Abstract
We report preliminary results obtained from the AstroSat observation of the newly discovered X-ray binary MAXI J1803-298. The transient X-ray source was discovered with MAXI/GSC on 01 May, 2021 (ATel #14587) and subsequently detected by NICER (ATel #14588, ATel #14602, ATel #14606), Swift (ATel #14591), INTEGRAL (ATel #14598, ATel #14601), NuSTAR (ATel #14609), and Insight-HXMT (ATel #14613) in soft and hard X-ray bands. Follow-up observations are made in optical (ATel #14594, ATel #14597) and radio wavebands (ATel #14607) too. AstroSat observed the source from 11 May 2021 12:08 (UTC) to 12 May 2021 17:24 (UTC) for exposure of ~50 ks. During the observation, we detected four absorption dips in the 3-20 keV LAXPC20 light curve with the count rate dropped from ~1050 counts/s to ~850 counts/s. The dips are found to be of ~5000 s duration and repeated after an interval of ~7 hours. The mid-points of the dips observed in the LAXPC light curve are 15:50 UT and 22:50 UT on 11 May 2021, and 05:50 UT and 12:50 UT on 12 May 2021. Similar kinds of dips is also observed in the light curves obtained from the NuSTAR and NICER observations (ATel #14606, ATel #14609). The periodic dips are likely to be associated with the orbital period of the system.
The time-averaged SXT and LAXPC spectra in the 0.7-20 keV range can be fitted by an absorbed disk blackbody and power-law with disk temperature ~0.6 keV and photon index ~ 1.8. The value of equivalent hydrogen column density was fixed at 3.2e21 cm^-2 (as estimated from NICER observation) at Wilm abundances. Evidence of a broad iron line is also present in the residuals. The unabsorbed flux is estimated to be ~7.94e-09 erg/cm^2/s in the 0.7-20 keV energy range. We found a softer photon index as compared to NuSTAR observation by fitting the combined spectra of dip and non-dip durations in our study. Along with the change in photon index, detection of a strong disk component in the AstroSat observation indicates that the source is undergoing a spectral state transition. The spectral transition of the source is also reported from the MAXI/GSC (ATel #14627).
The preliminary spectral analysis suggests that the source is likely to be a black hole X-ray binary and is in the hard-intermediate state during the AstroSat observation. Detailed investigation of the source using data from the AstroSat observation is underway. We thank the AstroSat team for quickly scheduling the ToO observation.
AstroSat/LAXPC light curve can be found at https://www.arghajit.net/maxi-j1803-298-astrosat-lightcurves
The time-averaged SXT and LAXPC spectra in the 0.7-20 keV range can be fitted by an absorbed disk blackbody and power-law with disk temperature ~0.6 keV and photon index ~ 1.8. The value of equivalent hydrogen column density was fixed at 3.2e21 cm^-2 (as estimated from NICER observation) at Wilm abundances. Evidence of a broad iron line is also present in the residuals. The unabsorbed flux is estimated to be ~7.94e-09 erg/cm^2/s in the 0.7-20 keV energy range. We found a softer photon index as compared to NuSTAR observation by fitting the combined spectra of dip and non-dip durations in our study. Along with the change in photon index, detection of a strong disk component in the AstroSat observation indicates that the source is undergoing a spectral state transition. The spectral transition of the source is also reported from the MAXI/GSC (ATel #14627).
The preliminary spectral analysis suggests that the source is likely to be a black hole X-ray binary and is in the hard-intermediate state during the AstroSat observation. Detailed investigation of the source using data from the AstroSat observation is underway. We thank the AstroSat team for quickly scheduling the ToO observation.
AstroSat/LAXPC light curve can be found at https://www.arghajit.net/maxi-j1803-298-astrosat-lightcurves
| Original language | English |
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| Publication date | 15 May 2021 |
| Publication status | Published - 15 May 2021 |
| Series | The Astronomer's telegram |
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| Number | #14629 |