BLACK HOLES (continued) and GAMMA RAY BURSTS

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BLACK HOLES HAVE NO HAIR!

Well, actually, they do have just 3 hairs;
the 3 things that characterize them are:

  • MASS (most important);
  • Electric Charge (unlikely to be important for astronomy--BH's are usuallly neutral);
  • ANGULAR MOMENTUM (SPIN) -- most BHs will be rotating so we study it.

  • A rotating (Kerr) BH will have a SMALLER EVENT HORIZON than
    the same mass non-rotating (Schwarzschild) BH.
  • BUT, outside the Event Horizon there will be an ellipsoidal
    STATIONARY LIMIT: inside of it, everything MUST rotate w/ BH;
    outside the Stationary Limit, a powerful enough rocket could stand still.

  • The region between the Event Horizon and the Stationary Limit is
    called the ERGOSPHERE: (it is sort of donut shaped)
  • In principle (and maybe in practice too!) the ROTATIONAL ENERGY
    of a BH can be EXTRACTED by PARTICLES or MAGNETIC FIELDS that
    penetrate the ERGOSPHERE (Penrose effect).
  • A way to make a great garbage disposALL plus power plant!

    If the SPIN of a BH is too large it could become a NAKED SINGULARITY,
    with no EVENT HORIZON; but the COSMIC CENSORSHIP HYPOTHESIS argues
    this never happens and BH's stay clothed with horizons.

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    QUANTUM GRAVITY

    Physicists have not yet properly combined QUANTUM MECHANICS,
    the way to describe the very small, with GENERAL RELATIVITY,
    the way to describe the very massive.

  • Quantum gravity probably means that SINGULARITIES are
    replaced by REALLY small smears (like 10^{-35}cm !!!)
  • It also means that BHs actually have a TEMPERATURE and RADIATE:
    the HAWKING EFFECT; however for a normal BH with 10 M_sun
    this temperature is < 0.0001 K -- basically ABSOLUTE ZERO,
    so the Hawking radiation is COMPLETELY UNDETECTABLE and NEGLIGIBLE.
    Except for the possible existence of mini-BHs formed in the early universe.

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    HOW ARE BHs MADE?

    1 -- Direct COLLAPSE of the CORE of VERY MASSIVE (> 30 M_sun or so) STARS:

  • -- after Fe core with more than 2-3 M_sun forms it cannot
    make a NS since nuclear forces and neutron degeneracy pressure
    cannot support anything more than roughly 2 M_sun --
  • the UPPER MASS LIMIT for NSs (this is DEFINITELY < 5 M_sun).

    2 -- A NS in a BINARY SYSTEM:

  • could ACCRETE enough mass to drive it over the NS upper mass limit.
  • Similar to WD in binary collapsing and exploding as SN I.
  • But these NS collapses to BHs will not yield supernovae!

    Both 1 (usually) and 2 (occasionally, probably) make "ORDINARY" BHs
    -- roughly 3 - 20 M_sun.

    3 -- COLLISIONS and collapse of stars in a DENSE STELLAR CLUSTER:

  • Such very massive, dense clusters should form in most young galaxies.
  • Within about 10^8 years, such clusters could evolve into massive BHs with 1000 M_sun
    and then grow into SUPERMASSIVE BHs with millions to billions of M_sun!

    4 -- PRIMORDIAL FLUCTUATIONS in the VERY EARLY UNIVERSE:

  • During the first 10^{-43} second after the Big Bang (end of course)
    Quantum Gravity ruled and the universe was crazy/frothy;
    BHs of all kinds of masses could have formed then.
  • Low mass (< 10^17 g) PRIMORDIAL BHs would have EVAPORATED via the
    Hawking mechanism by now, producing lots of GAMMA-RAYS
  • Since not so many gamma-rays are seen, few, if any, were made.

    Both 3 and 4 (possibly) make SUPERMASSIVE BHs -- roughly 10^6 - 10^9 M_sun

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    How do we DETECT BLACK HOLES?

