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D. Pandel, F. Cordova (UCSB), K. Mason (MSSL), T. Sasseen, R. Shirey, J. West (UCSB)
We report the discovery of an optical transient in images taken with the optical monitor onboard XMM-Newton during an observation of the Lockman hole. The transient, which was observed with a visual magnitude of 18.7, was in a state of high brightness for the 9 hours of the observation. The object was not detected in other observations of the same region several days earlier and later, from which we infer an upper limit of 22 on the visual magnitude during quiescence. Magnitudes obtained in four filters from visual to UV are consistent with a steep power-law spectrum of spectral index -4.3 for F\nu. The large change in brightness of more than 3 magnitudes, the length of the outburst between 9 hours and 10 days and the steep spectral slope are consistent with the signature of a dwarf nova outburst observed during the rise to outburst. We present our analysis of the optical and UV data and give an upper limit on the X-ray flux.
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