NICER detection of Type-B QPOs in follow-up observations of MAXI J1803-298

ATel #14660; S. Ubach (SAO/CfA), J. Steiner (SAO/CfA), J. Homan (Eureka Scientific and SRON), K. C. Gendreau (NASA/GSFC), Z. Arzoumanian (NASA/GSFC), D. Altamirano (University of Southampton), P. Uttley (Amsterdam), D. Pasham (MIT)
on 25 May 2021; 20:14 UT
Distributed as an Instant Email Notice Transients
Credential Certification: James F Steiner (jsteiner@cfa.harvard.edu)

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Following the MAXI/GSC alert of the newly discovered X-ray transient MAXI J1803-298 (ATel #1458; see also ATels #14588, #14591, #14594, #14597, #14598, #14601, #14602) and subsequent to the observations reported in ATel #14606, the Neutron Star Interior Composition Explorer (NICER) has continued observing the outburst.

Since the last NICER report (ATel #14606), NICER has monitored MAXI J1803-298 for an additional 57 ks over the time period May 18 17:58 UTC to May 25 11:24 UTC. No further X-ray dips have been seen. Midway through this recent interval, from May 21 onward, we detect the presence of a low-frequency Type-B QPO at ~6 Hz along with a broad sub-harmonic at ~3 Hz.

As an example, in the linked Figure, we present one representative 1.4 ks NICER observation. The 0.3-12 keV PDS continuum follows a broken power law and the QPO and its sub-harmonic have been fitted as two Lorentzian components. The fractional rms amplitude of the QPO and sub-harmonic in the example shown are, respectively, 1.8+/-0.3% and 1.5+/-0.3% at 90% confidence. The corresponding Q-factors are ~10 and ~2, respectively. Over the several days of monitoring, the fundamental frequency is variable, observed to move between ~6.5 Hz and ~5 Hz. The final few observations on May 24-25 show only intermittent presence of the QPO, perhaps indicating evolution in the source. This recent detection of a Type-B QPO reveals that the source has remained in an intermediate ''Steep Power Law'' (SPL) state for an extended period of time. We confirm this spectroscopically.

The associated energy spectrum of the system has been fitted from 0.25-12 keV with an absorbed Comptonized disk-blackbody model (tbabs * simpl x diskbb), with Nh = 3.3e21/cm^2. During the May 21-24 period with the prominent QPO, the spectrum of MAXI J1803-298 is generally consistent with the properties of the SPL state, i.e., the photon index is high, Gamma ~ 2.7-2.9, and the disk component is prominent and hot (kT ~ 0.5-0.6 keV), but the scattering fraction is somewhat larger than typical, f_sc ~0.6-0.8. Data obtained on May 25 show a hotter disk component (kT ~0.8 keV) and somewhat steeper power-law Gamma >~3.

MAXI J1803-298 Power Density Spectrum on May 22