VLBI observations of the radio counterpart of IGR J20187+4041/2MASX J20183871+4041003 in the error box of the gamma-ray source AGL2021+4029/3EG J2020+4017

ATel #1597; A. Trejo (CRyA-UNAM, Mexico), E. Giacani (IAFE, Argentina), Z. Paragi (JIVE, Netherlands), H. J. Langevelde (JIVE, Leiden Univ., Netherlands), G. Dubner (IAFE, Argentina), A. M. Bykov (Ioffe Inst., Russia)
on 2 Jul 2008; 10:38 UT
Credential Certification: Alfonso Trejo (a.trejo@astrosmo.unam.mx)

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We report on European VLBI Network (EVN) observations of 2MASX J20183871+4041003, which was earlier detected with the VLA by Dubner et al. (ATel#1518). 2MASX J20183871+4041003 was identified as a counterpart to IGR J2018+4043 (=IGR J20187+4041) by Kennea et al. (ATel #788) and Bykov et al (2006, ApJL 649, L21), where both AGN and X-ray binary interpretations were discussed. IGR J2018+4043 is the only known hard X-ray source near to the so far unidentified EGRET source 3EG J2020+4017. The AGILE transient recently detected by Longo et al. (ATel#1492), that showed a re-brightening in June 2008 (Giuliani et al., ATel#1547) may be related to 3EG J2020+4017, and in turn to IGR J20187+4041/2MASX J20183871+4041003, although this relation has been debated by Halpern (ATel#1498). Chen et al. (ATel#1585) recently pointed out a possible positional discrepancy between 2MASX J20183871+4041003 and the AGILE detections.

Our goal was to test the compactness of 2MASX J20183871+4041003 in a short, exploratory project using the e-VLBI technique. The observations took place on 25 June 2008 at 1.6 GHz using seven telescopes of the e-EVN: Cambridge, Effelsberg, Jodrell Bank (MkII), Medicina, Onsala, Torun and the phased array Westerbork. The target was phase-referenced to J2015+3710, the total on-target time was 1.7 hours. The data rate was 512 Mbps, which resulted in 8*8 MHz subbands using 2-bit sampling.

Preliminary analysis of the data show a partially resolved source at the position of RA (J2000) 20 18 38.7347, Dec (J2000) 40 40 59.984 with a peak brightness of 0.72 +/- 0.05 mJy and an integrated flux density of 2.3 +/- 0.2 mJy. The angular resolution was about 30 milliarcseconds. The estimated position error is two milliarcseconds. We note that scatter broadening in this region and at this frequency may play a significant role. Full-track observations are needed at higher frequencies to determine the source structure. The presence of a compact radio component makes 2MASX J20183871+4041003 an eligible candidate for being the counterpart to the variable gamma source in the Cygnus region, however the detailed structure, spectra and variability of the compact component still have to be established. The observed low flux density however does not suggest recent flaring activity in the source. The lack of significant variability was noted by Cheung (ATel#1584) as well.

e-VLBI is a technique where the signals from far away radio telescopes are directly streamed to the central data processor for real-time correlation, instead of recording. The data are immediately available to the astronomers for further processing. e-VLBI developments in Europe are supported by the EC DG-INFSO funded Communication Network Developments project 'EXPReS' Contract No. 02662. The European VLBI Network is a joint facility of European, Chinese, South African and other radio astronomy institutes funded by their national research councils.