basically what we call power laws straight lines in these flux spectrum plots so that's a way of distinguishing a black hole by its spectrum in contrast the pulsar the neutron star systems are green they end up being very relatively faint and low energies peaking in the middle of the Fermi band and then fading away as you go to high energies by the way they're very green and a sort of ecological sense as well turns out that this flywheel that's going along can put tens of percents of its power into gamma rays so they're the most efficient gamma-ray sources that we know of in the sky the Priuses of the astrophysics world if you will okay so here we're starting to come to a bit of a mystery when I got working on this mission a few years ago I decided what I really wanted to do is understand the content of the gamma-ray sky and so I sent ourself a task we took the bright source list the ones that came right off the the font when the missions first started the brightest 250 so the things dots that were there in the sky and said I want to understand what every darn one of these is and so here's what I did I took the radiation in gamma rays from these two kinds of objects and plotted it as scientists do when you got two numbers you plot one where to the other so variability goes up this way getting more variable in this direction and color we'll call it greenness increases in this direction and if you look on this plot you can see a bunch of dots those 250 brightest gamma-ray sources in the sky and they kind of fall into two general clumps there is this white issue or well really black outline set of clumps those are the Blazers they're very variable and they're relatively white not very green in contrast there's a set of clumps out this direction which are not variable at all but they're relatively green relatively curved spectrum those are the pulsars but when I started this project after we done all the standard techniques which I will review for you briefly of checking what kind of object we have we found that out of these 250 there were six that we didn't know what they were six remained unidentified after about three years of work on the gamma-ray sky now those are these things in a red dots and if you're been following the story I've been telling you the fact that they lie in this clump of green dots makes you suggest there at least related to pulsars but as I'll show you shortly all the standard ways of finding pulsars didn't work for these there unidentified I got excited because it's usually true when you work very hard to check for normal things and the objects don't turn out to be normal you've got a real chance of discovering something new unusual and possibly important and so that was the Questor take the and full of objects that were remaining and work incredibly hard to see if we can figure out what they are now that's a couple years of work already and I'm sorry to say that were only half done but it's been a fun half already so I'll tell you about that so the six remaining unidentified we're going to call Fermi's missing half dozen and here they are on the gamma-ray sky again here we plotted the positions of those six objects and if the and this theater were really dark you could probably see their little dots within these those are the actual gamma-ray sources and they lie spread across the sky some near the plane in the galaxies some farther away that's those half-dozen sources that I'll be talking about for the next few minutes so we've already described you how from the color and the lack of variability they look like pulsars so the natural thing we all did when we first saw these things was to try to find pulsars at the positions of these objects now how does one do that well for the specialists among you those who amateur astronomers know that pulsars are very traditionally found by radio emission and what you do is you take a relatively large radio telescope like the Green Bank telescope 100 meters across or even bigger the Arecibo telescope in Puerto Rico 305 metres across and you look for pulsating radio signals coming from it that's a great technique and it's worked all these gamma ray sources worked about a hundred times already so we found pulsing radio sources in a large number of the gamma ray sources on the sky but these six did not yield radio pulsations okay so it failed one test well one of the great things Fermi taught us is that some pulsars the radio beam seems to miss the earth while the gamma ray beam still sweep to crossed us so perhaps they're pulsing only in the gamma rays that too has been a subject of search and it's kind of difficult because as excited as I am about these gamma ray photons you should probably remember that gamma rays such as Fermi seeds are enormous ly energetic about a billion times the energy of the visible light in this room but they're very rare even one of these powerful pulsars delivers to Fermi this you know car size objects this car size telescope in the sky it only gets of order one photon a day and during that one day the object may have spun around thousands or perhaps even millions or even tens of millions of times and you got to figure out how to stack up all those photons to see if you can find out a pulse it's a challenge and it requires enormous supercomputer searches so a gamma-ray supercomputer search is kind of tricky I've got a little animation that may illustrate that for you let's see if I can get my cursor yeah sure why not so here we go so what you need to do is you need to move to a position on the sky take all the photons from that position stack them up and just try a whole mess of periods that doesn't look like much does it and so you move the to another position on the sky recompute the arrival times of all the photons stack them up and try a whole bunch of periods again and that doesn't look like much either and you continue to do this for with the largest supercomputer clusters that we've been able to get to for weeks months