We searched for X-ray flashes ( XRFs ) – which we defined as \sim 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 % upper limit on the number of flashes per sky per year at two different incoming flash fluxes : 4.0 \times 10 ^ { 9 } events sky ^ { -1 } yr ^ { -1 } for a flux of 7.1 \times 10 ^ { -13 } erg \mbox { $ cm$ } ^ { -2 } \mbox { $ s$ } ^ { -1 } and 6.8 \times 10 ^ { 7 } events sky ^ { -1 } yr ^ { -1 } for 1.4 \times 10 ^ { -11 } erg \mbox { $ cm$ } ^ { -2 } \mbox { $ s$ } ^ { -1 } , both assuming a spectral power-law photon index \alpha =2 .
These limits are consistent with an extrapolation from the BeppoSAX /WFC XRF rate at much higher fluxes ( \sim a factor of 10 ^ { 5 } ) , assuming an homogenous population , and with a previous , more stringent limit derived from ROSAT pointed observations .
This is a preprint of an Article accepted for publication in Monthly Notices of the Royal Astronomical Society © 2004 The Royal Astronomical Society