Optical flare at RA 15:16:21.2 DEC -20:08:16

ATel #7601; Roberto Nesci (INAF/IAPS), Vincenzo Falasca and Stefano Fantaulli (Foligno Observatory)
on 7 Jun 2015; 01:38 UT
Credential Certification: Roberto Nesci (Roberto.Nesci@iaps.inaf.it)

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On June 4 2015, while monitoring the occultation of the star HD 132885 by the asteroid 322 (Phaeo) in a session open to the public at the Foligno Observatory (IAU K56), we detected an optical flare with our Mintron intensified camera, mounted in parallel to the main telescope as electronic finder, with a 135mm F/2.5 objective. The camera was set to acquire images at a frame rate of 4 fps, which were projected on a screen to allow the public to see the occultation in real time. Star down to V=10 were detectable on the screen. The screen images were recorded with a camcorder at 25 fps for documentation of the occultation event. The scale of the images on the camcorder is 8"/pixel. Shortly after the asteroid occultation event (midtime 20:49:27.5 UT) a bright flare was detected, lasting about 1s, with a smooth increasing and decreasing light curve covering 5 frames of the Mintron camera. Astrometry with nearby Tycho-2 stars gave coordinates 15:16:21.2 -20:08:16. Apparent magnitude at the peak, derived from a calibration with the Tycho-2 stars, was about 5 +/- 0.4 mag. Our time estimate of the event is 20:49:39 UT with an accuracy of a few seconds. The shape of the flare image was quite star-like, from a comparison with the two stars of similar magnitude in the field (iota Lib and 25 Lib), so an electronic interference explanation seems unlikely. No other similar episodes were ever detected by us while using this camera as electronic finder. Within our error-box on the NED there is an unclassified X-ray source, 2XMM J151620.2-200828. No references of specific studies on this source is available on NED. No obvious optical counterpart is present on the DSS and we are not aware of a GRB reported at this position at the time of our observations. Searches in the operating Gamma ray satellite archives might be useful to determine the nature of the source.