in some cases years and not much is often seen but when you hit it it's obvious because all the photons coming from this object stack up into narrow peaks narrow pulsations you gets excited you write your paper you get tenure if you're a young assistant professor there you go okay let's stop that so that technique has been employed to search the gamma-ray photons of a large of a number of those sources and that too has worked in fact about 40 times before on the gamma-ray sky but it failed for those six huh so now we're getting suspicious that didn't work perhaps those unidentified sources are something weird something special something different so now we come to the part of the story that is connecting a little more to classical astronomy how do we go about chasing these things down so once again remember this map of the gamma-ray sky that I showed you it turns out that if you're going to be looking for the counterpart of the gamma-ray source to try to figure out what it is at lower energies life gets a lot easier if you're not too close to the Milky Way because as you all know the Milky Way is just crowded with interesting stuff and so objects they're close to the Milky Way you have to sift through thousands and thousands of possible counterparts before you can find something that's plausible makes it very difficult so in this quest we started with the objects furthest from the Milky Way and that of the six is the furthest one down there so what we did is we took the furthest object that had the best localized smallest uncertainty region and tried to figure out what was inside that now the words uncertainty region may seem a little bit weird to you I think you're used to when you see an object on the sky you know where it is you've already done the problem but alas gamma-ray telescopes don't quite work that way it turns out the gamma-ray telescopes can't focus the light at all after all gamma rays are even more powerful even more penetrating the NEX rays and so they go smashing right through any device that you build there's essentially no way of bringing the gamma rays to a focus and making a direct image instead what you do with the gamma-ray telescope like the Fermilab is you use that smashing power to actually have the gamma rays penetrate the telescope collide with dense material tungsten in this case and that tungsten in the presence of the tungsten the gamma rays materialized into an electron-positron pair which goes smashing through the rest of the detector and since the electrons and positrons are charged you can track them so what we do is we have a tracker that actually sees the materialized matter antimatter pair from this gamma ray and follows the shower of particles coming out from it projects back along that shower to say aha my gamma ray came from there in the sky it's a great idea it works but alas it doesn't work incredibly sharply our best gamma ray vision still gives us a large region of uncertainty and in a typical gamma ray astronomical optical photograph they would you can see I think given the lighting here a handful of objects in it but there are many many thousands of stars with this uncertainty region which is a good fraction the size of the moon so we have a large area to discover to end a survey to see what's inside of it now we can help things a bit by using lower energy photons x-rays in this particular case and it focuses our attention on a handful of objects dozen or so but then what we really want to do is go to optical visible light telescopes and chase down what the counterpart and now I want to show you how we did that for this particular object has which has this strange telephone number its position on the sky J 23:39 - oh five 33 this one was a lot of fun because I got to do a bit of a stunt I got to use the very largest telescope I had a Stanford and the smallest one that we had access to I took our teaching Observatory you can see this little photograph of the observatory where our students learn how to do astronomy and how to measure images it's a 24 inch relatively small 0.6 meter telescope but it was just big enough to see the star corresponding to this object so we choose that telescope to watch the star and watch it changing it turns out at brightens and fades dramatically over the over a period of hours unfortunately in a physics astrophysics really you need to not just look at objects and measure the colors you really have to break up the light into a spectrum to understand what they're all about and the spectrum of an object takes a lot more light and a lot larger telescope so I used our larger telescope the Hobby everly telescope it's about a 10 meter telescope down in West Texas that we are partners of at the time and I use that and here's the relative collecting power of the point 6 meter and this is the area of the mirror of the point nine point two meter HGT and I use that to measure the spectrum of the star well let's see what we found first of all let me show you some images that I took at the Kitt Peak National Observatory of this object this is a sequence of images stacked up over the orbital spin this orbital period of this object which is about four and a half hours I've actually gone ahead and color-coded them so they look a little more attractive the filters in the original photographs that the Peak Observatory were green red and infrared but I've converted it to blue green and red so it makes a common photograph so here's a hulking bright star this one's about 8th magnitude bright enough that for you amateur astronomers you know it's quite visible a nice pair of binoculars and here's the target that we're gunning for right there that's the variable object now if you look carefully you'll notice that first it fades and gets redder those of you at least up front can hopefully see it and then gets brighter and bluer I don't know if anybody's else's notice something peculiar take a look what's going on up here see a little object marching through a little