NuSTAR Observation of MAXI J1803-298 with Periodic Dips
ATel #14609; Yanjun Xu, Fiona Harrison (Caltech)
on 7 May 2021; 02:08 UT
Credential Certification: Yanjun Xu (yanjun@caltech.edu)
Subjects: X-ray, Transient
We report preliminary data analysis results from an NuSTAR observation of the new X-ray transient MAXI J1803-298 (ATels #14587, #14588, #14591, #14594, #14597, #14598, #14601, #14602, #14606). The NuSTAR observation started at 2021-05-05 16:45 (UTC) and ended at 2021-05-06 10:25, with an exposure time of 26.5 ks. We detected three dips during this observation, at around 2021-05-05 16:50, 2021-05-05 23:50, and 2021-05-06 6:50 (UTC), each lasting about 5000 s. The dips are prominent; the NuSTAR (FPMA+FPMB) count rate dropped from ~220 ct/s to ~100 ct/s around the center of the absorption dips. Including the dips caught by NICER around May 5 09:07, and May 5 16:48 (ATel #14606) there have been so far four consecutive dips observed at ~7 hour intervals. The 7 hour period is likely associated with the orbital period.
The time-averaged NuSTAR energy spectrum can be approximated by a cutoff power-law with the photon-index, Gamma~1.5, and a high energy cutoff, E_cut~50 keV. Strong spectral residuals are observed relative to the cutoff power-law model, indicating the presence of relativistic disk reflection features (a broad Fe K line around 4-8 keV and Compton reflection hump peaking around 20 keV). The average X-ray flux during the observation is ~6.6e-09 ergs/cm^2/s in 3-79 keV. We found a QPO around 0.4 Hz in the power spectrum, and no coherent pulsations are detected.
The X-ray spectral and timing properties suggest the source is a low-mass black hole X-ray binary candidate viewed close to edge on, and is currently in the hard state during the rising phases of an outburst. Further observations are encouraged.
