A search for X-ray flashes with XMM-Newton

Abstract

We searched for X-ray flashes (XRFs) - which we defined as ~10-s duration transient X-ray events observable in the 0.4-15 keV passband - in fields observed using XMM-Newton with the EPIC/pn detector. While we find two non-Poissonian events, the astrophysical nature of the events is not confirmed in fully simultaneous observations with the EPIC/MOS detectors, and we conclude that the events are anomalous to the EPIC/pn detector. We find a 90 per cent upper limit on the number of flashes per sky per year at two different incoming flash fluxes: 4.0 × 10⁹ events sky^-1 yr^-1 for a flux of 7.1 × 10^-13 erg cm^-2 s^-1 and 6.8 × 10⁷ events sky^-1 yr^-1 for 1.4 × 10^-11 erg cm^-2 s^-1, both assuming a spectral power-law photon index α= 2. These limits are consistent with an extrapolation from the BeppoSAX/WFC XRF rate at much higher fluxes (~ a factor of 10⁵), assuming a homogenous population, and with a previous, more stringent limit derived from ROSAT pointed observations.