EXTraS discovery of an 1.2-s X-ray pulsar in M 31

Abstract

During a search for coherent signals in the X-ray archival data of XMM-Newton, we discovered a modulation at 1.2 s in 3XMM J004301.4+413017 (3X J0043), a source lying in the direction of an external arm of M 31. This short period indicates a neutron star (NS). Between 2000 and 2013, the position of 3X J0043 was imaged by public XMM-Newton observations 35 times. The analysis of these data allowed us to detect an orbital modulation at 1.27 d and study the long-term properties of the source. The emission of the pulsar was rather hard (most spectra are described by a power law with < 1) and, assuming the distance to M 31, the 0.3-10 keV luminosity was variable, from ∼3 × 10 37 to 2 × 10 38 erg s −1 . The analysis of optical data shows that, while 3X J0043 is likely associated to a globular cluster in M 31, a counterpart with V 22 outside the cluster cannot be excluded. Considering our findings, there are two main viable scenarios for 3X J0043: a peculiar low-mass X-ray binary, similar to 4U 1822−37 or 4U 1626−67, or an intermediate-mass X-ray binary resembling Her X−1. Regardless of the exact nature of the system, 3X J0043 is the first accreting NS in M 31 in which the spin period has been detected.Key words: galaxies: individual: M 31 -X-rays: binaries -X-rays: individual: 3XMM J004301.4+413017. I N T RO D U C T I O NEXTraS (Exploring the X-ray Transient and variable Sky) is a project to explore systematically the serendipitous content of the XMM-Newton European Photon Imaging Camera (EPIC) pn (Strüder et al. 2001) and MOS (Turner et al. 2001) data in the temporal domain. The results will be released to the astronomical community in an easy-to-use form. The project includes a search for fast transients missed by standard image analysis, as well as the search and characterization of variability (both periodical and aperiodical) in hundreds of thousands of sources, spanning more than nine orders of magnitude in time-scale (from <1 s to >10 yr) and

Identification of the Hard X-Ray Source Dominating the E > 25 keV Emission of the Nearby Galaxy M31

“…We also note that Swift J0042.6+4112 could be an ultracompact binary (i.e., 4U 1626-67 and 4U 1822-37) with a white dwarf companion (Savonije et al 1986) that is under the HST detection limit. This kind of system has a hard power spectrum below 10 keV (see Esposito et al 2016), which is similar to Swift J0042.6+4112. We point out that this type of system has a very short orbital period (on the order of subhours).…”

“…Recently, Esposito et al (2016) discovered the first accreting pulsar in the direction of an external arm of M31 with detection of its 1.2s spin, thanks to its relatively high pulse fraction (∼50%), using XMM-Newton observations. They also found that its orbital modulation is about 1.3 days.…”

The Slowest Spinning X-Ray Pulsar in an Extragalactic Globular Cluster

“…This is the first persistently accreting X-ray pulsar hosting a neutron star detected in M31 2 , and it also has the longest period among all known pulsars (rotation-powered and accreting) in globular clusters, being more than an order of magnitude slower than the mildly recycled accreting pulsar from Terzan 5 globular cluster (Papitto et al 2011). We note that this pulsar has also recently been detected by Esposito et al (2016), but these authors interpret the nature of this source quite differently. In this paper, we discuss possible evolutionary scenarios that may produce such slowly rotating neutron stars in globular clusters.…”

“…We note that when correcting photon arrival times using the orbital solution of Esposito et al (2016), we were only able to get sufficient pulsations significance in three observations (0112570101, 0650560301, 0690600401), whereas in all remaining datasets the pulsating signal was not detected because it was smeared. At the same time the orbital solution obtained in this study allows to significantly detect pulsations in all observations listed in Table 2.…”

“…We first estimate the chance association probability of this X-ray source with the globular cluster B091D projection without being physically associated, as Esposito et al (2016) favor an interpretation with the pulsar not associated with the GC. The association was first sug- gested by Supper et al (2001) based on ROSAT data and confirmed by Trudolyubov & Priedhorsky (2004) when the first XMM-Newton and Chandra data became available.…”