TITLE: GCN CIRCULAR NUMBER: 41443 SUBJECT: LIGO/Virgo/KAGRA S250818k: AstroSat CZTI non-detection and upper limits DATE: 25/08/20 13:06:27 GMT FROM: Gaurav Waratkar at IIT Bombay gauravwaratkar@iitb.ac.in
G. Waratkar (IITB), V. Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao (TIFR), S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration:
We have carried a search [1] for X-ray candidates in Astrosat CZTI data in a 100 sec window around the trigger time of the event S250818k (UTC 2025-08-18 01:20:06; LVK Collaboration, GCN Circ. 41437). We use the Bilby.offline0.multiorder.fits,0 map (https://gracedb.ligo.org/api/superevents/S250818k/files/Bilby.offline0.multi...) for our analysis.
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 the merger, Astrosat's nominal pointing is RA,DEC = 00:46:53.8, 42:11:36.4 (11.7242,42.1934), which is ~88 deg away from the maximum probability location, which severely reduces the effective area of CZTI. At the time of the merger event, the Earth-satellite-transient angle corresponding to maximum probability location is ~33 deg and hence is occulted by Earth in satellite's frame. In a time interval of 100 sec around the event, the region of the localisation map which is not occulted by Earth in the satellite's frame has a total probability of 0.59 (59%).
We note that the source AT 2025ulz (Stein et al., GCN Circ. 41414) was occulted behind Earth for AstroSat at the time of the merger event.
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. Quadrant C being noisy was excluded from further analysis. Searches were undertaken by binning the data in 0.1s, 1s, and 10s respectively. Statistical fluctuations in background count rates were estimated by using data from 5 preceding orbits. We selected confidence levels such that the probability of a false trigger in a 100 sec window is 10^-3, after excluding one noisy quadrant. 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 use a detailed mass model of the satellite to calculate the direction-dependent instrument response for points in the visible sky. We then assume the source is modelled as a power law with photon index alpha = -1, and convert our count rate upper limits to direction-dependent flux limits. We obtain the following upper limits for source flux in the 20-200 keV band by taking a probability weighted mean over the visible sky:
| Bin (s) | Flux Limit (ergs/cm^2/s) | Fluence Limit (ergs/cm^2) | |---------|--------------------------|---------------------------| | 0.1 | 1.47e-06 | 1.47e-07 | | 1.0 | 3.78e-07 | 3.78e-07 | | 10.0 | 8.00e-08 | 8.00e-07 |
CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project.
CZTI EMGW detections are reported regularly on the payload site at: http://astrosat.iucaa.in/czti/?q=emgw
[1] Waratkar et al. ApJ 976, 123 (2024) doi:10.3847/1538-4357/ad84e6
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