4XMM J175136.8−275858: a new magnetar candidate?

ABSTRACT Magnetars are very rare astrophysical objects, with $\sim$31 known to date. They are best understood as highly magnetized neutron stars, but a greater number need to be found to constrain their role in stellar evolution pathways. We apply a novel approach for the detection of fast, transient X-ray sources, using a revised version of the EPIC XMM–Newton Outburst Detector with the aim of detecting and identifying new and rare variable compact objects. We detect a transient, variable source notable for its strong variability and hard spectrum. The emission from 4XMM J175136.8−275858 is well characterized by a blackbody, with temperatures between $\sim$1.8 and 5 keV during its lower luminosity phase. Its temperature is poorly constrained during its brightest phase, and we observe an increase in luminosity by two orders of magnitude over time-scales of a few ks. This is driven by increased emission of X-rays at energies above 2 keV, with a luminosity decay potentially over weeks or months. Derived luminosities for 4XJ1751−2759 range up to $\sim 10^{35}\, \text{erg s}^{-1}$ at 8 kpc at the Galactic centre, but neutral hydrogen column densities are greater than predicted Galactic values possibly implying a greater distance to the source, still within our galaxy, further increasing its luminosity. A consideration of optical and IR information in combination with the X-ray observations allows us to exclude the possibility that 4XJ1751−2759 is a star, rotationally powered pulsar or supergiant fast X-ray transient. This rapid, hard, variability is closest to that of outbursts in magnetars than any other known class of X-ray transient.