Gamma Ray Burst Afterglows and Host Galaxies

1
Gamma Ray Burst Afterglows and Host Galaxies

• James E. Rhoads
• Space Telescope Science Institute

2
The Quest for Counterparts

• Most classes of astronomical object can be
  studied at a range of wavelengths. For 24 years
  GRBs were an exception.
• Difficulties due to the difficulty of deriving
  accurate positions from gamma rays, especially in
  short times.
• Potential payoff Large, as we shall see.

3
Fast Debris from GRBs

• GRBs release a lot of energy (about 1052 ergs,
  corresponding to 1 of the Suns rest mass
  energy) very quickly (0.1 to 100 sec).
• Their gamma ray brightness varies strongly in
  milliseconds implying sizes lt 100 km.
• This much power in so small a region requires
  fast outflow. In fact, the ejecta are highly
  relativistic, with G gt 100.

4
The Prediction of Afterglows

• GRBs require relativistic ejecta.
• Sometime, these ejecta must encounter an ambient
  medium.
• gt Afterglow!
• This argument led to the prediction of afterglows
  years before they were observed (Paczynski
  Rhoads 1993, Katz 1994, Mészáros Rees 1997).

5
Probable Sequence of GRB Events

• The central engine emits a large amount of energy
  (in almost any form).
• Most of that energy accelerates a small mass
  (about the mass of the Earth) to speeds gt 99.99
  of lightspeed.
• Collisions between different shells of ejected
  debris creates the gamma rays.
• Collisions between ejected debris and
  interstellar gas create the afterglow.

6
Afterglows are decoupled from the central engine

• The prediction of GRB afterglows requires only
  that there be highly energetic, highly
  relativistic ejecta.
• It does not matter what the original source of
  the energy is.
• Also, it natural that a large part of the GRB
  energy goes into kinetic energy of ejecta, given
  the sizes and energies involved.

7
Possible Central Engines

• The Central Engine refers to the monster in
  the middle the ultimate source of the burst.
• Current interest focuses on two possibilities
• The collapse of a massive, rotating star into a
  central black hole or
• Two neutron stars that merge in a cataclysmic
  explosion.
• Either way, the immediate progenitor is a
  rotating black hole surrounded by a massive ring
  of matter. This can provide sufficient energy for
  the burst.

8
The Central Engine (speculative!)
Afterglow properties are decoupled from the
central engine.
Martin Reess best buy model.
B 1e15 Gauss
Neutron Torus
Can tap gravitational, rotational, or magnetic
energy.
9
Precise GRB Positions and the First GRB
Counterparts

• The Italian-Dutch satellite BeppoSAX began
  providing accurate GRB locations in 1997 (good to
  a few arcminutes and available within a few
  hours).
• This led to the first counterparts at X-ray (GRB
  970111), optical (GRB 970228), and radio (GRB
  970508) wavelengths.
• These have been named afterglows.

10
How Afterglows are Found

• Gamma ray detectors see the burst.
• Fairly good positions come from gamma ray
  instruments (via triangulation) and/or X-ray
  images within hours of the burst.
• These positions are searched for variable objects
  using X-ray, optical, and/or radio telescopes.
• The brightest optical afterglow yet peaked at 9th
  magnitude, during the GRB itself!

11
Example GRB 990123
BeppoSAX X-ray images of GRB 990123
12
GRB 990123 in Visible Light
ROTSE optical images of GRB 990123
13
GRB 990123 Hubble Images
Hubble Space Telescope images of GRB 990123 and
its host galaxy at 16, 59, and 380 days after the
gamma ray burst.
14
Triumphs of Afterglow Studies

• Afterglows have demonstrated that
• Fireball models describe GRBs reasonably well
• GRBs are at cosmological distances
• GRB ejecta move relativistically
• GRBs occur in galaxies.
• GRBs may be associated with the deaths of some
  (but not all!) high mass stars
• GRBs may be collimated (search lights rather
  than flood lights).

15
Afterglow Models Ingredients

• Initial conditions Energy, ejecta mass, Lorentz
  factor, jet opening angle.
• Ambient medium density profile
• Relativistic shock physics
• Distribution of energy among radiating electrons
  and magnetic fields in highly relativistic shocks
• Note, this cannot be tested in the lab!

16
Fireball Models Basic Results

• Blast wave structure
• Reverse Contact Forward
• shock discontinuity shock
• Unshocked shocked material
  unshocked
• ejecta G 2 G
  100 ambient gas
• Predict broken power law spectra and light
  curves, with a predicted relation between
  spectral and light curve slopes.

17
Afterglow Spectral Energy Distribution

• Spectral energy distribution for the simplest
  afterglow model.
• Measurement of whole spectrum can determine the
  energy, ambient density, and some shock physics
  parameters.

(Figure from Sari, Piran, Narayan 1998)
18
Afterglow Light Curve for GRB 970508

• The Rc band light curve of GRB 970508.

19
Prompt Optical Followup

• Several experiments (LOTIS, ROTSE, TAROT, )
• Only one detection.
• Several GRBs have low optical to gamma ratios.
• Implications for initial Lorentz factor

Fig. 2 of Akerlof et al 2000 ROTSE data rescaled
by GRB fluence.
20
The GRB Distance Scale (GRB 970508)
Metzger et al 1997, Nature 387, 878 GRB
970508 z gt 0.835
21
Afterglow Distance Determinations
HST spectrum of GRB 000301C. (From Smette et al
2000.)
22
Proof of Relativistic Speeds

• Interstellar gas in our Galaxy causes small radio
  sources to twinkle (like stars seen in visible
  light through our atmosphere).
• Larger sources do not twinkle (like planets).
• Measuring the time when an afterglow stops
  twinkling at radio wavelengths reveals its
  speed of expansion to be near light speed.