sequence of our green and B red green and blue that's an asteroid it turns out that this particular object is right on the ecliptic and darn it every time I've observed this thing there's asteroids coming through the field all the time in fact quite depressingly last time I observed it down at Sarah to Lowell Observatory this past summer an asteroid went within the seeing disc right through the object during the middle of observations destroying the photometry so observing this one is always a bit of a challenge because you get these little funny extra objects to discover in the background as you're as you're watching it but in any case the main phenomenon I want you to pay attention to is the brightening and fading of this object it's pretty spectacular it brightens and fades by about a factor of 30 as a over two and a half hours so the full orbital period four and a half hours takes it from very bright and blue to very faint and red now remember the physics which we won't go to an in detail the undetailed understanding of these objects come from the spectra and for amateur astronomers you know that star spectra are closely coupled to the temperature we have the famous sequence OB AF OB a fine girl kiss me that we all remember from our student days well this object gives us a little lesson in stellar spectroscopy because it sweeps from the m-class up to the F class and back again in four and a half hours so as we observed it with a telescope we saw a sequence of spectra which varied from M Class stars up to an F class and right back down again and you can just watch it changing stellar spectral type over the period of couple of hours now with those spectra what you really want to do is ask your physicists is you want to measure the composition from the Doppler shifts the motion the masses of the objects and that's what we did here's what we found the Doppler shifts of the object that is going around this gamma ray source shows us two things one it shows us that the object going around the gamma ray source is being heated on one side now the story is pretty straightforward you saw it very hot and blue at half of the phase and when you go half a period later it's very faint red so the object itself this companion of the heater of the gamma ray source is some little low mass pathetic red star but it's being blasted and heated up to very high temperatures on the front side and the spectra show that what they also do is by measuring the Doppler shift of that heated object if it swirls around the star it gives you an idea of the masses and in fact it turns out that the heating source is about 1.8 times the mass of our Sun and the other objects is about a third of the mass of our Sun ok hold on to that thought that's going to be useful in a few minutes when we come onto the next few objects so we measured the temperature and we saw it going from about 3,000 degrees up to 7,000 degrees or so that was the first of the six objects I want to tell you about the second now we have to work our way a little closer to the plane of the galaxy and the second one turned out to be not so bright so I had to use both the wind telescope at Kitt Peak National Observatory and another 4-metre class telescopes or down in Chile to observe this object when we observed it this is what we found once again it's varying dramatically over a short period now a really short period 93 minutes 93.7 minutes this turns out to be the shortest orbital period of any spin powered object known and it's great fun at the telescope because over the period of an hour and a half the thing comes and goes and comes and goes varying in flux by a factor of in this case a factor of 50 or so so once again you can kind of make pictures of it at maximum when it's hot and blue and at minimum where I bet out there you can't even see it but it's really faint and red so there we go in this case is heated up to the incredible temperature of 14,000 degrees from nearly invisible at minimum now just like in the case of the first object where you could see it with a modest sized telescope to really study it you needed a big gun here it took a four meter telescope just to measure it we needed the biggest telescopes on the planet to measure the spectrum so I and some colleagues oh yes you're going to see Alex next time you come here so this is work I've done with Alex Filippenko and in his postdoc Brad Shenko we use the Keck telescopes in Hawaii to discover this to study the spectrum of this object it's an ongoing project that Alex and I have and this one turned out to be really really remarkable even stranger than the other first thing the object going around the heater the power source was incredibly light only about a hundred the mass of our Sun or ten times the mass of Jupiter really small second the power source itself seemed to be remarkably heavy this has been tough to pin down but it seems to be more than about two and a half times the mass of our Sun even heavier than the previous one and the spectra were absolutely bizarre this is of course kind of for the specialist when you look at stellar spectra it's not for the standard audience but let me tell you one thing we found that was really remarkable well the first thing is that it has essentially no hydrogen now remember hydrogen is by far the most common element in the universe and the vast majority of stars are mostly made up of hydrogen somehow this remnant this one hundredth of a solar mass got stripped of almost all of its hydrogen essentially all of its hydrogen and all main thing left behind was helium and heavier elements essentially no hydrogen is there the normal emission lines of hydrogen visible on the top blue trace are invisible on the bottom black trace less than 1/100 of thousands of the hydrogen that you expect the other thing of course is that we've out at masses as well as the composition so now we're going to ask ourselves we've found this thing making gamma rays which is heating up this little thing