Nov 4 (the class before midterm 2, to be tested on Midterm 3) FIRST, REVIEW: # Last week, we talked about the discovery of PULSARS, a type of NEUTRON STAR that's spinning and magnetized. # Pulsars emit in the radio. # Neutron stars are created in *core-collapse supernovae.* # Pulsars SPIN DOWN as they turn rotation energy into light. # We also talked about the basic properties of neutron stars. # The pulsars we talked about Tuesday were ISOLATED. NOT BINARY. ALL PULSARS ARE NEUTRON STARS, BUT NOT ALL NEUTRON STARS ARE PULSARS. (Just like all tigers are cats, but not all cats are tigers.) For most of this lecture, we're going to talk more about neutron stars, and neutron stars in binary star systems. First, a couple other cool topics. ############################################################################# PULSAR "KICKS": # If supernovae explosions aren't exactly symmetric (ie, they're a little off-center), then the supernova goes slightly one way, and the neutron star remnant goes the otherway. The neutron star doesn't weigh much (compared to the whole >10 solar masses of the expoded star), so it can get "kicked" pretty strongly away from the explosion. So astronomers search for pulsars around supernova remnants, and measure how fast the neutron stars are flying away from the explosion. That tells you something about how symmetric supernova explosions are. The answer seems to be that neutron stars can get kicked pretty strongly by the explosion. ######################################################################## WHY IS THE SUPERNOVA REMNANT <--> NEUTRON STAR CONNECTION SO WEAK? # there are about 700 known pulsars, but only a dozen are associated w/ supernova remnants (all the gas that got blown up in the supernova) # searches for pulsars near ~100 SNRs havn't found new pulsars Example 1: # Chandra x-ray telescope, in it's first-ever picture, discovered a compact object in the supernova remnant named "Cas A". This is a weird compact object -- it's brightness and temperature aren't quite right for a neutron star. It may be a neutron star with "hot spots". It may also be an accretion disk around a black hole, though Cas A's parent star was probably only ~20 solar masses, so we'd expect a neutron star. Still trying to figure this one out. photo of the remnant: http://chandra.harvard.edu/photo/0237/more.html Example 2: # The CRAB nebula definitely is a supernova remnant with a pulsar. So why is the neutron star <--> supernova renmant connection so weak? # Do pulsar kicks carry the pulsars away from the SNRs? # Are the lighthouse beams of pulsars narrow, so that most pulsar beams never intersect the Earth? # Is it really true that core-collapse SNe usually make NS? We think we know this, but the lack of pulsars in SNRs makes us uneasy! This is a topic of current research! ######################################################################## PLANETS AROUND A PULSAR: # Surprisingly, the first planets discovered outside our solar system were discovered around a pulsar. # They are the only Earth-mass planets yet discovered outside the solar system. # There are 3 planets. Masses known for 2: 4.3±.2 and 3.9±.2 earth-masses # Very unlikely that the planets survived the supernova that formed the pulsar. # More likely that the planets formed in a disk after the supernova. # How did this happen? Why was there a disk? Why did planets form? # This is a very rare thing! no other pulsars found have planets # Is this important, or just a freak show? ######################################################################## NEUTRON STARS IN BINARIES MOST NEUTRON STARS ARE PROBABLY ISOLATED. No companions. Either the supernova destroyed their companions, or the supernova kicked the neutron star away from its companions. BUT A *SMALL FRACTION* OF NEUTRON STARS ARE IN MULTIPLE STAR SYSTEMS. - can learn lots! (like masses OF neutron stars) - get some weird binary behavior Now i'm going to talk about different kinds of binary neutron star systems. THE BINARY PULSAR # 1975, 2 astronomers (Taylor and Hulse) discovered a pulsar in a binary orbit. # It's companion is ANOTHER neutron star (not a pulsar). # 11 binary radio pulsars have been discovered to date. # THESE ARE RARE! ALMOST ALL RADIO PULSARS ARE SINGLE-STARS!!!!!! # WHY? well, the "kick" probably disrupts many orbits. and the SN destroys companions and mass transfer may short out the radio spark. # The Binary Pulsar is a binary! Can measure the masses of the 2 neutron stars and the separation between them. # the orbits of the binary pulsar are decaying. WHY? There's no mass being transferred, so mass transfer isn't the reason. There's no thick medium they're travelling through, so it's not a "Jello" inspiral. The answer is "Gravitational radiation". - a weird effect predicted by Einstein's theory of gravity. - as bodies orbit very close to each other, they give off waves of gravitational energy, which travel through space at the speed of light. - Gravitational Energy removes energy away from the system, so the neutron stars orbit ever closer and closer. # The decay of the orbit matches the predictions of grav. radiation. # This is the strongest validation of Einstein's General Theory of Relativity yet. - binary pulsar has much stronger gravity than on Earth, so it's a better test. # Discovery of binary pulsar, and working out the gravitational radiation, was worth a Nobel prize. This time, the grad student shared it with his advisor. ########################################################################### MILLISECOND PULSARS # The CRAB is spinning 30 times every second. It's 1000 years old. # About 12 pulsars discovered that are spinning much faster (up to 600 times/second) # Q: HOW can nature make such fast-spinning neutron stars? A (wrong answer): It was born fast -predicts they're brand-new (but none are near supernova debris) -predicts strong magnetic fields (but fields are weak) -predicts slowing down rapidly. (not seen spinning down. seen spinning up!) A (right answer): It got spun up by a binary, dumping mass onto it! -predicts