Discovery of the radio counterpart for 2MASX J20183871+4041003 within the error-box of IGR J2018-4043 and of the AGILE variable gamma-ray source in the Cygnus region

ATel #1518; G. Dubner and E. Giacani (IAFE, Argentina), A. M. Bykov, A. M. Krassilchtchikov and Yu. A. Uvarov (IOFFE, Russia)
on 13 May 2008; 13:51 UT
Credential Certification: Gloria Dubner (gdubner@iafe.uba.ar)

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Based on C-configuration VLA observations carried out on December 9 and 12, 2006 at 1423 and 4885 MHz (program ag728), we report on the presence of a radio point source centered exactly at the position of 2MASX J20183871+4041003 within the error box of the discovered AGILE variable gamma-ray source (ATEL #1492, ATEL #1497 and ATEL #1498).

The image at 1.4 GHz has a HPBW=25.6¨x 15.8¨, PA=-62.39, and rms= 0.15 mJy/beam; and at 4.8 GHz, HPBW=7.1¨x 4.7¨, PA =-63.21 and rms=0.08 mJy/beam.

By fitting an elliptical Gaussian to the peak, we have derived characteristic parameters for this source. Position: RA(J2000) 20 18 38.73 , Decl. (J2000) 40 41 00.2 , peak intensity at 1.4 GHz: 8.7 +/- 0.87 mJy/beam; integrated intensity at 1.4 GHz: 31.3 +/- 1.9 mJy; peak intensity at 4.8 GHz: 2.0 +/- 0.05 mJy/beam; integrated intensity at 4.8 GHz: 3.20 +/- 0.12 mJy .

The observations were carried out in dynamic scheduling of the VLA through different short intervals. The source did not show variability during the observing periods, which were: at 1.4 GHz from December 9, 2006, 23:15:40 to December 10 06:54:40, and at 5 GHz on December 12, 2006, between 06:05:50 and 07:48:50 and between 22:32:50 and 23:16:10 UT.

Dedicated study of the source variability is required as well as multi-frequency radio observations to get a reliable estimate for the radio spectrum. These observations will help to constrain the nature of a possible counterpart for IGR J2018-4043, particularly discriminating between AGN and X-ray binary interpretations as were discussed by Bykov et al. (ApJ v.649, L21, 2006). We especially encourage high frequency radio observations to search for high-frequency flattening or upturn shape spectra as was recently shown by Tucci et al. (MNRAS, v.386, p. 1729, 2008) for a sample of AGNs.