FRB181228: AstroSat CZTI upper limits

ATel #12370; A. Anumarlapudi (IITB), E. Aarthy (PRL), V. Bhalerao (IITB), D. Bhattacharya (IUCAA), A. R. Rao (TIFR), S. Vadawale (PRL)
on 9 Jan 2019; 10:19 UT
Credential Certification: Varun Bhalerao (varunb@iitb.ac.in)

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We have carried a search for X-ray candidates in Astrosat CZTI data in a 20 sec window around the de-dispersed arrival time of the event FRB181228 (UTC 2018-12-28-13:48:50.1, Farah W. et al, ATel #12335 ). 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 time of FRB emission, Astrosat's nominal pointing is (RA=53.47916667, DEC=-27.924), approximately 35.5 deg away from the transient. The FRB direction was not occulted by earth in Astrosat's frame. CZTI data were de-trended to remove orbit-wise background variation. We then searched data from three of the four independent, identical quadrants to look for coincident spikes in the count rates. We ignored the data from quadrant A as it was saturated by detector noise in this 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 20s 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 flux by assuming that the source spectrum is a power law with alpha = -1.0. We then iterate over alpha such that the source spectrum is consistent with observed radio flux and then use this power law index to calculate the flux limits. We use a detailed mass model of the satellite to calculate the instrument response in the direction of FRB181228. We get the following upper limits for source flux corresponding to power law index of -1.37 in the 20-200 keV band: 0.01 s: flux limit= 1.2e-7 ergs/cm^2/s 0.1 s: flux limit= 2.0e-7 ergs/cm^2/s 1.0 s: flux limit= 2.7e-7 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.