Gamma Ray Burst

Gamma-ray burst

Gamma-ray bursts are the universe’s most energetic cosmic explosions.  They are among the most distant objects known. They hold promise for understanding the most distant parts of the universe, first, as “backlights” allowing absorption spectroscopy studies of the matter distribution at extreme redshifts, and second, as probes of the earliest star formation.

Here is the light curve of a gamma-ray burst from the BATSE instrument from ~ 20-60 keV photon energy:


The time scale, the extreme apparent energetics, and the clear change in the long-term light curve to non-relativistic behavior tells us that this emission is from internal shocks in a highly relativistic jet.

However, there is much we do not know about GRBs, including even the origin of the light we see from them.  There is no consensus on the emission mechanism.

THE NUTTelA-TAO

To learn about the emission mechanism of GRBs, we want to directly measure the optical emission from the gamma-ray burst – NOT the afterglow. The GRB lasts only some ~ 60s, so we have to start with a very fast telescope: The NU Transient Telescope at Assy-Turgen Astrophysical Observatory – the NUTelA-TAO. This telescope can point anywhere above the horizon in < 8 s.

 

We commissioned the telescope in 2018, and here is our “first light” picture:

Burst Simultaneous Three-Channel Instrument

In order to measure the predicted “optical end” of the GRB emission, we designed and are building the Burst Simultaneous Three-channel Instrument, the BSTI:

  • Separate waveband to each camera via dichroics
  • High time resolution without noise penalty via Electron Multiplied CCD cameras – allows cross-correlation with gamma-ray signal to determine if from same emission source
  • Three filters, Sloan i’,r’,g’ allows measurement of power law slope, determination of synchrotron self-absorption frequency.

Through 2019 we (especially Research-Engineer Zhanat Maksut) are working hard on fabricating the BSTI and automating the measurement response of the telescope to GRB position alerts.

Synchrotron models which reproduce the gamma-ray spectrum of GRBs have clear predictions for optical emission (e.g. Shen & Zhang 2009). By measuring the optical slopes with multi-channel cameras, we can verify or eliminate this important model. The spectral slope of prompt GRB emission has never been measured in the optical bands before.

For more details, see Grossan, Kumar and Smoot, 2019 (submitted to JHEA https://arxiv.org/abs/1909.02152).