Discovery of periodic dips in the brightest hard X-ray source of M31 with EXTraS

Abstract

We performed a search for eclipsing and dipping sources in the archive of the EXTraS project - a systematic characterization of the temporal behaviour of XMM-Newton point sources. We discovered dips in the X-ray light curve of 3XMM J004232.1+411314, which has been recently associated with the hard X-ray source dominating the emission of M31. A systematic analysis of XMM-Newton observations revealed 13 dips in 40 observations (total exposure time $\sim$0.8 Ms). Among them, four observations show two dips, separated by $\sim$4.01 hr. Dip depths and durations are variable. The dips occur only during low-luminosity states (L${}_{0.2-12}<1\times 10^{38}$ erg s${}^{-1}$), while the source reaches L${}_{0.2-12}\sim 2.8\times 10^{38}$ erg s${}^{-1}$. We propose this system to be a new dipping Low-Mass X-ray Binary in M31 seen at high inclination (60${}^{\circ }$-80${}^{\circ }$), the observed dipping periodicity is the orbital period of the system. A blue HST source within the Chandra error circle is the most likely optical counterpart of the accretion disk. The high luminosity of the system makes it the most luminous dipper known to date.

arXiv comment11 pages, 2 figures, 5 tables, accepted for publication in ApJL

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• The Ultraluminous X-ray sources population of the galaxy NGC 7456
• Exploring X-ray variability with unsupervised machine learning I. Self-organizing maps applied to XMM-Newton data
• A Multi-wavelength search for Black Widows and Redbacks counterparts of candidate $\gamma$-ray millisecond pulsars
• Discovery of 3 s pulsations from the Brightest Hard X-ray Source in M31
• NuSTAR view of the central region of M 31
• 4XMM J181330.1-175110: a new supergiant fast X-ray transient