NICER follow-up observations of MAXI J1803-298

ATel #14602; P. M. Bult, K. C. Gendreau (NASA/GSFC), T. Enoto (RIKEN), J. Homan (Eureka Scientific and SRON), D. Altamirano (University of Southampton), A. Sanna (Univ. Cagliari) , G. K. Jaisawal (DTU Space), M. Ng (MIT), W. Iwakiri (Chuo U.), T. E. Strohmayer, Z. Arzoumanian (NASA/GSFC), J. Steiner (CfA)
on 5 May 2021; 14:19 UT
Credential Certification: Peter Bult (p.m.bult@nasa.gov)

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Continuing the ongoing efforts to observe the new X-ray transient MAXI J1803-298 (ATels #14587, #14588, #14591, #14594, #14597, #14598, #14601), we have collected further observations with the Neutron Star Interior Composition Explorer (NICER). We observed the source on May 2nd (1 ks) and May 4th (0.5 ks), finding the 0.5-10 keV count-rate to be increasing from 770 ct/s to 1100 ct/s, respectively.

The energy spectra could be reasonably well described with an absorbed disk plus power-law continuum model. The absorption column density is measured at 3.2 x 10²¹ cm^-2. Between the two sets of observations we further observe the disk temperature to decrease from 0.34 +/- 0.01 keV to 0.29 +/- 0.01 keV, while the power law photon index increases from 1.56 +/- 0.02 to 1.63 +/- 0.02. Strong residuals are observed between 6-8 keV likely associated with Fe emission lines. Further investigation of these features is under way. The 1-10 keV flux was 2.5 x 10^-9 and 3.3 x 10^-9 erg/s/cm², respectively.

Considering the temporal variability in the 0.5-10 keV band, we find that on May 2nd the power spectrum showed band limited noise below 10 Hz with an amplitude of about 23% rms. Additionally, we see a sharp QPO at ~0.13 Hz. By May 4th, the QPO frequency had increased to ~0.26 Hz, while the amplitude of the noise component decreased to 18% rms.

The spectral and temporal properties are more in line with those of a hard state accreting stellar-mass black hole than with those of an accreting neutron star. However, further observations are needed to unambiguously identify the nature of the compact object.

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.