Fermi
Gamma-ray Space Telescope
Observations of Gamma-ray
Bursts
Julie McEnery
NASA/GSFC and University of
Maryland
On behalf of the Fermi-LAT and
Fermi-GBM collaborations
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Fermi Observatory
Large Area Telescope (LAT):
• 20 MeV - >300 GeV
• 2.4 sr FoV (scans entire sky every
~3hrs)
Gamma-ray Burst Monitor (GBM)
• 8 keV - 40 MeV
• views entire unocculted sky
Launched on June 11, 2008
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All Sky Coverage
LAT sensitivity on 4
different timescales:
100 s, 1 orbit (96
mins), 1 day and 1
year
•
•
•
In survey mode, the LAT observes the entire sky every two orbits (~3
hours), and covers 20% of the sky at any time.
Can also perform pointed observations of particularly interesting regions
of the sky.
• Autonomously repoint for 2.5 hours following on-board detection of a
bright, hard GBM burst
GBM covers the entire unocculted sky
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GRB090902B - Autonomous repoint
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LAT
–
–
–
–
–
pointing in celestial coordinates from -120 s to 2000 s
Red cross = GRB 090902B
Dark region = occulted by Earth ( z>113°)
Blue line = LAT FoV (±66°)
White lines = 20° (Earth avoidance angle) / 50° above horizon
White points = LAT events (no cut on zenith angle)
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Fermi and GRB
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•
•
LAT: <20 MeV to >300 GeV. With both onboard and ground burst triggers.
GBM: 12 NaI detectors— 8 keV to 1 MeV. Used for onboard trigger, onboard and
ground localization, spectroscopy: 2 BGO detectors— 150 keV to 40 MeV. Used for
spectroscopy.
Total of >7 energy decades!
Good spectral
observations of the
prompt phase of lots of
GRB
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Alerts and Data Flow
1s
Instrument
Trigger(s)
Fast
signal
GBM>LAT
10 s
Alerts and
updates
100s
1000 s
103 s
Repoint Repoint
to GCN with request request
localization GBM- LAT->SC
>LAT
Science processing
parameters reviewed by
Users Committee
•
104 s
Slew to keep burst
within LAT FOV
(dwell time 2.5 hrs)
105 s
Regularly-scheduled
data downlinks (10-12/day)
Planned repoint frequency (adjustable):
• bursts starting within LAT FOV ~2/month
• bursts starting outside LAT FOV ~2/year
Onboard processing (both LAT and GBM) - GCN alerts: location, intensity (cnts),
hardness ratio, trigger classification (GRB, solar flare etc)
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GBM Prompt ground processing (10-30 mins): updated location, lightcurve.
Quicklook GBM products available at FSSC
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GBM Burst Advocate localization
LAT ground processing (5-15 hours): updated location, high energy spectrum, flux
(or upper limit), afterglow search results. LAT count data available.
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Final ground processing (24-48 hours): GBM model fit (spectral parameters, flux,
fluence), joint LAT-GBM model fit, raw GBM data available.
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Science Data Availability
No proprietary gamma-ray data - Everyone gets access to
the data at the same time
Latency
requirement is
72 hours, typical
latency is much
less ~<10 hours
•
LAT and GBM instrument teams generate additional high level data
(lightcurves, transient alerts, pulsar timing solutions etc) which are served to the
community by the FSSC
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GBM Triggers/Month
• Nov 9, 2009 - add new TGF trigger
• TGF trigger rate increased by factor of ~10 to 1 per 3.7 days
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Fermi GRB detections
GRB 130427A
GRB 131108A
14 LAT bursts with measured redshift from z=0.145 (GRB130702A via iPTF!) to 4.35
(GRB080916C)
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GBM Spectra - Thermal Components?
GRB100724B - bright GBM burst
Non
thermal
Expect thermal/blackbody component
from photosphere
• Black body components have been fit in
several GRB - GRB080916C,
GRB090902, GRB100724B…
Guiriec et al, 2011, Ryde et al 2011
Total
Non
thermal
Thermal(kT≈
38keV)
kT~constant, evolving nonthermal component
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Continuous high time resolution data
• The initial GBM configuration was to return count rates binned
in time and energy.
