Identification of the optical/nIR counterpart of IGR J13402-6428 as an AGN at z=0.9202

ATel #16099; Francis Fortin (APC, Paris), Sylvain Chaty (APC, Paris), John Tomsick (SSL/UCB)
on 28 Jun 2023; 08:50 UT
Credential Certification: Francis Fortin (ffortin.sci.edu@gmail.com)

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IGR J13402-6428 currently has two Chandra candidates reported, and the nature of the true counterpart is still ambiguous (Tomsick et al. 2012, ApJ, 754, 145). We have acquired VLT/X-Shooter R-band images and spectra from 350 to 2500nm on both Chandra candidates CXOU J133935.8-642537 (a) and CXOU J133959.2-642444 (b) to lift the ambiguity on the true counterpart to IGR J13402-6428 and identify its nature.

(a) CXOU J133935.8-642537 :
The Chandra error circle does not contain any detectable source, neither in the R-band images nor within the spectral acquisitions of X-Shooter. We were able to extract the spectrum of the nearest source, 2MASS J13393589-6425363, located 1.4" away from the Chandra position. The spectrum is featureless, and the continuum can be fitted by a ~2000K blackbody assuming no extinction. We conclude that this source is very unlikely to be the counterpart to IGR J13402-6428 and that the Chandra candidate (a) is spurious.

(b) CXOU J133959.2-642444 :
The Chandra error circle lands between two 2MASS sources (2MASS J13395936-6424410 and 2MASS J13395909-6424481). From the X-Shooter R-band images, we confirm that the Chandra error circle contains a single source, Gaia DR3 5864147448949045632, separated by at least 2.5" from the nearest 2MASS sources. The X-Shooter spectrum of Gaia DR3 5864147448949045632 shows a single prominent emission line around 1260nm, that can be accurately reproduced by a lorentzian with a FWHM of 11.5(2) nm (~2736 km/s) with an equivalent width of -39.4 nm. We identify this feature as a redshifted Halpha line coming from an active galactic nuclei located at z=0.9202(1).

We conclude that the first Chandra candidate (a) CXOU J133935.8-642537 is spurious, and that the true counterpart to IGR J13402-6428 is (b) CXOU J133959.2-642444 ( = Gaia DR3 5864147448949045632). We thus identify IGR J13402-6428 as an AGN located at z=0.9202(1).

The VLT/X-Shooter acquisitions were carried out at ESO on 19/02/2019 under the program ID 0102.D-0918(A) (P.I. S. Chaty). S. Chaty is grateful to the CNES (Centre National d’Etudes Spatiales) for the funding of MINE (Multi-wavelength INTEGRAL Network).