that's going around it and the nature of that power source is the key question well if you've been paying attention you know I already have a hint that it's probably a spinning neutron star a pulsar and indeed that's going to be the case after we figured out what these binaries were doing doppler shifting it took that supercomputer project which failed repeatedly before and narrow down the possible range of things the Doppler shifts and positions that they had to check enough that was possible for both these objects to confirm that there actually are gamma-ray pulsars but the remarkable thing is that their pulsar is spinning very fast spin periods of only a few milliseconds so they're very very powerful as well as being incredibly fastly rapidly Doppler shifted as they go around the companion stars shortest periods known okay finally we did manage to also detect the other classical signature of these kind of pulsars radio pulsations but here at 2:00 it was also very peculiar we only found them after going back to the telescope repeatedly in one case on 20 times before we saw it even once so the radial pulsations are almost never visible what's that about well hang on there the solution seems to be that these are millisecond pulsars but they're incredibly highly obscured there are a class of objects that we call millisecond pulsars that because they're fast but they're a special class of millisecond pulsars the black widow is the subject of today's lecture so before I do that it turns out every pulsar astronomers got to show this plot and even for a non-technical audience I think it teaches you a lot what we're plotting on the horizontal axis is how fast the thing is spinning here's a period of one second over in this ends of the period of a thousandth of a second a millisecond and on this axis is how fast the spin is changing it turns out that those two quantities map to how powerful the source is if you're up near these top lines they're high power sources and then over time they fade away in this direction the colored dots represent the objects that Fermi has seen and they come in two classes it seems that when pulsars are first born they're born at periods of a few hundredths of a second here high-power low-power they start off at high power and then they spin down over perhaps ten million years for the first million years or so they can give off gamma rays and then the spinning dynamo gets weak enough that they can't predict Celler eight particles and it fades yeah they still met radio waves until after about ten million years or so they disappear completely into what we call the Pulsar graveyard the dead center of the star just sits there unless it's fortunate to have a companion if it's mourn in a binary as many stars are sometimes that companion can resurrect it give it life again by transferring matter on and spinning the thing back up again very much like those black holes in the Centers of galaxies that we showed you earlier if it spins up the thing can be reborn as a millisecond short period pulsar and then spin itself down over periods in this case of billions of years lasting a very long time so remember the red dots Fermi sees the most energetic pulsars of this class of things so here's that pulsar Resurrection scenario take a look at our little animation here we've got a neutron star buried in here a companion star with an accretion disk materials flowing down onto that companion star and as it flows down onto the companion star the beams of radiation which here denote the poles the magnetic poles have material flowing into them and it spins the star up it's kind of like the reverse of you know when you have a sprinkler in the and the water is coming off the end of it slowing it down here the material is flowing in and spinning it up so it's going faster and faster and faster and during the secretion phase this material flows onto the star you can get it up to these millisecond kind of periods and that powers up the battery powers up the flywheel and once the dust and lightning disc have cleared away the remaining object can be a reborn millisecond pulsar so the story of the discovery the first these millisecond pulsars was a great one I remember this very well I was an undergraduate at the time and it was very exciting to me because I knew enough about pulsars to know that they came in that when they were born there were maybe a few hundredths of a second you know a thirtieth of a second was the fastest one known in fact the Pulsar in the Crab Nebula there was the fastest pulsar known so I'll give you a little audio for a second if you take a radio telescope when we study things professionally we make all sorts of plots but when you think of a radio antenna you usually think of hooking it up to a speaker right if you do that when you're pointing it at the crab pulsar this is what you hear that's enough of that that was rather loud and that is the fastest known pulsar at the time in 1982 when this discovery was made so that's the crab in the center of the Crab Nebula however if you hook up to an antenna and look at the direction of this guy 1937 plus 2 1 4 here you go hold yours ok that was enough of that you almost couldn't hear the beats it's a very high pitch because the frequency it's so fast 642 Hertz it's well within the audio zone it's not just a set of pulses it's a tone it was shocking it was amazing it was exciting when this object was discovered but the thing that was really amazing is we had some inkling of how you could get a pulsar fast and that's the story I just told you material flows onto a star and spins it up for that to work it better be a double star a binary this object was single in splendid isolation it was sending there spinning at 642 Hertz 642 times a second an object heavier than the Sun a very remarkable thing indeed okay so we puzzled our heads a bit and in about five years ten years later an example of an object that showed us the way in which this might come to be