FRB180311: AstroSat CZTI upper limits and correction to FRB180301 upper limits

ATel #11417; A. Anumarlapudi (IITB), E. Aarthy (PRL), B. Arvind (IISER Pune), V. Bhalerao (IITB), D. Bhattacharya (IUCAA), A. R. Rao (TIFR), S. Vadawale (PRL)
on 14 Mar 2018; 09:41 UT
Credential Certification: Varun Bhalerao (varunb@iitb.ac.in)

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We carried out offline analysis of data from Astrosat CZTI in a 200 second window centred on the FRB 180311 (Parkes discovery - Oslowski, S. et al., ATEL #11396) trigger time, 2018-03-11 04:11:54.80 UTC, to look for any coincident hard X-ray flash. CZTI is a coded aperture mask instrument that has considerable effective area for about 29% of the entire sky, but is also sensitive to brighter transients from the entire sky. At the instant of the FRB, AstroSat was pointing at (RA = 189.2, DEC = 62.3): about 157 degrees away from the nominal FRB direction.

CZTI data were de-trended to remove orbit-wise background variation. We then searched data from the four independent, identical quadrants to look for coincident spikes in the count rates. Searches were undertaken by binning the data in 0.01s, 0.1s, and 1s respectively. Statistical fluctuations in count rates were estimated by using data from 10 neighbouring orbits. We selected confidence levels such that the probability of a false trigger in this 200s window is 10^-4. We do not find any evidence for any hard X-ray transient in this window, in the CZTI energy range of 20-200 keV.

We convert our count rates into fluence and flux limits by assuming that the source spectrum is a power law with alpha = -1.0. We use a detailed mass model of the satellite to calculate the instrument response in the direction of FRB180311. We get the following upper limits for source fluence and flux in the 20-200 keV band:
0.01 s: Effective fluence limit= 7e-7 ergs/cm^2; flux= 7e-5 ergs/cm^2/s
0.1 s: Effective fluence limit= 2e-6 ergs/cm^2; flux= 2e-5 ergs/cm^2/s
1.0 s: Effective fluence limit= 5e-6 ergs/cm^2; flux= 5e-6 ergs/cm^2/s

Update / Erratum in ATEL #11413 (We are reporting corrections in the flux limits with some recalculations) We carried out offline analysis of data from Astrosat CZTI in a 200 second window centred on the FRB180301 (Parkes discovery - Price, D. C. et al., ATEL #11376) trigger time, 2018-03-01 at 07:34:19.76 UTC, to look for any coincident hard X-ray flash. Based on the pointing direction of Astrosat at the time of the event (RA=216.5, DEC=-11.6), the FRB was 124 degrees off axis.

CZTI data were de-trended to remove orbit-wise background variation. We then searched data from three independent, identical quadrants to look for coincident spikes in the count rates. We ignored a quadrant which was very noisy in this time interval. Searches were undertaken by binning the data in 0.01s, 0.1s, and 1s respectively. Statistical fluctuations in count rates were estimated by using data from 10 neighbouring orbits. We selected confidence levels such that the probability of a false trigger in this 100s window is 10^-3. We do not find any evidence for any hard X-ray transient in this window, in the CZTI energy range of 20-200 keV.

We convert our count rates into fluence and flux limits by assuming that the source spectrum is a power law with alpha = -1.0. We use a detailed mass model of the satellite to calculate the instrument response in the direction of FRB180301. We get the following upper limits for source fluence and flux in the 20-200 keV band:
0.01 s: Effective fluence limit= 3e-7 ergs/cm^2; flux= 3e-5 ergs/cm^2/s
0.1 s: Effective fluence limit= 6e-7 ergs/cm^2; flux= 6e-6 ergs/cm^2/s
1.0 s: Effective fluence limit= 2e-6 ergs/cm^2; flux= 2e-6 ergs/cm^2/s

CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, ISAC, IUCAA, SAC and PRL. The Indian Space Research Organisation funded, managed and facilitated the project.