Download Gamma Ray Burst Afterglows and Host Galaxies

Gamma Ray Burst Afterglows
and Host Galaxies
James E. Rhoads
Space Telescope Science Institute
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.
Fast Debris from GRBs
• GRBs release a lot of energy (about 1052
ergs, corresponding to 1% of the Sun’s rest
mass energy) very quickly (0.1 to 100 sec).
• Their gamma ray brightness varies strongly
in milliseconds… implying sizes < 100 km.
• This much power in so small a region
requires fast outflow. In fact, the ejecta are
highly relativistic, with Γ > 100.
The Prediction of Afterglows
• GRBs require relativistic ejecta.
• Sometime, these ejecta must encounter an ambient
medium….
• => Afterglow!
This argument led to the prediction of afterglows
years before they were observed (Paczyński &
Rhoads 1993, Katz 1994, Mészáros & Rees 1997).
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 > 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.
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.
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.”
Example: GRB 990123
BeppoSAX X-ray images of GRB 990123
GRB 990123 in Visible Light
ROTSE optical images of GRB 990123
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.
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”).
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!
Fireball Models & Basic Results
• Blast wave structure:
Reverse
shock
shocked material
Unshocked
ejecta
Contact
Forward
discontinuity shock
Γ~2
unshocked
Γ ~ 100
ambient gas
• Predict broken power law spectra and light curves,
with a predicted relation between spectral and
light curve slopes.
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)
Afterglow Light Curve for GRB 970508
The Rc band light curve of GRB 970508.
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.
The GRB Distance Scale (GRB 970508)
Metzger et al
1997, Nature
387, 878
GRB
970508:
z >= 0.835
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.
Gamma Ray Burst Host Galaxies
• Optical and radio afterglow observations
can pinpoint GRB locations to an accuracy
of < 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.
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)
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.)
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.
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.
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
color…These could be supernovae also.
• The “smoking gun” will be spectroscopy of
a light curve “bump”.
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.
Afterglows that faded fast:
GRB 990510
Afterglows that faded fast:
GRB 000301C
Collimation Corrected Energies
• Gamma ray
energies before and
after collimation
correction.
• From Frail et al
(2001)
• See also Kumar and
Panaitescu 2002.
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)
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.
GRBs at Extremely High
Redshifts
• GRBs and their afterglows could be
detected at very high redshifts (at least to
z>>5) if GRBs occur there.
• GRBs should occur at high redshift, if they
are really associated with the deaths of
massive stars.
=> Probe of the earliest stars and the
universe in which they formed.
GRB Redshift Distribution
• From
Bloom,
Frail, &
Sari
2001
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.
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.
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?
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”).
=> GRB energy scale is determined, 1052 ergs.
• GRBs occur in star-forming galaxies;
• GRBs may be associated with supernovae;
=> Progenitors are probably massive stars of some
kind.
Tomorrow’s 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?