Preliminary spectral fitting and QPO evolution in NICER observations of black hole candidate Swift J1727.8-1613

ATel #16219; Paul A. Draghis (University of Michigan), Jon M. Miller (University of Michigan), Jeroen Homan (Eureka Scientific), Phil Uttley (University of Amsterdam), Niek Bollemeijer (University of Amsterdam), James F. Steiner (Center for Astrophysics | Harvard & Smithsonian), Jeremy Hare (NASA GSFC/CUA/CRESST), Francesco Tombesi (University of Maryland, College Park; Tor Vergata University of Rome), Keith C. Gendreau, Zaven Arzoumanian, Tod E. Strohmayer (NASA GSFC), A. Sanna (University of Cagliari), D. Altamirano (University of Southampton), Douglas Buisson (Independent), Andrew C. Fabian (University of Cambridge)
on 30 Aug 2023; 18:58 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Paul Draghis (pdraghis@umich.edu)

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X-ray transient Swift J1727.8-1613 was initially detected as a bright X-ray source on 2023 August 24 (GCN #34540, ATel #16205, #16206) and identified as a candidate low-mass black hole X-ray binary based through subsequent optical (ATel #16208), X-ray (ATel #16207), and radio (ATel #16211) observations. Here, we report on preliminary spectral analysis of the NICER observation obtained on 2023 Aug 28 with an exposure of 2.6ks, and timing properties from observations between August 25 and August 30. Our analysis was restricted to data collected during orbit-night.

We binned the spectrum obtained on August 28 to achieve a minimum SNR=10, and fit it in the 1-10 keV band with a model accounting for the thermal emission from a disk blackbody (diskbb) as well as a power-law component to characterize the emission from the compact corona, all absorbed by Galactic neutral ISM-abundance gas with column density as a free parameter. The complete model, TBabs×(diskbb+powerlaw) in XSPEC, produces a poor fit, with χ²/dof=3822.66/894. The residuals between 5-7 keV are compatible with a relativistically broadened Fe K emission line generated at the inner edge of the accretion disk. Replacing the power-law component with a model that accounts for relativistic reflection features (relxill) produces a significantly improved fit. Furthermore, the residuals indicate the presence of an absorption feature around 7 keV indicative of an accretion disk wind, so we included a Gaussian absorption line to the model. The full model, TBabs×(diskbb+relxill+gauss) returns χ²/dof=1504.66/883. The measured column density is N_H=4.1±0.1 ×10²¹ cm^-2, and the disk temperature is ~0.37 keV. Assuming an inner disk radius consistent with the innermost stable circular orbit, the BH spin and inclination are a=0.995^+0.001_-0.004 and θ=47.9±0.03 degrees, respectively. The measured power-law index is Γ=1.40 and the ionization parameter log(ξ)=3.40±0.01. The energy of the Gaussian absorption line is 7.00±0.03 keV with a width of 0.36±0.03 keV, consistent with an ionized Fe outflowing absorber. All reported errors are at the 1 sigma level and only include statistical uncertainties. A full spectral analysis of the source will be reported in a future paper.

The 1-10 keV observed flux during the observation was 1.9×10^-7 ergs/s/cm², which given the measured column density corresponds to an unabsorbed flux of 2.1×10^-7 ergs/s/cm². For a 10 solar mass BH located at a distance of 8 kpc, this flux corresponds to an Eddington fraction of 1.24. However, for a much closer distance of 3 kpc, the flux corresponds to an Eddington fraction of 0.17.

A timing analysis of the early observations shows a prominent QPO at ~0.44 Hz, with a strong harmonic. This QPO shifted to 0.9 Hz and then to 1.2 Hz during later observations. We also detect a broad feature around 8 Hz, which in other sources has been found to evolve into a high-frequency QPO. The timing properties suggest that the source is evolving from the hard to the hard-intermediate state (HIMS). Note that due to fragmentation of good-time intervals (GTI) because of the high count rate, earlier NICER observations show spurious high-frequency timing features (e.g., at 26 and 55 Hz). These effects have been mitigated in recent observations by operating with fewer detectors.

NICER observations are continuing. A schedule of upcoming observations can be found here: https://heasarc.gsfc.nasa.gov/docs/nicer/schedule/nicer_sts_current.html

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.