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ATEL # 2429; C. Knigge (University of Southampton), D. Steeghs (University of Warwick), B. Gaensicke (University of Warwick), J.E. Drew (University of Hertfordshire), P.J. Groot (University of Nijmegen), A.A. Zijlstra (University of Manchester), M. Torres (Harvard/CfA), P. Rodriguez Gil (IAC), J. Casares (IAC), M. Santander Garcia (IAC), J. Drake (Harvard/CfA), K. Verbeek (University of Nijmegen), S. Sale (PUC/Universidad de Valparaiso), on behalf of the IPHAS and UVEX collaborations
on 10 Feb 2010; 14:01 UT
Password Certification: Danny Steeghs (firstname.lastname@example.org)
Subjects: Optical, X-ray, Gamma Ray, Transients
The position of the recently discovered transient gamma-ray source J0109+6134 (ATEL #2414) lies within the footprints of the IPHAS (Drew et al. 2005, MNRAS, 362, 753; http://www.iphas.org) and UVEX (Groot et al. 2009, MNRAS, 399, 323) surveys of the northern Galactic plane.
Paredes et al. (ATEL #2421) have already suggested an optical counterpart to this transient source, which is located just over 1" from the position of the suggested radio counterpart to this transient (VCS2 J0109+6133) and 4" from the Swift/XRT counterpart (ATEL #2420). However, several IPHAS images obtained between 2003 and 2006 show another optical source whose position is a much better match to the radio counterpart. Our best estimate of the position of this source is RA = 01:09:46.33, Dec = +61:33:30.5, which puts the source within 0.1" of the radio counterpart and also within 4" of the Swift counterpart. As an indication of the internal uncertainty on this position, we note that our single best detection of this source (in 2006) yields a position estimate of RA = 01:09:46.30, Dec = +61:33:30.7, within 0.25" of our best overall estimate. This is also comparable to the absolute positional error associated with our astrometry, which is tied to the 2MASS frame. We dub this new candidate counterpart IPHAS J010946.33+613330.5 and suggest it is more likely to be the true counterpart for the radio source, and, by extension, the gamma-ray transient. A finder chart showing both proposed counterparts along with the X-ray and radio positions is available at http://www.astro.soton.ac.uk/~christian/fermi_finder.html
The IPHAS and UVEX data also provide estimates of the optical brightness and colours of both candidate counterparts. Both sources have similar magnitudes and both are red. Our best overall estimates for IPHAS J010946.33+613330.5 are i' = 19.7, r'-i' = 1.5, while the corresponding numbers for the counterpart proposed by Paredes et al. are i' = 19.7, r'-i'= 1.2. Neither source shows significant evidence for an Halpha excess in the IPHAS photometry. In order to check that both sources are presently visible, we have obtained a 120s r'-band image with ACAM on the William Herschel Telescope on Feb 5, 2010, 19:54 UT. This yielded estimates of r' = 21.2 for IPHAS J010946.33+613330.5 and r' = 20.9 for the Paredes et al. counterpart, in excellent agreement with our IPHAS-based estimates.
Closer examination of the full IPHAS data set has yielded marginal evidence for optical variability of IPHAS J010946.33+613330.5. Perhaps more convincingly, the faintness or absence of this source in the I-band image shown by Paredes et al. 1993, A&ASS 102, 381 -- which does clearly show their proposed counterpart -- also seems to suggest that our new candidate counterpart for the gamma-ray transient may be optically variable as well.