Péter Veres' research page on gamma-ray bursts

For the first binary black hole merger detected by LIGO, GW150914, Fermi-GBM detected a tantalizing weak signal (GW150914-GBM), 0.4 s later. If GW150914 and GW150914-GBM were associated, how many LIGO-Virgo binary BH mergers would Fermi-GBM have to follow up to detect a second source? A large majority of the models considered here predict a BBH-to-GRB ratio in the range of 5 - 20, but for optimistic cases it can be as low as 2, while for pessimistic assumptions it can be as high as 700.

The plot shows a simulated inclination distribution of binary BHs that produce GW signal with signal to noise greater than 8. It follows the theoretical expectation (red line).

1.7 seconds after the first neutron star binary merger was detected by LIGO-Virgo, Fermi-GBM triggered on GRB 170817A. The GRB is short, close by, and has the same source as the gravitational wave signal. This is the first unambiguous gravitational wave - electromagnetic radiation detection.

The plot shows from top to bottom the GBM lightcurve in soft and hard channels, INTEGRAL-ACS lightcurve and the LIGO/Virgo binary neutron star merger signal.

0.4 seconds after the first gravitational wave signal observed by LIGO (GW150914), Fermi GBM detected a weak short GRB, having a loosely consistent position with the GW event (GW150914-GBM). Gamma-rays from binary black hole mergers are unexpected, and we need more observations to confirm or to refute such a scenario. In this paper we assume the two are related and analyze how GW observations constrain GRB models. We find that internal shocks and non-dissipative photosphere models can be marginally ruled out.

This plot shows the constraints GW 150914-GBM places on the GRB physics in the context of the external shock model (gray histogram). Brown lines show the peak energy, red is the peak flux. Also shown are R-deceleration radius, where the shock is the strongest, Click on the image for a larger version

A GRB similar to GRB 170817A

Long term LAT upper limits

Digging deep into the GBM data

GRB 090510A: modeling of

Fermi GBM's 6 year catalog

V404 Cygni during the 2015 bright state

Swift observes an ultralong GRB

Relation between GRB Lorentz factor

VERITAS observes GRB 130427A

GRB jet composition

The Cherenkov Telescope Array

Link between prompt and

Very Large Array observations 4 hours after the GRB onset provided useful insights into the nature of the GRB afterglow. We model this burst with a wind medium at early times, which then changes to a constant density interstellar medium. We explore weather there is a reverse shock present and conclude that if it is, we cannot distinguish its signature from the purely forward shock dominated outflow.

The figure shows radio observations at different frequencies and the possible physical models.

Some short gamma-ray bursts show extended emission that is softer than the initial short pulse. This might be the signature of material injected at later times into the decelerating shock complex. The late injection can be modeled in two ways: either an instantaneous injection releases material with a range of Lorentz factors (assumed to be power-law distributed with index s), or a central engine that is active for a long time, e.g. a magnetar. We calculated the high energy component which can be expected from such a late injection. The main source of the high energy emission is the synchrotron self-Compton process.

The figure shows synchrotron self-Compton spectrum for late injection models and the sensitivity of current and future Cherenkov telescopes.

After a qualitative analysis, we have shown that the model with dominant magnetic fields can interpret the brightest GRBs observed by Fermi. We have developed a model with general acceleration with several free parameters (luminosity, coasting Lorentz factor, ambient density, etc.). We then folded this model through the Fermi detector's response matrices to match the observations. We found that the model is capable to interpret the data.

The figure shows a spectral fit of the Fermi GBM and LAT data of GRB 090510A with a magnetic model. Click on the figure for a larger version.

GRB 110721A was observed by Fermi and after careful spectral analysis the peak at the beginning of the burst was observed to be 15 MeV, one of the highest ever measured. This poses problems for the standard internal shock interpretation. We showed that in the framework of a photospheric model, this high peak energy can be achieved by synchrotron emission.

The figure shows the allowed region by the synchrotron model for the peak energy (below the dashed and dash-dotted thick lines for different types of acceleration). Diamonds mark the evolution of the peak energy with luminosity for this burst.

We worked out the details of a model, where the Lorentz factor of the jet increases more gradually (as radius^1/3) than in baryonic models which were mostly considered so far. The prompt emission originates from the photosphere, the GeV emission observed by Fermi-LAT is from the external inverse Compton upscattering of the prompt photons off the ambient medium's electrons. This model gives a qualitatively correct picture of the observations. Take a look at my presentation of this topic at the Munich conference in 2012 here or here.

The figure shows a qualitative model spectrum for magnetically dominated dynamics with photospheric emission at MeV and external inverse Compton components at GeV. Thick gray lines show the total emission with or without considering the reverse shock.