Updated orbital ephemeris and hard X-ray spectrum of EXO 0748-676

ATel #16673; D. J. K. Buisson, R. M. Ludlam, M. Middleton, M. T. Wolff, J. van den Eijnden, A. H. Knight, M. Sudha, G. K. Jaisawal, and J. Homan
on 26 Jun 2024; 13:34 UT
Credential Certification: Gaurava Kumar Jaisawal (gaurava.jaisawal@gmail.com)

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EXO 0748-676 is a neutron star X-ray binary which has recently begun an outburst, observed in X-rays (GCN #36653; ATel #16654) and optical bands (ATel #16646,16648,16649). Several Type I bursts have been observed in the current outburst (ATel #16654,16655). EXO 0748-676 was discovered in February 1985 by the EXOSAT satellite (Parmar et al. 1986) and remained X-ray active until August 2008 (ATel#1812). The source has remained in quiescence until it recently returned to X-ray activity (ATel#16646).

NuSTAR observed EXO 0748-676 on 2024 June 17, with an average net count rate of 5.0 cts/s (3-78 keV; 2 arcmin extraction radius; both FPMs combined) with clear detections of five eclipses and a Type I X-ray burst (peak time MJD 60478.2548; TDB).

The mean spectrum may be described to within 10% by an absorbed (NH=11+-2x10²² cm^-2, covering fraction=0.79-0.05+0.12) cut-off power law (Xspec norm 0.025+-0.002, Gamma=1.42+-0.03, kT=62+-7 keV), typical of a hard state. This corresponds to an observed flux of (4.14+-0.04)x10^-12 ergs/cm²/s over 3-78 keV. Additionally, a quasi-periodic oscillation at 0.14 Hz is present in the NuSTAR co-spectrum from 3-79 keV, which is consistent with the low-frequency features expected for this source in the hard state (Homan et al. 2015).

Five consecutive eclipses occur during the Good Time Intervals of the observation. The mid-eclipse times are MJDs 60478.24606, 60478.40549, 60478.56450, 60478.72408, 60478.88345 (TDB); uncertainties are around 2s (1-sigma). Eclipse cycle count has been maintained from the original work of Parmar et al. (1991). This is around 50s before the projection of the final segment of the ephemeris of Wolff et al. (2009); this deviation is of similar duration to the changes observed during the previous outburst (ibid.).