VLA Observations of the AGILE Cygnus Region Source Field Following its May 2008 Re-brightening in Gamma-rays

ATel #1584; C. C. Cheung (NASA GSFC)
on 20 Jun 2008; 23:30 UT
Credential Certification: Teddy Cheung (ccheung@milkyway.gsfc.nasa.gov)

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Following the recent announcement of renewed gamma-ray activity in the AGILE Cygnus region source (ATel #1547; see also ATel #1492), we obtained VLA observations of the field in search of a variable radio source which would implicate a likely counterpart. Observations at 1.4 GHz were obtained on May 30 and June 3, 2008 (both at 09hr UT), a few days after the AGILE detection of the high gamma-ray state (from data collected between May 22-27).

In the central ~20' portion of the reported r=0.5 deg error circle of the AGILE source, we found several interesting bright compact radio sources, but none varied by more than ~10% from their average between the four days. This includes the recently discovered VLA radio counterpart (Dubner et al. ATel #1518) of 2MASX J20183871+4041003 (Bykov et al. 2006 ApJL 649, L21) for which the details are reported here; the others sources will be described elsewhere. The 2MASX/radio source is a bright X-ray source detected by the Swift XRT (ATel #788, #1497), making it the likely counterpart of the hard X-ray source IGR J2018+4043 (Bykov et al. 2006; see ATel #1497 for the Swift BAT detection). It has been further suggested as the likely low energy counterpart of the AGILE transient, as its association with the hard X-ray source makes it a possible blazar or X-ray binary. However, the 2MASX/radio source may simply be a nearby Seyfert galaxy (Atel #1498), and a different source in the field may be responsible for the variable gamma-ray emission detected by AGILE.

The radio source discovered by Dubner et al. is detected at both our epochs. In our analysis/search for compact radio sources, we expunged the shortest baseline data (<1 kilo-lambda) to avoid contamination from the bright surrounding diffuse emission. We model the source with an elliptical Gaussian using the JMFIT task in AIPS and the resultant parameters (we quote the average between the two epochs) are: RA=20h18m38.90s (0.12s), Dec=+40d40'58.5" (0.8"), Peak = 11.1 (0.9) mJy/bm, Integrated = 17.4 (2.1) mJy (the restoring beam is 40.6"x13.7', PA=-80.4 deg and 1 sigma errors are in parentheses). The source is not variable between the 4 days to within the given uncertainties.

To gauge longer timescale variability, we note that the Dubner et al. detection of 2MASX J20183871+4041003 was from comparably deep VLA observations (from December 2006) but with slightly higher resolution data. They reported a radio source with a 8.7 (0.9) mJy/bm peak (25.6" x 15.8" beam @PA=-62.4 deg) and 31.3 (1.9) mJy total flux. We re-analyzed their archived data in a consistent manner with our analysis (expunging the <1 kilo-lambda baseline data and restoring with our larger beam) and measured 8.9 (0.7) mJy/bm peak and 10.3 (1.3) mJy total. Our two analyses give consistent results for the peak, but there is a large discrepancy in the integrated intensity. This may be due to the fact that we removed the shortest baseline data, thus are less sensitive to more extended emission which would explain their higher value. Based on the peak flux values, there is little evidence for dramatic variability of the compact source between the ~1.5 years span of the observations.

In summary, we find no dramatic variability of the radio source (on 4 day and ~1.5 year timescales) associated with 2MASX J20183871+4041003, even after the gamma-ray event in the AGILE source. Additionally, we find that the 2MASX radio source is <1% polarized (3 sigma limit) at 1.4 GHz from an analysis of the Dubner et al. data. Together with the fact that there is also no dramatic X-ray variability detected in the Swift observations (ATel #1497) and that a host galaxy was resolved (ATel #1498), makes it unlikely that this source is a blazar or even an X-ray binary. The question of a plausible low energy counterpart for the AGILE Cygnus source remains open.

VLA images