Continued X-ray activity in EXO 0748-676 from NICER observations
ATel #17024; D. J. K. Buisson (Independent), D. Altamirano (University of Southampton), Z. Arzoumanian, K. C. Gendreau (NASA GSFC), E. C. Ferrara (University of Maryland/CRESST II/NASA GSFC), T. Boztepe (Istanbul University), G. K. Jaisawal (DTU Space), M. Sudha (Wayne State University), A. Sanna (University of Cagliari), M. Ng (McGill University), M. T. Wolff (NRL), J. Homan
on 10 Feb 2025; 20:04 UT
Credential Certification: Mason Ng (masonng@mit.edu)
Subjects: X-ray, Binary, Neutron Star
EXO 0748-676 is a low mass X-ray binary (LMXB) system at a distance of about 7.5 kpc that shows X-ray bursts, quasi-periodic oscillations, absorption dips in the X-ray light curves, and full eclipses with a known orbital ephemeris. In June 2024 this system returned to X-ray activity after about 15 years of quiescence (GCN #36653; ATel #16654). EXO 0748-676 is a unique binary system in that it shows so many phenomena associated with LMXBs.
Recently, NICER observed EXO 0748-676 for 6 snapshots over 2025-01-30 to 2025-02-03, totalling 3077 s of good exposure in both ISS day and night, specifically arranged to include eclipse times.
The NICER count rate in the 0.3-10 keV energy range outside of eclipses is approximately 55 cts/s. This is much higher than NICER observations taken during 2024 (typically 10-15 cts/s), with the increase being greater at softer energies.
The 0.3-10 keV spectrum is fit to within 10% by a power law (Gamma=1.33 ± 0.03, Xspec norm=0.0198 ± 0.0005) with Galactic absorption. This corresponds to a 0.3-10 keV flux of (1.76 ± 0.03) × 10-10 erg/cm2/s. The power law index is slightly harder than seen in the NuSTAR observation in 2024 (ATel #16673), although we note that the energy bands measured differ. An absorbed disc (kT=0.69 ± 0.06 keV, Xspec norm=13 ± 4) plus black body (kT=1.6 ± 0.1 keV, Xspec 'bbody' norm=(1.62 ± 0.08) × 10-3) model can also provide a good fit. This, along with the low level of variability, implies that a transition to a softer state is possible, with harder X-ray coverage being required to confirm this. In either model, the strong absorption (NH~1023 cm-2) seen in 2024 (ATel #16673) is no longer present.
NICER detected an eclipse in each snapshot taken, with the mid-eclipse time of the first at MJD 60705.30329 (TDB). The eclipse ingress/egress times are consistent with the ephemeris implied by the eclipse times of the 2024 NuSTAR observation (ATel #16673), and show jitter of a few seconds as seen in the previous outburst of this source (Wolff et al. 2002).
No Type I X-ray bursts were observed in these NICER data, although such bursts have been observed recently by SVOM/ECLAIRs (ATel #17015) and earlier in this outburst by various instruments (ATels #16654, #16655, #16673, #16678, #16703, #16753, #16790). The higher count rate contrasts with the drop in optical flux from ATLAS data (ATel #17004) that suggested that EXO 0748-676 was returning to quiescence; it is not clear whether the X-ray activity is still steady or declining from a peak between these and previous observations in 2024.
Future NICER observations are planned provided EXO 0748-676 remains active.
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.
