Archival Infrared Data on the New Transient MAXI J1621-501

ATel #10876; Peter Pessev (GRANTECAN, IAC, ULL)
on 20 Oct 2017; 10:48 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Peter Pessev (peter.pessev@gtc.iac.es)

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MAXI/GTC nova alert system detected a bright uncatalogued X-ray transient at 05:45 UT on 2017 October 19 (Hashimoto et al. 2017; ATel #10869). The MAXI error region was observed with Swift/XRT, enhancing the coordinates and significantly reducing the positional uncertainty (Bahramian et al. 2017; ATel #10874). A comprehensive search in the available archives resulted in a detection of a probable progenitor in the VVV Survey (vvvsurvey.org; Minniti et al. 2010, New Astronomy, 15, 433) archival data and Spitzer GLIMPSE catalog. All distances are reported with respect of the Swift/XRT position, reported by Bahramian et al. (ATel #10874).

RA: 16:20:22.01
Dec: -50:01:11.6
With radial uncertainty of 3.4''

In the VVV data, there are three objects within the uncertainty circle: VVVJ162021.84-500112.94, VVVJ162021.76-500110.27 and VVVJ162022.12-500113.86. The closest (and brightest) object is located 2.1 arcseconds away and has the following magnitudes:

Z = 19.443 +/- 0.086
Y = 18.339 +/- 0.049
J = 17.124 +/- 0.027
H = 16.056 +/- 0.029
Ks = 15.351 +/- 0.035

The photometric flags indicate a stellar object. The other two objects are detected only in H and Ks and are about two magnitudes fainter. The VVV data are in the natural VISTA Vegamag system.

The GLIMPSE source G333.4561+00.0474 is located 2.7 arcseconds away and has the following magnitudes and colors:

[3.6] = 14.269 +/- 0.131
[4.5] = 13.989 +/- 0.285
[3.6 - 4.5] = 0.28 +/- 0.314

There is a good positional match between both detections within 0.969 arcseconds.

Note that according to the Schlafly & Finkbeiner 2011 (ApJ 737, 103) and Schlegel et al. 1998 (ApJ 500, 525) extinction maps the visual extinction values are Av = 43.99 and 51.14 magnitudes respectively. (For a two degrees region around the transient position.) This to a large extent explains the absence of a Gaia source and probably will hamper the visual counterpart detection. The reported VVV magnitudes are well below the limits of 2MASS and the absence of a 2MASS source is not surprising as well. Follow-up observations, particularly in the infrared (taking into account the high extinction) are needed to better constrain the nature of the transient.

This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. I gratefully acknowledge use of data from the ESO Public Survey programme ID 179.B-2002 taken with the VISTA telescope, and data products from the Cambridge Astronomical Survey Unit.