– After an onboard trigger enhance time and energy
resolution of the binned data for 10 mins and collect
individual time-tagged events for ~5.5 minutes.
• On November 26, 2012, we transitioned to continuously
collecting time-tagged event data
– This allows for sensitive ground-based analyses on short
timescales
• Excellent for TGFs, which typically have durations less
than the shortest onboard trigger integration time.
• Also helpful for searches for sub-threshold short GRB,
precursors etc.
• Pipeline to search for SGRB offline is currently under
development by the GBM team.
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GBM Localization Contours
• GBM’s strength is its broadband spectral coverage of the prompt
emission of GRB, its weakness is relatively poor localization
– 68% containment: 15o (1st FSW), 11.5o (last FSW), 7.5o (GA), 5o (HitL)
• The GBM team have developed new data products to support
follow-up observations of GRB locations.
– Location of the GRB along with 1, 2 and 3 sigma contours that
include both the statistical and the current best-guess
systematic uncertainties on the GBM localizations.
• Already two successful follow-ups with iPTF using the contour files
(130702A & 131011A)
• These localization products will routinely be made publicly
available from the FSSC.
• Please see Adam Goldstein’s poster (or talk with Adam) for more
details.
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Fermi-LAT Observations of GRB
• Onset of >100 MeV delayed w.r.t. keV flux
• Durations of high energy emission longer than keV emission
– Spectral and flux variability greatest at early times
– Spectral index ~constant at later times
• Hard power-law components seen in bright LAT GRB
• Cutoffs -> fewer detected GRB than hoped
• MeV-GeV counterparts to X-ray flares?
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Energetics of Fermi-LAT GRBs
130427A
• LAT GRBs are among the most energetic both intrinsically and
observationally
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Comparing MeV to GeV emission
• The GeV fluence is
typically ~10% of the
MeV fluence.
• The two short GRB
may be exceptions to
this, but need more
short GRB to confirm.
100%
10%
1%
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Highest Energy Photons
Highest energy photons
provide constraints on
the
acceleration/emission
processes (need to be
able to produce the
photons) and on
conditions in the
emission region (need
to be able to get them
out)
• The highest energy photons are not correlated with spikes in
the prompt emission lightcurve, and often arrive after the end
of the prompt emission
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Delayed onset of >100 MeV Emission
• The LAT >100 MeV
emission starts after the
keV emission, sometimes
by up to 80 seconds
• Seen for both long and
short GRB
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Duration Distributions
•
We measure a systematically
longer duration in the LAT
– Emission at GeV energy
lasts longer than the
emission at MeV energy
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Extra power-law components in most bright
bursts
Ackermann et al ApJ
Abdo, A. A., et al. 2009, ApJ, 706, L138
GRB 090902B
Best fit spectrum to interval b
(T0+4.6 s to T0 + 9.6 s) is a band
function (smoothly broken powerlaw) + power-law component.
GRB090510. First bright short GRB
Clear detection of an extra component,
inconsistent with the Band function.
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GRB090926B
• Sharp spike seen at all
measured gamma-ray
energies
– Strongest below 15
keV and above 10
MeV
– Clear correlation
between keV and
MeV/GeV
lightcurves
Ackermann et al, 2011
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GRB090926 - extra spectral component
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•
Hard power-law component emerges during the bright spike, with
cutoff at 1.4 GeV
Power-law index remains constant through the afterglow (~5000
seconds)
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Long-lived Emission with powerlaw temporal decays
GRB 090510
GRB 090902B
De Pasquale, M., et al. 2010, ApJ, 709, L146
GRB 080916C
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Breaks in the extended emission
• Significant (>3 sigma) breaks
seen in 3 bright GRB
• Transition from prompt to
afterglow?