was found and it was the first of this class of objects now known as Black Widow pulsars snow in 1957 plus 20 and it's a binary system there's a companion going around the neutron star with a period of about nine hours now every nine hours the neutron star Pulsar disappears behind that companion and it disappears in a kind of blobby variable way it's clear the Campania it's got some kind of wind coming off of it material is being stripped off of the companion somehow by the radiation of this pulsar well now so that's the idea perhaps this stripping or ablation takes the companion whittles it down until there's nothing left and you end up with a single millisecond pulsar alas when you try to look at what's happening with this object it can't seem to do it it can't get the job done well we're not going to give up on such a beautiful idea yet I mean yes theories can be destroyed by the ugly truth but we're going to hold on to them as long as possible and so it's possible that more violent and more powerful black widows exists that may still be the way in which you actually make single pulsars so this story of cosmic and gratitude of having a companion star donate the mass that regale the pulsar life and then in horrible retribution being fried and evaporated back to nothing is in fact I would argue the story the objects that I've been showing you let me show you another animation an animation that we generated at Goddard showing the particular object 13 11 minus 34 30 that I described you second it's a very short period system member 94 minutes about an hour and a half the pulsar the spinning object here slowed down for your own enjoyment you know it's going around almost a thousand times a second is slamming radiation beams of radiation into the companion star which is heated white-hot on the front but it's very cold on the back it's also being stripped a very powerful wind is being driven off this companion which we see in our spectra as well finally that powerful wind seems to pile up in sort of a torus a thick disc technical name in the field is a excretion disc instead of an accretion disc and that excretion disc surrounding the system shrouds it hides it well you remember what gamma rays are they're more powerful than x-rays they're incredibly penetrating it hides it from radio it hides it from the low frequency way in which you would normally find a pulsar but the gamma rays can punch right through and so this object this very tight object is shown up in the form of gamma rays whereas the radio signal is almost always blocked once in a blue moon as we found by staring at it repeatedly a little gap in that wind excretion disk occurs and a little bit of radio can get through but we can almost never find it in the radio gamma rays were very much the key okay that's enough of that animation onward so that's the story the power from the pulsars is evaporating the Companions ripping material off and perhaps even in this case wildung it down to nothing the gamma-ray pulsar is destroying it's made so this got some nice attention there's an article in sky and telescope a few a few months ago which kind of just in time for Halloween I guess maybe that's why they liked it anyway you can see this nice graphic of this fanciful spider ish looking pulsar ripping apart its companion star okay so that's part of the story I think we've got a handle on what's going on with these mysterious half-dozen objects two of them are incredibly tight binary shrouded with this wind hidden from the elderly normal ways in which you would find a pulsar but the penetrating gamma rays get through and give us the signpost of what's there is that the story for the rest well remember the task gets harder and harder as you get close to the Milky Way we figured out that one first we figured out that one second and if you're looking down at our plot well he's next so last summer when we went back down to Chile we observed this particular object and this is not even published yet so it's hot off the press this is even faster period it looks like seventy under seventy-five minutes is the binary for this guy and a quick look with the Keck telescope in September it's lost behind the Sun but Alex and I will hit it again in March when it comes back out shows it looks like it's another really high mass pulsar so great looks like the patterns holding and this one's the new record holder so we're continuing to dig our way down to the shortest period things ever found okay so maybe this is the origin of those single millisecond pulsars our objects are so powerful and such short periods that they seem to get the complete evaporation done and correct may end up with something single but there's some other hints that that may happen many of you probably forget you've heard over the last 5-10 years many many talks I hope and at least news stories about the discovery of exoplanets planets around other stars we in the Pulsar world are eager to remind you all that the first exoplanets ever found we're actually going around one of these millisecond pulsars my colleague Alex Walsh on found these and there are two three actual planets two planets in a moon sized thing going around in millisecond pulsar with this code name here so they were the first known exoplanets they may well be the dregs of this evaporation process more recently does anybody remember some news reports about the so-called diamond planet that was kind of a sexy little story but really what's going on here is it seems to be a planet made mostly of carbon which if it's cold enough will crystallize and once again it's around one of these millisecond pulsars so the stripping and vaporators 'single seems very effective the occasion it leaves a little debris behind so that may indeed be the story now we're coming on towards the end of the talk and I promised you that this story of destruction and retribution of this vengeful pulsar would have a little upside at the end and that is a chance to actually do