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Sub-MeV spectroscopy with AstroSat-CZT imager for gamma ray bursts

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Abstract

Cadmium–Zinc–Telluride Imager (CZTI) onboard AstroSat has been a prolific Gamma-Ray Burst (GRB) monitor. While the 2-pixel Compton scattered events (100–300 keV) are used to extract sensitive spectroscopic information, the inclusion of the low-gain pixels (\(\sim \)20\(\%\) of the detector plane) after careful calibration extends the energy range of Compton energy spectra to 600 keV. The new feature also allows single-pixel spectroscopy of the GRBs to the sub-MeV range which is otherwise limited to 150 keV. We also introduced a new noise rejection algorithm in the analysis (‘Compton noise’). These new additions not only enhances the spectroscopic sensitivity of CZTI, but the sub-MeV spectroscopy will also allow proper characterization of the GRBs not detected by Fermi. This article describes the methodology of single, Compton event and veto spectroscopy in 100–900 keV combined for the GRBs detected in the first year of operation. CZTI in last five years has detected \(\sim \)20 bright GRBs. The new methodologies, when applied on the spectral analysis for this large sample of GRBs, has the potential to improve the results significantly and help in better understanding the prompt emission mechanism.

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Notes

  1. http://astrosat.iucaa.in/czti/?q=grb.

  2. Research Systems Inc. (1995). IDL user’s guide: interactive data language version 4. Boulder, CO: Research Systems.

  3. The CZTI spectral data fit is considered to be reasonable when (a) the obtained residuals are roughly randomly distributed around zero, (b) the reduced chi-square \(\chi ^2 < 2\) and the (c) normalization of the band function is found to be consistent with what is obtained from Konus-wind and Fermi spectral analysis.

  4. https://fermi.gsfc.nasa.gov/ssc/data/analysis/scitools/gtburst.html

  5. https://swift.gsfc.nasa.gov/analysis/threads/bat_threads.html.

  6. https://heasarc.gsfc.nasa.gov/ftools/caldb/help/grppha.txt

  7. The scatter is the standard deviation of the Gaussian fit to the distribution of the displacement of the CZTI measured flux from the Fermi flux and is found to be \(\sigma =0.21\).

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Acknowledgements

This publication uses data from the AstroSat mission of the Indian Space Research Organization (ISRO), archived at the Indian Space Science Data Centre (ISSDC). CZT-Imager is built by a consortium of Institutes across India including Tata Institute of Fundamental Research, Mumbai, Vikram Sarabhai Space Centre, Thiruvananthapuram, ISRO Satellite Centre, Bengaluru, Inter University Centre for Astronomy and Astrophysics, Pune, Physical Research Laboratory, Ahmedabad, Space Application Centre, Ahmedabad: contributions from the vast technical team from all these institutes are gratefully acknowledged. We acknowledge the use of Vikram-100 HPC at the Physical Research Laboratory (PRL), Ahmedabad and Pegasus HPC at the Inter University Centre for Astronomy and Astrophysics (IUCAA), Pune. This research has also made use of data obtained through the High Energy Astrophysics Science Archive Research Center Online Service, provided by the NASA/Goddard Space Flight Center.

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Authors and Affiliations

  1. Kavli Institute of Astrophysics and Cosmology, 452 Lomita Mall, Stanford, CA, 94305, USA

    Tanmoy Chattopadhyay

  2. The Inter-University Centre for Astronomy and Astrophysics, Pune, 411 007, India

    Soumya Gupta, Vidushi Sharma, Shabnam Iyyani, A. R. Rao & Dipankar Bhattacharya

  3. Department of Physics, KTH Royal Institute of Technology, AlbaNova, 10691, Stockholm, Sweden

    Vidushi Sharma

  4. Dipartimento di Fisica, Universita di Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133, Roma, Italy

    Ajay Ratheesh

  5. INAF Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere, 00133, Roma, RM, Italy

    Ajay Ratheesh

  6. Dipartimento di Fisica, Universita La Sapienza, 00185, P. le A. Moro 2, Roma, Italy

    Ajay Ratheesh

  7. Physical Research Laboratory, Ahmedabad, 380 009, India

    N. P. S. Mithun, E. Aarthy, Abhay Kumar & Santosh V. Vadawale

  8. Indian Institute of Technology Bombay, Mumbai, 400 076, India

    Sourav Palit & Varun Bhalerao

  9. Tata Institute of Fundamental Research, Mumbai, 400 005, India

    A. R. Rao

Authors
  1. Tanmoy Chattopadhyay
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  2. Soumya Gupta
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  3. Vidushi Sharma
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  6. N. P. S. Mithun
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  7. E. Aarthy
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  10. Santosh V. Vadawale
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  11. A. R. Rao
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  12. Varun Bhalerao
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  13. Dipankar Bhattacharya
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Corresponding author

Correspondence to Tanmoy Chattopadhyay.

Additional information

This article is part of the Special Issue on “AstroSat: Five Years in Orbit”.

Appendix A

Appendix A

Plots for the Bayesian block analysis conducted on single event data of the GRBs are shown in Fig. A1.

Figure A1
figure 17figure 17

The Bayesian block binning of the single event CZTI light curve of the bursts are shown above in black solid lines. The time interval of the integrated emission of each burst is marked by the vertical dotted lines on the respective plots. The red dashed horizontal line marks the background level. The basic light curve is plotted in the background in pink colour. We note that here the 0 marks the start of the \(T_{90}\) region of the burst.

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Chattopadhyay, T., Gupta, S., Sharma, V. et al. Sub-MeV spectroscopy with AstroSat-CZT imager for gamma ray bursts. J Astrophys Astron 42, 82 (2021). https://doi.org/10.1007/s12036-021-09718-2

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