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Discovery of eclipses in the X-ray transient IGR J17451-3022

ATel #7361; Gaurava K. Jaisawal (Physical Research Laboratory, Ahmedabad, India), Jeroen Homan (MIT & SRON), Sachindra Naik (Physical Research Laboratory, Ahmedabad, India), and Peter Jonker (SRON & Radboud Universiteit)
on 10 Apr 2015; 11:31 UT
Credential Certification: Jeroen Homan (jeroen@space.mit.edu)

Subjects: X-ray, Binary, Transient

Referred to by ATel #: 7570

We report on a Suzaku observation of the X-ray transient IGR J17451-3022. The observation was made on 2014 September 23/24, several weeks after the source was discovered with INTEGRAL/JEM-X (ATel #6451). The observation covers an interval of ~80 ks with a total effective exposure of ~34 ks.

Light curves in 0.5-10 keV energy range from the XIS-0 and XIS-1 detectors were combined. The resulting light curve shows strong dipping activity, including a series of intervals during which the count rate drops to ~10% of its maximum (non-dip) value. The count rate remains at a low level for periods of up to more than 6 ks, with only occasional small and brief increases. Four such low count rate periods can be identified in the Suzaku observation, with a spacing of roughly 22 ks. At the end of three of these periods we observe sharp egresses with durations of less than 20 s, which are suggestive of eclipses. From these egresses we derive a period of 22623+/-5 s (~6.3 hr). Given the highly regular occurrence of the egresses, we identify this period as the binary orbital period of the source. A link to a light curve of the source is provided below; egresses are marked by red lines.

The duration of the eclipses is difficult to constrain, for two reasons. First, the light curve contains a lot of data gaps. Given that the binary orbital period is close to four times the Suzaku orbit, the data gaps keep occurring at similar binary orbital phases during the observation. The phase at which ingress occurs may therefore simply be missed. Second, a light curve from a higher energy band (~7-12 keV) reveals considerable variability during some of the low-count rate intervals (indications for this are also seen in the full 0.5-10 keV light curve). This suggests that parts of these low count rate intervals are due to very strong (but variable) dipping rather than eclipses. We therefore suspect that the eclipses in this system are preceded by deep absorption dips that may effectively 'hide' the eclipse ingress. Due to the aforementioned data gaps the extent of the dipping is unclear, however. Based on the shortest observed time interval between the low-count rate variability and eclipse egress, we constrain the duration of the eclipse to be less than 2600 s. We note that if the eclipse duration would cover the entire low count rate period, this would imply an unrealistically large mass ratio.

The presence of X-ray dips and eclipses in the light curve with a ~6.3 hr periodicity strongly suggest that the X-ray source IGR J17451-3022 is a highly inclined binary system (>70 degrees). This had been suggested earlier based on the unusually low soft-state luminosity of the source (if a Galactic center location is assumed) and its varying N_H (ATel #6459). A preliminary spectral analysis reveals an Fe absorption line at ~6.63 keV with an equivalent with of ~77 eV, similar to what has been seen in other high inclination X-ray binaries.

No X-ray bursts were detected in the Suzaku data and the nature of the compact object remains unclear. Further analysis of the XIS data is on-going. Swift observations made in late March 2015 show that the outburst of the source is still continuing.

Suzaku light curve of IGR J17451-3022