Optical follow-up and archival X-ray/optical observations of the new X-ray transient MAXI J1631-479
ATel #12504; A. K.H. Kong (National Tsing Hua University, Taiwan)
on 14 Feb 2019; 21:47 UT
Credential Certification: Albert Kong (akong@phys.nthu.edu.tw)
Subjects: Optical, X-ray, Transient
We conducted optical follow-up observations of the new X-ray transient MAXI J1631-479 (ATels #12320, #12340, #12418, #12438, #12440) with the iTelescope.Net T17 0.43m telescope in Siding Spring and the 1m telescope of the CHILESCOPE Observatory. The optical object reported in ATel #12438 is clearly seen in all observations although the T17 observations have low S/N. By comparing with the AAVSO Photometric All-Sky Survey (APASS) r-band data, we estimated the following magnitudes.
Date Telescope Magnitude
2019-02-03 17:44 UT 0.43m clear filter 20.0+/-0.5
2019-02-09 17:38 UT 0.43m clear filter 19.86+/-0.15
2019-02-10 07:43 UT 1m SDSS-r filter 19.36+/-0.06
The object is also seen in the Digitized Sky Survey although it is not in the USNO catalogue. By comparing with nearby USNO stars, the object has an R magnitude of 18.13+/-0.03 on 1992 August 21. The position measured with the 1m telescope is R.A.=16:31:14.1, Decl.=-47:48:22.8 which is 1.4 arcsec from the radio position (ATel #12396). Given that the seeing during our 1m observation is about 1.4 arcsec, it is unclear if there is a fainter object at the radio position. In summary, the optical object near the radio source is likely a persistent source although it shows some level of variability.
We also note that the position of MAXI J1631-479 was observed with the Chandra X-ray Observatory for 9.6 ks with ACIS-I on 2009-06-22. There is no X-ray source detected at the radio and optical positions. We estimated a 3-sigma unabsorbed flux (0.3-8 keV) limit of 5.4e-14 erg/s/cm^2, equivalent to 1.6e32 (d/5 kpc)^2 erg/s by assuming an absorbed power-law model (nH=2e22 cm^-2, photon index=2).
Spectroscopic observations of the optical source will show the physical nature and its association with the X-ray/radio transient. High-resolution optical imaging observations may also help identify the real optical counterpart of MAXI J1631-479.
