Detection of low-frequency QPO in MAXI J1803-298 by AstroSat

ATel #14630; Swadesh Chand (GGV, Bilaspur), G. C. Dewangan (IUCAA, Pune), Parijat Thakur (GGV, Bilaspur), Prakash Tripathi (IUCAA, Pune) and V. K. Agrawal (URSC, ISRO, bangalore)
on 16 May 2021; 05:07 UT
Credential Certification: Gulab Dewangan (gulabd@iucaa.in)

Tweet

We report the detection of a low-frequency QPO at ~5.4 Hz along with a sub-harmonic at ~2.7 Hz in the AstroSat follow-up observation of the newly discovered X-ray Transient source MAXI J1803-298 (ATels #14587, #14588, #14591, #14594, #14597, #14598, #14601, #14602, #14606, #14607, #14613, #14627). The follow-up observation of the source was conducted by AstroSat from May 11 at 01:09 UTC to May 11 at 11:59 UTC. The centroid frequency of the QPO remains the same in both the 3-15 keV and 15-30 keV bands. However, the QPO in the 3-15 keV cannot be fitted by a single Lorentzian, and hence an additional Lorentzian component centred at ~6.3 Hz is required. The fractional rms amplitude of the QPO at ~5.4 Hz decreases from ~6% in the 3-15 keV band to 3.5% in the 15-30 keV band, and the fractional rms amplitude of the QPO at ~6.3 Hz is found to be ~3.7% in the 3-15 keV band. Additional Lorentzian at ~6.3 Hz for the QPO and the subharmonic were absent in the 15-30 keV band, which may be due to lower S/N in this hard band. The QPO detected by AstroSat is found to be shifted towards a higher frequency with respect to that reported for Insight-HXMT (ATel#14613), NuSTAR (ATel#14609) and NICER(ATel#14602) follow-up observations. The time-averaged broadband X-ray spectral analysis in the 0.8-60 keV band was carried out by fitting the SXT and LAXPC spectral data jointly. The spectra can be well described by an absorbed multi-coloured disk blackbody and a power-law component with a photon index ~2.3. The value of the absorption column density (n_H) is found to be ~4.5e21 cm^-2, which is consistent with that reported in ATel#14602. We obtain the inner disk temperature (kT_in) to be ~0.89 keV, and the estimated inner disk radius (R_in) is ~168.4km by assuming D = 8 kpc and i=60 deg (ATel#14606, #14609, #14627). This value of the R_in is consistent with those reported in ATel#14627. A residual in the ~5-8 keV indicates the presence of the iron line in the X-ray spectra. The estimated unabsorbed flux in the 0.8-60 keV band is found to be ~3.5e10^-9 erg/cm^2/s. The strong disk component and relatively weaker power-law component indicate that the source is in the state of hard-to-soft transition during this particular AstroSat observation. The hard-to-soft spectral transition of the source is also reported by ATel#14627 and ATel#14629. The large inner disk radius and other spectral and temporal properties of the source suggest that the source may be an accreting black hole X-ray binary. Further follow-up observations are encouraged. Further analysis of this AstroSat observation is in progress. We thank the AstroSat team for granting the ToO observation, and ISSDC, ISRO for availing the data.