Swift J1658.2-4242 - new counterparts around the reported ATCA position

ATel #11334; Peter Pessev (GRANTECAN, IAC, ULL)
on 22 Feb 2018; 14:50 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Peter Pessev (peter.pessev@gtc.iac.es)

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The X-ray transient Swift J1658.2-4242 was detected on 18.02.18UT with initial coordinates RA=16h58m11s and Dec=-42d42'31" with an uncertainty of 3 arcmin (GCN #22416). (All coordinates in this contribution are in equinox J2000.) The subsequent analysis of the XRT/UVOT data resulted in more precise coordinates RA=16h58m12.64s and Dec=-42d41'54.5'', with error radius 1.6'' at 90% confidence (ATel #11307). Within the error circle two known sources were encountered VVV J16581262-42415478 and GLIMPSE G343.2514+00.0537 (ATel #11310). The ATCA radio observations of the transient (ATel #11322) yield RA=16h58m12.700+/-0.004, Dec=-42d41'56.09+/-0.25 (statistical errors only). Both positions agree within the uncertainty of the measurements, but the reported counterparts are outside of the error circle of the radio position. A dedicated search around the new reported position resulted in the detection of a counterpart in three catalogs: 2MASS J16581268-4241563 (located 0.33" away, but consistent within the positional uncertainty of the 2MASS PSC), VVV J165812.68-424156.25 (0.21" away) and GAIA 5966232529369270784 (0.25" away). The 2MASS magnitudes of the source are J=16.038+/-0.106, H=15.116, Ks=15.055 with photometry flags AUU, which means that H and Ks magnitudes are reported as upper limits. The VVV source have magnitudes J=16.046+/-0.012, H=15.381+/-0.016, Ks=14.996+/-0.027. Taking into account the J band data in the epoch of 2MASS and VVV, along with the H and Ks magnitudes it is obvious that there are no significant changes in the near-IR flux of the source. The GAIA DR1 magnitude of 5966232529369270784 is 19.604. According to the Schlafly&Finkbeiner 2011 reddening maps, the extinction in the V band is 58.6 magnitudes, corresponding to Aj=15.7, Ah=9.98, Ak=6.7 magnitudes. Applying these will result in unreasonably blue near-IR colors. Obviously the extinction estimates are unreliable due to the proximity of the position to the Galactic plane. Based on the near-IR colors from the VVV data (without extinction correction): J-Ks=1.05+/-0.03, J-H=0.67+/-0.02, H-Ks=0.38+/-0.03, the source could be identified as a M6-M8 main sequence or giant star (Pecaut&Mamajek 2013, Ducati ey al. 2001). If we consider a main sequence star, the resulting V band magnitude will be in the order of 23. If a M6III object is considered, the resulting V~20 mag is broadly consistent with the G band magnitude reported by Gaia. (Note that the Gaia G band is fully encompassing the widely used V band filter.) Based on the reported characteristics of the counterparts, the uncertainty of the available extinction estimates and the interesting nature of the transient further optical and near-IR follow-up observations are strongly encouraged.