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NICER Detection of a Transition to the Hard State in MAXI J1803-298

ATel #14994; J. F. Steiner (SAO, CfA), S. Ubach (SAO, CfA), J. A. Tomsick (UC Berkeley, SSL), B. Coughenour (UC Berkeley, SSL), J. Homan (Eureka Scientific)
on 22 Oct 2021; 16:55 UT
Distributed as an Instant Email Notice Transients
Credential Certification: James F Steiner (jsteiner@cfa.harvard.edu)

Subjects: X-ray, Black Hole, Transient

MAXI J1803-298 is an X-ray binary system hosting a black-hole candidate which was detected in outburst by MAXI GSC on 2021 May 01 (ATel #14587). The first month after discovery presented an initial hard state rise and transition through intermediate and steep-power law states. Subsequently, over the last five months it has remained in a protracted, declining soft/thermal state. These phases of discovery and evolution have been described in numerous ATels discussing its X-ray, optical, or radio behavior (ATels #14588, #14589, #14591, #14594, #14597, #14598, #14601, #14602, #14606, #14607, #14609, #14613, #14627, #14629, #14630, #14650, #14660, #14706). In this update we report the discovery that transition to a faint hard state is presently underway.

NICER observed MAXI J1803-298 for 770s on 2021 October 19. This observation shows definite evidence for a state transition. Notably, the NICER count rate dropped by a factor of 6 compared to the previous observation on 2021 October 13. Both observations have been fitted using a Comptonized-disk model: tbabs*simplcut(ezdiskbb). Comparing the older to the more recent observation, the fits show hardening of the Compton power law from Gamma=2.7+/-0.3 to Gamma=1.92+/-0.04 while the fraction of disk photons Compton-scattering in the corona increased from 6%+/-2% to 34%+/-1% and the disk temperature decreased from 0.388+/-0.003 keV to 0.22+/-0.01 keV.

The 0.5-10 keV absorbed flux during the October 19th observation is 1.3e-10 erg/cm2/s (corresponding to a 52-detector count rate of ~60/s), which is ~100 times lower than the outburst peak. The October 13th observation preceding the transition had an 0.5-10 keV flux of 4.2e-10 erg/cm2/s. Assuming that the peak occurs near the Eddington limit, this indicates that the transition to the hard state occurred at several percent Eddington, as is typical of black hole X-ray transients.

In the Fourier domain, the October 19th observation's power-density spectrum exhibits strong power in the 0.1-10 Hz band with a fractional rms amplitude of ~10-20%, structured like band-limited noise breaking at ~0.1 Hz. No significant power had been detected in prior recent observations. This is separately indicative of the state transition underway. Observations at other wavelengths are encouraged, as this phase may progress rapidly.

NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.