some fundamental physics that's what I want to tell you about next and it comes up back to a question well we have binary star and the companion disappeared but what happened to its maths we need to get a full budget of what happened to the mass that started off in the pulsars companion well we knew some of it had to fall in the neutron star to spin it up and we also see that in some cases some of its being blown away in this wind that's evaporating the remnants of the companion the critical question is how much goes into each of those two channels it turns out that these hidden black widows that I'm finding with Fermi are ultra short period and they have to accrete an immense amount of matter before they can turn on powerfully enough to start ripping the companion away because they're so tight the accretion is very very strong and this gives us an important hint that the Fermi Black Widows may accrete lots of mass before they turn on as millisecond pulsars you've already seen a bit of a hint of that those of you who may remember there's a classical number and astrophysics called a Shandra shaker mass one point for solar masses it's the classical and typical mass that we find for most neutron stars so your garden-variety neutron star is about 1.4 for reasons I'll tell you in just a minute the Fermi detected Black Widows that we see so far seem to be quite heavy the standard ones are 1.7 and we even found one is heaviest 2.4 solar masses but what about these shrouded deeply buried short period ones 13 11 minus 34 30 seems to clock in almost three solar masses that number is still a little bit uncertain but it's really really heavy for a neutron star twice as heavy as we expected it to be and the critical question is how much more mask and these things take it's critical for a physics reason and let's see if I can explain it to you here's a diagram plotting for you the logarithm of the density at the center of one of these neutron stars now waters got a density of one so this is a hundred thousand times the density of water ten to the tenth ten billion times the density of water and on up but then when you get up about here the density suddenly increases really fast with the mass highly hardly growing at all and then there's an unstable branch here so let's imagine we have an object here we'll go back back back we have an object here and we start piling mass on it the mass increases increases but when it hits that period it plummets it drops off and becomes a neutron star it reaches the next branch the electrons disappear they get sucked into the protons you end up with a pure ball nearly pure neutrons and it appears on this branch of the diagram that magic number is the famous Shandra shaker mass the maximum mass of white dwarf can have 1.4 solar masses this number is known exquisitely well it depends on the physics of electrons which we really understand very well indeed so well that in advanced undergraduate courses we show them how to calculate this number to a few percent precision we know the Shandra shaker maths what we don't know is the equivalent number for neutron stars you see if you've been looking ahead you'll notice that the same kind of curve occurs from the neutron stars you pile more mass on gets heavier and heavier and denser and denser in the center but somewhere between a low mass probability and a high mass possibility it collapses and becomes a black hole so the critical question then is how heavy can it get and that in turn determines the physics at ultra high densities many times the density of a nucleus of an atom denser than any material we can react create here on earth it determines the physics of what's going on at those densities the so-called equation of State so deep inside neutron stars things are very mysterious we don't know what's going on and the only way to study it is to see the neutron star that are given us by nature and here's where I think Fermi has given us a great gift for the reasons that I tried to explain you it seems that the neutron stars that Fermi finds those missing half-dozen seem to have suffered extra mass accretion they've gotten more mass than your typical neutron star and have been pushed as close to that death limit of becoming a black hole as possible and so the scientific quest I now have is to try to use measurements in the neutron stars to figure out this equation of state to see if something really exotic is going on at the ultra high density limit in this part of the diagram so the last couple diagrams here are a little bit technical but I think it's fairly it's easy to understand if you think about this as being the size of an object and this being its mass it's related to that other diagram with density I showed you but once again here are theoretical curves the so-called equation of state of nuclear matter what happens when you put a bunch of nuclei together and here is a bunch of theorist predictions there's a wide range of predictions and at most really at most one of those can be true we don't know which one and it turns out the study of neutron stars winnows a theoretical field immensely and puts many theorists out of business and gives a few a final lease on life so here's how it works the heaviest neutron star is known to date before we started this project clocked in at just under two solar masses so a lot of exotic models hit the dustbin the Fermi Black Widows that we're working on now as I've told you are coming in at about two point two two up to two point eight solar masses they're somewhere in this range we're down to the last couple possibilities that people have proposed now infinite is the flexibility of theorists and as soon as we narrow it down to this range I think that we will have a bunch of new models all clustered around that last remaining set of possibilities but the thing is it's exciting because it's experienced REME there's basic physics reasons that say you really can't get much heavier than this before you must become a