pulsars are speeding up their spin (observed) -predicts old B field, old NS (observed) -predicts a binary system (half of ms pulsars are in binaries) # How does accretion spin up an old neutron star? Angular momentum of gas in disk, spinning really fast by the time it falls onto the neutron star. This is like spinning a basketball on your finger, and using your other hand to spin it faster and faster. The accreting material spins up the neutron star. animation of how a millisecond pulsar might form: http://heasarc.gsfc.nasa.gov/docs/xte/Snazzy/Movies/millisecond.html ############################################################################## THE PULSAR-WITH-PLANETS AND THE BINARY PULSAR WERE 2 EXAMPLES OF NEUTRON STARS WITH COMPANIONS. MORE KINDS OF NEUTRON STARS WITH COMPANIONS: X-RAY BINARIES [see figure "showing the 2 types of x-ray binaries] NOTE: The naming of "high-mass x-ray binaries" and "low-mass x-ray binaries" refers to the mass of the COMPANION STAR, NOT the mass of the neutron star. Observationally, all neutron stars are about 1 to 1.4 solar masses. HIGH-MASS X-RAY BINARIES # neutron star whose companion is a high-mass with lots of mass-loss (a big wind) # optical telescopes see massive star, while x-ray telescopes see compact object # How does accretion happen? Mass is blown out of the massive star in a wind, and then the neutron star plows into the wind as it orbits the massive star. Not the neat Roche lobe overflow --> accretion disk. SEE FIGURE WHAT ARE X-RAY PULSARS? # a KIND of high-mass x-ray binary # *******DIFFERENT from radio Pulsars********* # about 20 x-ray pulsars are known to date # a x-ray pulsar is a binary system: a young neutron star and a massive,mass-losing star some eclipse, so you're SURE it's a binary in binaries, remember, you can measure masses and separation [see figure "measured masses of neutron stars. All ~1-1.4 solar masses] WHY DO X-RAY PULSARS PULSE? # X-RAY PULSARS ARE NOT RADIO PULSARS! DO NOT PULSE FOR THE SAME REASONS! # If the companion is a massive star, but it hasn't blown up yet, then the binary system has only been around for < a million years or so. So the neutron star is young, less than a few million years. This means the magnetic field of the neutron star is still strong. The magnetic fields affect mass falling toward the neutron star, and funnel the mass down onto the magnetic poles of the neutron star. So instead of falling down in a circle at the equator (as an accretion disk would do), the accreting mass all slams down onto the poles. This heats the poles up, created hot-spots at the poles. The hot-spots are much brighter than the rest of the star! The neutron star's rotation carries the hot-spots in and out of view. So you see pulses of x-ray light as now-you-see a hotspot, now-you-don't. # How are x-ray pulsars different from radio pulsars? See this table: x-ray pulsars radio pulsars --------------------------------------------------------------------- emitting in: the x-ray the radio companions? yes, all (high-mass stars) NO, for almost all accretion? yes, required! NO, b/c no companions energy source for pulses: accretion from companion rapid spinning spin changing: spinning faster! (accretes) slowing down ---------------------------------------------------------------------- # X-ray pulsars are SPINNING UP. same situation as a millisecond pulsar: accretion adds angular momentum x-ray pulsars can eventually become millisecond radio pulsars, as they get spun up really fast. ################################################################### LOW-MASS X-RAY BINARIES [see figure "2 types of x-ray binaries"] [see figure "Translation Guide for different types of compact object"] # A low-mass x-ray binary is a normal neutron star (~1 solar mass) orbiting a low-mass companion whose Roche lobe is filled # Matter spills down toward the NS, forms an accretion disk, falls onto the NS # Much like a cataclysmic variable, except compact object is a neturon star, not a white dwarf. # Neutron stars in these systems are *OLD*. They had to wait for their low-mass companions to leave the main sequence, swell up as red giants, and fill their roche lobes. This takes 10^8-10^9 years, so the NS is really old by the time mass transfer begins! # B/c NS in these systems are old, their magnetic fields are weak, too weak to funnel matter onto the poles. So big accretion disks can form. # Q - Knowing all this, would you expect to find x-ray pulsars with low-mass companions???? # More about the accretion disk in these systems: - disk is much brighter than the neutron star, so hot it glows x-rays - also get x-rays when the disk material slams into the neutron star surface - so when you look in x-rays, you mostly see the DISK. Bare neutron star only puts out as much energy as the sun (it's really hot, but very small. Rememeber L goes as R^2 T^4?) - in some of these systems, the disk or companion eclipses the NS. can measure masses, separations, the shape of the accretion disk.... - can get "X-RAY TRANSIENTS", a month-long flare in the xrays, which may be caused by an accretion disk instability (just like the dwarf novae in white dwarfs accretion disks we talked about earlier.) SOME LOW-MASS X-RAY BINARIES ARE ALSO "X-RAY BURSTERS". What are these? X-RAY BURSTERS [see figure "cartoon of how an x-ray burst happens] # low-mass x-ray binaries # build up accreted Helium on the surface. Eventually, it fuses explosively. # This is the same mechanism that made Classical Novae on white dwarfs (except that was usually hydrogen fusing explosively. small detail.) # the x-ray burst lasts for *10 seconds*, but it puts out 3 days worth of sun-energy [see figure "graph showing that the longer the wait...."] Do you understand this graph? SO HOW ARE X-RAY BURSTERS DIFFERENT FROM X-RAY PULSARS? x-ray bursters x-ray pulsars ---------------------------------------------------------- age? old neutron stars young neutron stars companion? low-mass high-mass magnetic fields: weak (old) strong (young) why build up Helium, then rotation brings spots in and out of view explode