  • It's not easy, since they don't emit any radiation.
    (Other than the incredibly faint Hawking radiation--irrelevant)
  • They bend light from stars behind them, but don't make a "black patch" in the sky
  • BUT their gravity means that they do exert a strong influence:
    if in a binary system, they can pull mass off of companions.
  • As for WD's and NS's, this gas will form an ACCRETION DISK (AD).

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    ACCRETION DISKS AROUND BHs

  • Because the BH's "graviational pit" is so deep, the inner parts
    of the AD get extremely hot and emit lots of X-RAYS.
  • Temperatures of 10^7 or even 10^8 K are found in the inner portions.
  • VISCOSITY drives mass in, angular momentum out and heats gas:
    this viscosity is not molecular (ordinary), but mainly:
  • MAGNETIC or TURBULENT or due to SPIRAL SHOCKS.
  • Magnetic FLARES mean extra heating: the X-ray emission varies RAPIDLY.
  • The Roentgen X-Ray Timing Explorer (RXTE) satellite can detect
    very fast (ms) X-ray flickering, common for BH accretion disks.
    (NS AD's more likely to yield periodic X-ray pulses since NS has a hard surface.)

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    WEIGHING BHs

    If the BH is in a binary, then

  • if ECLIPSING can get SUM OF MASSES;
  • if DOUBLE-LINE SPECTROSCOPIC, can get RATIO OF MASSES.
  • (If both, then of course, get both masses quite accurately.)
  • While BH doesn't emit, its AD does; but the disk mostly emits X-rays.
  • Usually: use visible spectrum of the companion star PLUS
    X-ray (and sometimes visible or UV) spectrum of BH AD to get:
  • decent Doppler shift ratios and therefore,
  • decent estimates of velocity and mass ratios.
  • The companion star spectral type gives a direct estimate of its mass.

  • So, putting this all together, astronomers have good cases
    that over 20 binary systems in the Milky Way and Large Magellenic Cloud
    have BHs with masses between 5 and 20 M_sun.
  • An example: Cyg X-1, with P = 5.6 days, an O9 supergiant companion
    with mass above 15 M_sun and
    BH mass exceeding 10 M_sun (most likely around 15 M_sun).

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    GAMMA-RAY BURSTS (GRBs)

  • Flashes of gamma-rays were detected in the 1960s by satellites designed to
    check on Soviet atomic weapons tests.
  • Analysis showed they were coming from space, not the earth, and astronomers started to study them.
  • The Compton Gamma-Ray Observatory discovered many of them during the 1980s and 1990s
  • But only very recently, with dedicated GRB telescopes like Swift, have we learned a lot about them.

  • GRBs are isotropically distributed on the sky and we now understand this is because they come from
    distant galaxies.
  • Since 1997 many have been identified with very distant galaxies, indicating that they are
    EXTRAORDINARILY POWERFUL.
  • The gamma-ray flashes last from 0.01 sec up to about 100 seconds.
  • Once localized in space they are observed by other telescopes at different wavelengths.
  • GRBs exhibit afterglows in the x-ray, optical and radio part of the spectrum that can be detected for hours, days or weeks.

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    GRB Models

  • The generic model involves a tremendous explosion that produces a relativistic fireball.
  • Much of the energy of this very hot gas is converted into ultrarelativistic motion, with
    Lorentz factors of ~ 100 in narrow jets.
  • This means we probably only see a small fraction (those beamed toward us) and the actual energies involved are not as extreme as first thought.
  • Still, they are very powerful and can be seen to immense distances.

  • Two leading competing models: Neutron-Star -- Neutron Star merger and Hypernova
  • NS-NS mergers could produce a flattened disk and then a BH. The disk would eject the jets.
  • Hypernova models involve a very massive star (> 30 M_sun) giving rise to a stalled SN,
    delayed BH formation, powerful jets and then a reactivated SN.
  • At least some (and maybe all) GRBs are pretty clearly associated with SN, so the hypernova hypothesis is currently better supported;
  • But some GRBs (maybe the shorter ones) could arise from NS-NS mergers.