The High-Altitude Water Cherenkov Gamma-Ray Observatory

Gamma Ray Bursts


Emitting over 1052 ergs in gamma rays, gamma-ray bursts (GRBs) are the most energetic phenomena in the known Universe. GRBs are transient, lasting from fractions of a second to nearly 15 minutes. Their light curves exhibit no particular pattern, and may include one or many bright flares (see figure below). Based on the length of their emission they are divided into short bursts (<2 seconds of gamma-ray emission) and long bursts (>2 seconds of gamma-ray emission). The prevailing model of short bursts is the coalescence of two neutron stars or a neutron star and a black hole. The long bursts are thought to occur when a massive star collapses to form a neutron star or black hole.

GRB light curve
Light curve of GRB 021206 measured by RHESSI between 20 keV and 3 MeV. From C. Wigger et al., ApJ 675:553, 2008.

The energy spectra of the majority of GRBs is well described by a Band function, an empirical combination of two power laws. Some GRBs also have an additional hard power law component in their spectra. This was observed first by the Compton Gamma Ray Observatory (M. Gonzalez et al., Nature 424:749, 2003) and RHESSI (C. Wigger et al., ApJ 675:553, 2008) and later by the Fermi sattelite (A. Abdo et al., ApJ 706:L138, 2009 and M. Ackermann et al., ApJ 716:1178, 2010).

In 2013, the intense GRB 130427A produced the highest-energy photon ever recorded from such an object, a 95 GeV gamma ray observed by Fermi-LAT. HAWC was under construction during this event, but was in partial data acquisition and provided the first very high-energy limits on the emission from GRB 130427A. With the full detector now in operation, HAWC would observe such a GRB should it occur within our field of view.

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Observations at High Energy

The measurements of high energy gamma-ray spectra from GRBs are rich with physics, providing strong constraints on the magnetic fields, the circumburst medium, and the bulk Lorentz factors in GRB models. However, detection of >100 GeV gamma rays from GRBs has proven difficult. The only evidence for TeV gamma ray emission available so far comes from Milagrito, the predecessor of the Milagro detector (R. Atkins et al., ApJ 533:L119, 2000). And even for this event the data provided marginal support for the detection at about the level of 3σ.

Observations at the highest photon energies for a large number of GRBs are necessary to completely understand of the acceleration processes and energy budgets of these extraordinary objects. Only an observatory with a wide field of view and high duty cycle can make these observations, since the key to success is the ability to catch a burst as it happens. In this respect there is significant complementarity between HAWC and the Imaging Air Cherenkov Telescopes (IACTs) used to conduct high-sensitivity, narrow-field surveys of the TeV sky. Because IACTs conduct pencil-beam observations of the sky, it is quite difficult for such observatories to catch a GRB "in the act." An IACT must slew into position to observe a flaring GRB, losing critical time when the highest-energy gamma-rays are arriving at Earth. By observing GRBs during the prompt phase, HAWC can alert the IACTs of extreme high-energy transients, allowing follow-up observations with these very sensitive observatories.

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Sensitivity of HAWC to GRBs

The figure below illustrates the complementary ability of HAWC to observe shorter time scale variations than similar observations with the Fermi satellite at GeV energies. HAWC can also be used to extend the energy range of observations beyond the upper limit of the Fermi-LAT detector.

Sensitivity to GRBs
Estimated sensitivity of HAWC to gamma-ray bursts using the two data aquisition systems in the detector: the main DAQ (black lines) and the scalers (red lines). The y-axis is the flux (prior to redshift attenuation) required in order to have a significant HAWC detection. The sensitivity is shown as a function of burst duration for a GRB with a spectral index of -1.6, a zenith angle of 20°, and several different redshifts. Also shown is the flux necessary for the observation of one photon above 10 GeV by Fermi-LAT.

Our calculations show that HAWC should be able to detect some of the most powerful GRBs. If no TeV emission is detected, upper limits will be placed that constrain GRB models.

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