Abstract
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.
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.
| Original language | English |
|---|---|
| Publication date | 10 Feb 2025 |
| Publication status | Published - 10 Feb 2025 |
| Series | The Astronomer's telegram |
|---|---|
| Number | ATel #17024 |
Keywords
- X-ray
- Binary
- Neutron Star
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