GCN Circular 34549

Brendan O'Connor (Carnegie Mellon University), Jeremy Hare (NASA/GSFC/CRESST/CUA), George Younes (NASA/GSFC), Keith Gendreau (NASA/GSFC), Zaven Arzoumanian (NASA/GSFC), Elizabeth Ferrara (NASA/GSFC/CRESST/UMCP):

Following reports of repeated bursts from Swift J1727.8-1613 (GCN #34540
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) — initially designated GRB 230824A (GCN #34536
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, GCN #34537
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; see also ATels #16205, #16206) — we have observed the source with a NICER ToO, prior to the telescope entering a Moon constraint. The observation started on 2023-08-25 at 00:25:40 UT and ended at 00:49:53 UT, with a total exposure of 1,055 s. 

The observation began during orbit-day, which, due to the high count rate and a recent light “leak” in NICER’s X-ray Timing Instrument, led to telemetry saturation that created fragmented good-time intervals (GTIs). This is a known issue that leads to a 55 Hz “signal” (and potentially other significant features) in a power-spectral analysis. We caution future observers against astrophysical interpretations of this signal. To minimize calibration complications due to visible-light loading, we restricted our spectral analysis to only the nighttime data, with an exposure of 198 s.

Throughout the observation, the source is prominently detected with a count rate varying between ~5,000 and 20,000 cts/s. Rapid (timescales less than one second) variability is observed, similar to the black-hole system V404 Cyg (ATel #16205, ATel #16206) and/or the neutron-star binary Swift J1858.6–0814 (ATel #12158). In the orbit-night-only data, the 55 Hz signal is no longer visible in power spectra, which lack narrow features (no obvious QPO or sharply periodic signal) but are dominated by a strong red-noise component. 

We modeled the time-averaged spectrum (0.5-10 keV) with an absorbed power-law model plus a blackbody. We derive a photon index of 1.47+/-0.01, blackbody temperature of kT = 0.269+/-0.006 keV, and hydrogen column density of N_H = (2.26+/-0.05)e21 cm^-2. The PL+BB model fits the data well with a chi-squared of 87 for 155 degrees of freedom. We identify a hint of an iron line at around 6.4 keV. The absorption-corrected X-ray flux in the 0.5-10 keV energy band is (5.34+/-0.02)e-8 erg/cm^2/s. Assuming a distance of 3 - 6 kpc this corresponds to an X-ray luminosity of ~5e37 to 2e38 erg/s, which suggests that this is a newly discovered X-ray binary in outburst (ATel #16205, GCN #34540
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).

Further analysis is ongoing and additional NICER observations are planned. 

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.