23
Gamma Ray Burst Host Galaxies

• Optical and radio afterglow observations can
  pinpoint GRB locations to an accuracy of lt 10,000
  light years.
• This is smaller than a typical galaxy.
• In most cases, a galaxy is indeed seen where the
  afterglow is found.
• These galaxies are reasonably typical of distant,
  star forming galaxies.

24
Locations of GRBs in their Host Galaxies

• Not all gamma ray bursts occur at the nucleus of
  their host galaxies.
• This rules out quasars and related objects (i.e.,
  the central black holes of galaxies) as the
  origin of GRBs.
• (Figure from Bloom et al 1999)

25
GRB Host Galaxies are Blue
Colors and magnitudes (brightness) of galaxies in
the Hubble Deep Field and of three GRB Host
Galaxies. Bluer is down, fainter is to the
right. (From Fruchter et al 1999.)
26
GRBs in Obscured Starbursts?

• Recently, two GRB host galaxies have been shown
  to have unusually high submillimeter wavelength
  brightness.
• This suggests strong star formation activity
  hidden by dust.

27
Gamma Ray Bursts and the Deaths of Massive Stars

• Afterglow data suggests that GRBs occur
• in galaxies with active star formation,
• often in regions with a lot of gas, which is
  where new stars form and where the most massive
  stars spend their entire brief lives.
• However, GRBs are so rare that only a tiny
  fraction of massive star deaths could produce
  them.

28
The GRB-Supernova Connection

• A nearby supernova, SN 1998bw, was found by
  searching the error box of GRB 980425.
• Some other GRBs show evidence for late time
  bumps in the light curve, often red in
  colorThese could be supernovae also.
• The smoking gun will be spectroscopy of a light
  curve bump.

29
Are Gamma Ray Bursts Searchlights?

• The extreme energy needed to produce a GRB could
  be reduced dramatically if the bursts are
  collimated searchlights.
• Three predictions for collimated GRBs
• There should be orphan afterglows, and
• Afterglows of collimated GRBs should fade more
  rapidly at late times.
• Afterglow light may be polarized.
• There are now several likely observations of
  rapid late time fading.

30
Afterglows that faded fast GRB 990510
31
Afterglows that faded fastGRB 000301C
32
Collimation Corrected Energies

• Gamma ray energies before and after collimation
  correction.
• From Frail et al (2001)
• See also Kumar and Panaitescu 2002.

33
Orphan Afterglows

• During the evolution of a GRB remnant,
• The ejecta slow down
• The characteristic photon frequency drops
• Collimation of the photons decreases.
• So, the observed transient rate should increase
  with wavelength if GRBs are collimated.
• (Rhoads 1997 Perna Loeb 1998)

34
Polarization

• Afterglows are thought to be produced by
  synchrotron emission, which is typically
  polarized.
• By symmetry, a spherically symmetric burst should
  have no net polarization.
• There is no corresponding argument for collimated
  bursts and net polarization is expected.

35
GRBs at Extremely High Redshifts

• GRBs and their afterglows could be detected at
  very high redshifts (at least to zgtgt5) if GRBs
  occur there.
• GRBs should occur at high redshift, if they are
  really associated with the deaths of massive
  stars.
• gt Probe of the earliest stars and the universe
  in which they formed.

36
GRB Redshift Distribution

• From Bloom, Frail, Sari 2001

37
Effects of Gamma Ray Bursts on their Environments

• Gamma ray bursts are not nice neighbors.
• The high energy photons they produce can destroy
  interstellar dust grains up to 100 pc away, and
  ionize interstellar gas at similar distances.
• The ionized gas will fluoresce as it gradually
  recombines, and can be used to look for GRB
  remnants in nearby galaxies.

38
The Shape of Things to Come

• Swift, a NASA MidEx mission, is approved and
  should fly in 2003. Yield 300 good positions
  per year?
• Swift will have hard X-ray, soft X-ray, and
  optical/ultraviolet instruments on board.
• Response time for the optical 20 to 70 seconds.
• This will open the way for systematic study of
  afterglows, including the still-mysterious short
  bursts.

39
The Niche for Amateurs in the Swift Era

• How can amateur observers complement the onboard
  optical capability of Swift?
• Red wavelength observations
• Light curve monitoring during Earth occultation
  of Swift
• Polarization information? (Hard)
• Orphan afterglow followup? (Faint)
• Monitoring of candidate lensed GRBs?

40
Triumphs of Afterglow Studies

• Afterglows have enabled explosive growth in GRB
  studies. In particular, they have shown that
• GRBs are at cosmological distances
• GRBs are likely collimated (search lights
  rather than flood lights).
• gt GRB energy scale is determined, 1052 ergs.
• GRBs occur in star-forming galaxies
• GRBs may be associated with supernovae
• gt Progenitors are probably massive stars of
  some kind.

41
Tomorrows Questions

• Upcoming space missions, better coordinated
  followup, and ongoing theoretical work all
  promise continued rapid progress in GRBs.
• Specific areas of enquiry
• Do the short GRBs have afterglows? Host
  galaxies?
• Do all GRBs have associated supernovae?
• Are all GRBs associated with massive stars, or
  are some caused by merging neutron stars?
• And, ultimately.
• What is the source of the Gamma Ray Bursts?