A four-year XMM-Newton/Chandra monitoring campaign of the Galactic centre: analysing the X-ray transients

Authors

N. Degenaar, University of Amsterdam
R. Wijnands, University of Amsterdam
E. M. Cackett, Institute of Astronomy, University of CambridgeFollow
J. Homan, Center for Space Research, Massachusetts Institute of Technology
J. J. M. in 't Zand, SRON National Institute for Space Research
E. Kuulkers, ISOC, ESA/ESAC, Urb.
T. J. Maccarone, University of Southampton
M. van der Klis, University of Amsterdam

Document Type

Article

Abstract

We report on the results of a four-year long X-ray monitoring campaign of the central 1.2 square degrees of our Galaxy, performed with Chandra and XMM-Newton between 2005 and 2008. Our study focuses on the properties of transient X-ray sources that reach 2-10 keV luminosities of L_X ≳ 10³⁴ erg s^-1 for an assumed distance of 8 kpc. There are 17 known X-ray transients within the field of view of our campaign, eight of which were detected in outburst during our observations: the transient neutron star low-mass X-ray binaries GRS 1741-2853, AX J1745.6-2901, SAX J1747.0-2853, KS 1741-293 (all four are also known X-ray bursters), and GRO J1744-28 (a 2.1 Hz X-ray pulsar), and the unclassified X-ray transients XMM J174457-2850.3, CXOGC J174535.5-290124 and CXOGC J174541.0-290014. We present their X-ray spectra and flux evolution during our campaign, and discuss our results in light of their historic activity. Our main results include the detection of two thermonuclear X-ray bursts from SAX J1747.0-2853 that were separated by an unusually short time interval of 3.8 min. Investigation of the lightcurves of AX J1745.6-2901 revealed one thermonuclear X-ray burst and a ~1600-s long X-ray eclipse. We found that both XMM J174457-2850.3 and GRO J1744-28 displayed weak X-ray activity above their quiescent levels at L_X ~ 10^33-34 erg s^-1, which is indicative of low-level accretion. We compare this kind of activity with the behaviour of low-luminosity X-ray transients that display 2-10 keV peak luminosities of L_X ~ 10³⁴ erg s^-1 and have never been seen to become brighter. In addition to the eight known X-ray transients, we discovered a previously unknown X-ray source that we designate XMMU J174654.1-291542. This object emits most of its photons above 2 keV and appears to be persistent at a luminosity of L_X ~ 10³⁴ erg s^-1, although it exhibits strong spectral variability on a time scale of months. Based on its X-ray properties and the possible association with an infrared source, we tentatively classify this object as a cataclysmic variable. No new transients were found during our campaign, reinforcing the conclusion of previous authors that most X-ray transients recurring on a time scale of less than a decade have now been identified near the Galactic centre.

Disciplines

Cosmology, Relativity, and Gravity | External Galaxies | Stars, Interstellar Medium and the Galaxy

Comments

NOTICE IN COMPLIANCE WITH PUBLISHER POLICY: Reproduced with permission from Astronomy & Astrophysics, ©2012 ESO. Available at: doi:10.1051/0004-6361/201219470

Recommended Citation

A four-year XMM-Newton/Chandra monitoring campaign of the Galactic centre: analysing the X-ray transients

N. Degenaar, R. Wijnands, E. M. Cackett, J. Homan, J. J. M. in ’t Zand, E. Kuulkers, T. J. Maccarone and M. van der Klis

A&A 545 A49 (2012)