MAXI J0637-430: Swift localization, optical counterpart

ATel #13257; J. A. Kennea (PSU), A. Bahramian (ICRAR), P. A. Evans, A. P. Beardmore (U Leicester), H. A. Krimm (CRESST/GSFC/USRA), P. Romano (INAF-IASFPA), K. Yamaoka (Nagoya U.), M. Serino (RIKEN), H. Negoro (Nihon U.)
on 3 Nov 2019; 19:10 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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At 13:39UT on November 3, 2019, Swift performed a target of opportunity observation to observe the location of the newly discovered soft X-ray transient, MAXI J0637-430 (Negoro et al., ATEL #13256). Swift performed a 7-point tiling of the MAXI error region with an exposure of 200s per tile. In the Photon Counting mode data, we find a previously uncatalogued bright X-ray source at the following location: RA/Dec(J2000) = 99.09828, -42.86780, which is equivalent to:

RA(J2000) = 06h 36m 23.59s,
Dec(J2000) = -42d 52m 04.1s,

with an estimated uncertainty of 2.3 arc-seconds radius (90% confidence). This position lies 28.4 arc-mins from the 2-4keV MAXI localization, and 18.5 arc-mins from the 4-10 keV band MAXI localization. We note that the XRT position is consistent with the hard band localization, but not the soft band. However, the XRT detected source is bright and uncatalogued, suggesting that this is MAXI J0637-430. This transient is bright, with a count rate corrected for pile up of ~310 XRT count/s.

The pile-up corrected X-ray spectrum can be well fit by an absorbed power-law + disk blackbody spectrum, with kT_in = 0.9 +/- 0.1 keV, and photon index = 2.3 +/- 0.8. Absorption with this model is ~8 x 10^20 cm^2. The observed flux is 1 x 10^-8 erg/s/cm^2 (0.5 -10 keV).

In addition UVOT detects a source inside the XRT error circle, with a brightness of u = 14.87 +/- 0.02 (Vega). A Vizier catalog search does not reveal a known star at this localization, suggesting that this optical source has brightened significantly. We note that significant brightening (~5 magnitudes) of the optical counterpart during X-ray outburst is common for BH-LMXBs. The UVOT position is RA/Dec(J2000) = 99.09881, -42.86785, which is equivalent to:

RA(J2000) = 06h 36m 23.23s
Dec(J2000) = -42d 52m 04.25s.

We encourage further observations of this source in optical and X-ray in order to determine the nature of the source.

This work was supported by NASA Grant 80NSSC19K1383, awarded by the Swift Guest Investigator program.