A bright X-ray flaring event from the intermediate supergiant fast X-ray transient IGR J00370+6122 observed with NuSTAR

Abstract

We present an analysis of a long νstar observation of the intermediate supergiant fast X-ray transient (SFXT) IGR,J00370+6122, obtained around the peak of its orbital modulation. The observation has a net exposure ∼93,ks. During the observation, the source displayed a bright flaring event that was resolved into numerous short flares with durations of 10-10^3,s. The 3-70,keV luminosity spanned a dynamic range of ∼200, from to -1 $; the maximum was reached for only $ /BAT monitoring, we refine the orbital period of the system to P_ in total. The spectrum is well described by an absorbed double-Comptonisation model, with parameters consistent with those found in other X-ray pulsars accreting at similarly low luminosities. This is the first detection of a double-hump continuum in which extends this spectral state to the class of intermediate SFXTs. We searched for the previously reported sim346,s and sim674,s periodicities and find no significant signal at either period, which makes their interpretation as the neutron star spin period less likely. Using sim21,yr of Swift ̊m orb =15.6654 Finally, we discuss the observed flaring activity in the framework of quasi-spherical settling accretion, which is expected at the observed luminosities.