MAX/GSC and Swift/XRT detection of an X-ray flare from 1RXS J043657.1-161258

ATel #6654; S. Nakahira (JAXA), N. Kawai (Tokyo Tech), T. Sakamoto (AGU), K. Yamaoka (Nagoya U), H. Negoro (Nihon U.), M. Serino (RIKEN), P. A. Evans (U. Leicester), K. L. Page (U Leicester), S. Ueno, H. Tomida, M. Kimura, M. Ishikawa, Y. E. Nakagawa (JAXA), T. Mihara, M. Sugizaki, M. Morii, J. Sugimoto, T. Takagi, A. Yoshikawa, M. Matsuoka (RIKEN), T. Yoshii, Y. Tachibana (Tokyo Tech), A. Yoshida, Y. Kawakubo, H. Ohtsuki (AGU), H. Tsunemi, D. Uchida (Osaka U.), M. Nakajima, K. Fukushima, T. Onodera, K. Suzuki, T. Namba, M. Fujita, F. Honda (Nihon U.), Y. Ueda, M. Shidatsu, T. Kawamuro, T. Hori (Kyoto U.), Y. Tsuboi, A. Kawagoe (Chuo U.), M. Yamauchi, Y. Morooka, D. Itoh (Miyazaki U.) on behalf of the MAXI team and the Swift-MAXI collaboration
on 1 Nov 2014; 10:32 UT
Credential Certification: Satoshi NAKAHIRA (nakahira@crab.riken.jp)

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The MAXI/GSC nova alert system triggered a bright uncatalogued X-ray transient source at 2014/10/29 04:32:09 UT . Assuming that the source flux was constant over the transit, we obtain the source position at
(R.A., Dec) = (69.413 deg, -16.481 deg) = (04 37 39, -16 28 51) (J2000)
with a statistical 90% C.L. elliptical error region with long and short radii of 0.28 deg and 0.23 deg respectively. The roll angle of long axis from the north direction is 170.0 deg counterclockwise. There is an additional systematic uncertainty of 0.1 deg (90% containment radius). The X-ray flux averaged over the scan was 137±24 mCrab (4-10keV, 1 sigma error). There was no significant excess flux in the previous transit at 02:59 UT and in the next transit at 06:05 UT with an upper limit of 20 mCrab.

The Swift ToO observation of this transient (GRB141029A; Nakahira et al., GCN16981) was performed starting from 2014-10-29T09:00:37 with 4-point tiling to cover the MAXI error circle.
We found three X-ray sources in the MAXI error circle as shown in the Swift observation summary page
http://www.swift.ac.uk/xrt_products/TILED_GRB00031/.
One of the sources ("source 1") showed suggestive fading between the two tilings centered at 15500 s and 20700 S after the MAXI detection with count rates of 2.7±0.3 c/s and 1.7±0.2 c/s respectively. Assuming a power-law spectrum with photon index of 2.4 the X-ray flux at the first Swift pointing is estimated as 6.8×10^-11 erg cm^-2 s^-1, two order of magnitude fainter than 5.6×10^-9 erg cm^-2 s^-1 at the MAXI detection.

The fading behavior was confirmed by additional Swift TOO observation conducted from 2014-10-30T10:14:59 with a detected count rate of 0.27±0.02 c/s.

This fading source matches a catalogued X-ray source 1RXS J043657.1-161258, whose position in the ROSAT All-Sky Survey Bright Source Catalog is 9.1" away from the Swift position with a 3-sigma positional error of 40.1". The source flux in the catalog corresponds to 9.3×10^-12 erg cm^-2 s^-1, indicating that the fluxes in the present Swift observations were in much elevated states. The source was also reported to be variable (Fuhrmeister and Schmitt 2003).
Thus, we conclude that 1RXS J043657.1-161258 is the most likely source of the MAXI transient.

This optical counterpart of 1RXS J043657.1-161258 has been studied, and found to be variable with with possible rotational period of P=0.3787d or P =2.74d (Kiraga et al. 2012).
These characteristics of the source suggest that the X-ray transient observed by MAXI was a strong flare from an active star system such as RS CVn star, dMe star, or a cataclysmic variable.

Non-detection of an optical counterpart of source 3 reported by Xu et al. ( GCN16994) further supports that this transient was not a gamma-ray burst.

We thank the Swift team for performing the observation.