black hole and that indeed seems to be what nature does so here's my last slide my conclusions this long story I've told you this story of surveying the sky to find mysterious things in the gamma rays tracing down the things we do find to see most of them are things we under and and a handful are weird in some way studying those weird ones intensively to try to see what's special about them and then learning something fundamentally new about the universe by studying those special objects that I believe is the grand quest of astrophysics is to look at the grand nature given us in the cosmos and to study it to study the experiments that we couldn't possibly yet do here on earth to learn the physics of the 22nd 23rd and perhaps even 24th century so with those thoughts I would like to conclude and I'd be happy to take a few questions yeah could you please explain the numbering mechanism on how you guys actually number these because it just seems a little abstract to me oh those names that we gave them yes is it worth it to your madness there certainly is those are simply the coordinates on the sky and what we call right ascension than ours in 1 hours and minutes in one direction and degrees north and south of the Equator and the other so it's the rather we refer them as telephone numbers and astronomy and that you know for the initiative problem but I probably should have explained that before I showed them to you there's nothing special about it except that it gives you a proximate idea where on the sky that thing is great thank you sure so I guess we're on the right now go ahead the white dwarfs is it actually what isn't actually possible for a white dwarf to get over Chandrasekhar limit or does it have to turn into a neutron star that's a very good question the basic theoretical modeling says that that Shandra shake our limit is the limit of what you can do but we've got to be just a little bit careful it turns out that the white dwarf is spinning really fast there you've heard of centrifugal force that centrifugal force can kind of hold up a little bit of the mass and you can get just a hair above Shanter Shakers limit but it doesn't buy you much time you pile a little more mass on it's all over blammo neutron star ok inevitable and I guess we're back on the left okay so the beams that came out of the magnetic poles like how narrowly focused are they like some of the earlier videos that showed them pretty why that picture there is pretty narrow you have any sense of we do and indeed that's been one of the main foci and my research is count calculating that it turns out that if you want to make pretty pictures they've got to be kind of narrow because you want to see them sweeping across the sky like a lighthouse in fact the radio beam seems to be narrow the gamma ray beam in contrast is what we call a fan beam it actually spreads out and covers a good fraction of the sky sweeping across with this thin the hollow cone so we see a double pulse as it comes by probably it varies by the age of the pulsar but between about half and down to perhaps a quarter of the sky is covered for every single pulsar so by each beam with two beams on each side that goes to 100% down to maybe 50% it's tricky to calculate detail but we think we know what's going on now yep they're quite wide thank you the pictures are ugly though so I didn't show you one of those back of the left one a new star is reborn um beak from another star that's already making it reborn what is flowing on to that star yeah okay so this resurrection or recycling process that I discussed is one of mass being drawn off by the gravitational field of the Pulsar very dense very strong gravity flowing from the surface of the other relatively normal star so most of that mass is probably like everything in the universe hydrogen as the hydrogen flows across and slams onto the neutron star the immense pressure exerted by the gravitational force of the surface will crush that hydrogen down to denser and denser and heavier and heavier elements going through a layer of helium carbon all the way up to Krypton and then iron and it finally dissolves into a sea of pure neutrons as it goes right down into the neutron star it's a very violent process and the skin of normal stuff on the surface is only about a centimeter thick I spent two years of my graduate student life calculating what happens in that one centimeter but it's still exciting things go on there so normal matter mostly at first neutrons by the end thanks hi you had that graph with the diagonal lines and the two groups of pulsars in all second in the regular why is it there's such a large gap between the lifetime of a regular pulsar in the lifetime of a millisecond you were talking like 10 million verses in the billions yep very good question so most pulsars when they're born seem to have enormous ly strong magnetic fields about a trillion times the Earth's magnetic field ten to the twelfth ish cows that field takes that flywheel in spins pretty fast about 10 million years to take yourself from a 30th of a second out to 5 or 10 seconds when the whole thing dies I think they still live in some sense because the objects there but they no longer met the beams of radiation that let us find it the accretion process now you're getting to something very near and dear to my own heart of some some theoretical work I did it quite a few years ago in addition to spinning it up it seems to crush the magnetic field to bury it in the star and lower that magnetic field by a large factor going from trillions only down to maybe as few as a hundred million a few as a hundred million times the magnetic field of our earth that lets the coupling to the outside will be weaker and you spin up the flywheel giving it even more energy than you started with originally and then it's dribbled out more slowly so the Pulsar can then last for billions in some case tens of billions the full age of the universe can the radio emission can still keep going