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LAT Detection during X-ray Flare Activity
•
GRB100728A
– Bright GBM burst->ARR
– No prompt LAT detection
(but was at edge of FoV,
58 deg)
– Hard spectrum (1.4+-0.2)
– Gamma-ray fluence
consistent with
extrapolation of the X-ray
flare spectrum
– Unable to distinguish
between afterglow or flare
emission due to weak
LAT detection
•
Sample of 140 Swift GRB
– 49 (35%) show flares at early times
– 12 with good LAT observations (in FoV and away from Earth limb)
– 29 flares with simultaneous Fermi/Swift observations, 1 detection!
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How fast is the emission region
moving?
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•
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Relativistic motion of the emitting shell:
– A relativistic motion of the shell allows higher energy events in dense
region to escape.
– Observing high-energy events correlated with the fast variability allows
us to constrain to the speed (Gmin) of the emitting shell.
For target photon spectrum
assume band function, or powerlaw.
Caveat : target photon field
assumed uniform, isotropic, timeindependent
– More realistic modeling yields
significantly (~3 times) lower
values
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GRB130427A
• One of the brightest
GRBs in gammarays ever detected
– Energy released
in gamma-rays
~1054 erg
• Highest energy
photon (95 GeV)
• Longest lasting GeV
emission – LAT
detected emission
for over 20 hours
• Redshift = 0.34
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GRB130427A Synchrotron emission?
Equate radiation and
acceleration timescales
• The high energy LAT-detected
photons challenge synchrotron origin
from shock accelerated electrons
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GRB131108A
• First onboard LAT detection in 4 years!
• Very bright burst in LAT
• Triggered Swift TOO based on LAT onboard location
– Swift found the X-ray afterglow
• Analysis is ongoing, but initial analysis suggests:
– Softer than usual LAT spectrum
– Highest energy LAT photon just 1.5 GeV
• The bright Fermi-LAT burst drought is over!
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The Future
• We are continuing to find unusual bursts
– GRB130427A, GRB131108A
• Planned improvements to LAT FSW to boost LAT onboard
trigger rate (by taking advantage of the GBM->LAT fast signal)
• New observatories coming online especially high energy and
multi messenger
– HAWC (wide-field TeV observatory)
– VERITAS, HESS2, MAGIC etc
– Unique opportunities for joint gravitational wave/photon
detections of binary mergers with advanced LIGO and
Fermi
• New GBM localization data products, possible localization
improvements
• Significant improvements to LAT data reconstruction will bring
increased effective area, field of view and angular resolution
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Updated event reconstruction
Analysis and configuration improvements coupled with robust
hardware means that we will continue to expand LAT science
capabilities
• The LAT collects a significant
amount of information for each
gamma-ray event
– Ground processing reduces this
to directions, energy, event type
(gamma-ray or background) and
associated errors
– Extensive scope for analysis
improvements tailored to specific
science questions or scenarios
– Extensive rework of low-level
algorithms is currently in
progress, ready for release in
2014/2015
Factor of two increase in acceptance
at 100 MeV
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GRBs and Gravitational Waves
Fermi-GBM and Advanced LIGO (>2015) should see coincident
Gravitational wave/Electromagnetic emission or rule out NS-BH mergers
as the progenitors of short GRB
Large rate of short bursts
in GBM is key to
coincident detections
GBM Short GRBs in ALIGO
horizon:
N(z<0.11, NS-NS) ~ 2+4
yr-1
-1
-1
N(z<0.22, NS-BH) ~ 8+6
-3 yr
• Both observations bring complementary information: ALIGO → inspiral
characteristics ; Fermi → jet properties & environment
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Summary
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250 GRB/year detected by GBM
~10 GRB/year detected at high energy by the Fermi LAT
Prompt emission observed over a wide energy range
• Delayed time onset between the LAT and GBM data
• Extra spectral components in most bright bursts
• High energy cutoffs in several GRB
• Thermal components in several GBM detected GRB
• Band model is no longer the best phenomenological model
Temporally extended high energy emission is common
• Likely related to early afterglow, very constraining to
emission/acceleration models
• Flux decreases as a power-law with time
• 3 bursts observed to have breaks in the temporal decay
Bright GRB challenge standard models
• GRB130427a challenges commonly accepted models
• Look out for new results from GRB131108A
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