Swift/XRT observations of the error region of MAXI J0758-456

ATel #8988; J. A. Kennea (PSU), P. A. Evans, A. P. Beardmore, K. L. Page (U Leicester), H. A. Krimm (CRESST/GSFC/USRA), P. Romano (INAF-IASFPA), K. Yamaoka (Nagoya U.), M. Serino (RIKEN) and H. Negoro (Nihon U.)
on 27 Apr 2016; 01:49 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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MAXI detected the new soft transient source MAXI J0758-456 on 15:05UT April 24th, 2016 (Nakagawa et al., ATEL #8983). Swift performed a follow-up 7-point tiling observation of the error region reported in that ATEL starting 20:02UT on April 24th, 2016, utilizing XRT in Photon Counting mode and 200s exposure per tile. The tiled region covered approximately 95% of the ATEL error circle, and 100% of a refined error region (Negoro, private communication). No new transient source was detected in those data.

The error region calculated by MAXI assumes a source of constant flux, analysis of the MAXI data showed that the source light-curve was likely not consistent with being constant. Therefore a more conservative error region was calculated. A further 7-point tiling was made of the southern part of this error region was performed starting 14:59UT on April 25th, 2016. In those tiles, and inside of the larger MAXI error region, we detect one point source, at RA/Dec (J2000) = 120.32057, -46.39084, which is equivalent to:

RA(J2000) = 08h 01m 16.94s,
Dec(J2000) = -46d 23m 27.0s,

with an error radius 3.2 arc-seconds radius (90% confidence). This source is consistent with the known source 1RXS J080114.6-462324, a CV which has been detected in Hard X-rays (Landi et al., 2010, MNRAS, 403, 945). This source is moderately bright (~0.15 XRT count/s) and hard, photon index = 0.9 (+0.8/-0.4). The estimate average flux of the source is 9 (+5/-4) x 10^-12 erg/s/cm².

We note that 1RXS J080114.6-462324 has been observed serendipitously by Swift as part of the field of GRB 070227, between February 28th, 2007 and March 14th, 2007. As reported by Landi et al. (2010), 1RXS J080114.6-562324 is detected in those data, at a position consistent with that reported for our latest observations above. The source shows a factor of 3 variations during those observations, and with fluxes between 1.6 - 5.1 x 10^-12 erg/s/cm², with a peak count rate of 0.12 c/s. The flux seen in the most recent observation is not significantly higher than the peak flux seen in 2007, and therefore we cannot determine from the XRT data if 1RXS J080114.6-462324 has been in outburst and is the origin of the flare seen by MAXI.