thank you thanks oh we do have somebody over here yes so do so neutron stars are primarily neutrons is there like a composition or a percentage of them that are neutrons compared to just normal matter okay yes um it well the detailed behavior of matter at high densities is one of the great puzzles that we're fighting for here but before you reach these magic central densities where we're not sure what's going on we think the theory is pretty solid and what happens there is that you get a sea of mostly but not 100% neutrons you get about 1% protons and for every proton you get an electron and so the relatively the mixture is mostly neutrons hence the name neutron star but this is little admixture of protons now the protons are great because protons and electrons unlike neutrons are charged and because they're charged they can hold that magnetic field which was the font of all the excitement and the power that we are describing so that teeny fraction of protons in there turns out to be very important indeed it's what makes that magnetic field be held into the star what makes it possible indeed they seem to be superconducting superfluid neutrons and protons in the center but until you get to the very central regions where the density gets about three to five times the density of normal nuclei that's where the puzzles that I'm trying to probe live up until then we think we know how to calculate it and it seems to be neutrons dominating with this little admixture of protons Thanks my question is about Neutron objects more generally you got atoms that are 1/2 neutrons roughly and you got neutron stars that are mostly neutrons with what an atomic weight of 10 to the 56 or something 57 yep 57 yeah Oh John Douglas so is there anything theoretically in between you know that's a super question and I my personal guess would be no because the key ingredient that takes you from normal atoms with atomic masses getting up to only well people are pushing it up closer to 300 but still 250 or so getting it up to the 10 to the 57 requires another force and that force is gravity and unfortunately gravity so darn weak that it takes a lot of stuff getting together to make it do anything whatsoever it seems the lightest neutron star you can make theoretically I don't know how to make it in the real universe very well but the lightest one you could make is just under a tenth of the solar mass so between about a tenth of a solar mass and the mass of the heaviest transuranium Newton you know nucleus I do not know any way of having stable nuclear matter which is a pity because the engineering things you could do with such stuff are fantabulous maybe someday we'll figure out how to do with the new force of some sort but at present sorry I don't know of one at least I'll work on it good is this spinning up by the is a spinning up of the stock neutron star after it takes in the matter from the partner star is it caused by when the mass is drawn in by the neutron star it spins up faster like when you draw on your arm so you're spinning you spend faster or is it actually caused by the particles physically hitting the neutron star well it's a combination of the two but the first thing you pointed to drawing in your arms and spinning up is really the key after all the binary star system itself the companion was orbiting so it had a certain amount of what we call angular momentum but as that material goes from the outside of the orbit and falls in towards the star from distances the size of the Sun down to distances of the size of a city that's an enormous pulling in of your arms and correspondingly it's an enormous increase in spin rate so I really think that your simile of the ice skater pulling in their arms to spin up is probably the key one as the material falls in for a given amount of angular momentum as it gets closer and closer it's got to spin faster and faster that's what brings the neutron star up to millisecond periods by adding a few tenths of the solar mass I think we're set there so I we have a final question not exactly a final question for me but it's the last one I'll ask tonight okay did what the source of the gamma rays because that seems like what is very important to be measured is that just the acceleration of the matter as it in the accretion or why why are gamma rays so prevalent from neutron stars golly I would love to talk about that since that's the main theme of my research but that's a tough very mathematical subject but I can tell you in a word or two the basic story it's actually not directly the accretion indeed if stuff is still falling on the star it seems to poison the gamma ray machine and stop it from emitting gamma rays at all it's only when that accretion stops and you're left with the bare flywheel with the magnetic field that that unipolar inductor as we talked about a spinning conducting sphere can turn the B field the magnetic field into an electric field and that electric field can take charged particles and accelerate them to enormous energies thereby producing gamma rays it's very much like you're having a slack in the sky that's spinning around really fast the power source is that flywheel the coupling is the magnetic field and you've got to keep that coupler pretty clean or you take the gamma ray process and shut it off and you end up with measly boring x-rays okay I think that's it so I thank you all for your time and attention thank you very much drive carefully and we'll see you in February for the lick crisis

Channel: SVAstronomyLectures

Views: 71,626

Rating: 4.8065524 out of 5

Keywords: astronomy, science, astrophysics, science news, Pulsar (Nobel Subject Area), stars, stellar evolution, neutron stars, gamma-rays, gamma-ray astronomy, Fermi Gamma-ray Space Telescope (Astronomical Observatory), Fermi Telescope, telescopes, high energy astronomy, pulsars, black holes, Star (Taxonomy Subject), neutron star, pulsar

Id: Fn-3G_N0hy4

Length: 61min 48sec (3708 seconds)

Published: Sat Apr 19 2014