Confirming the BL Lac nature of the low energy counterpart of the new VHE source VER J0521+211 with WISE

ATel #4128; F. Massaro (Stanford University), R. D'Abrusco (SAO), A. Paggi (SAO)
on 25 May 2012; 20:52 UT
Credential Certification: Francesco Massaro (francesco_mss@yahoo.it)

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Following the VERITAS discovery of a new VHE Gamma-ray Source VER J0521+211 (ATEL #2260) and consistent with the position of the Fermi LAT source 1FGL J0521.7+2114, we searched in the Wide-field Infrared Survey Explorer (WISE; Wright et al. 2010 AJ, 140, 1868) all-sky catalog at the position of the 1FGL J0521.7+2114 source for a gamma-ray blazar candidate using the association procedure outlined in Massaro, F. et al. 2011 ApJ, 740L, 48 and in Massaro, F. et al. 2012 ApJ in press (http://arxiv.org/abs/1203.3801). Our association procedure is based on infrared color analyses using WISE data. In our analysis, we used the position of the Fermi LAT source (R.A. (J2000): 80.4472 deg, Dec. (J2000): 21.235 deg) as reported in the First Fermi LAT Source Catalog (1FGL, Abdo et al 2010ApJS, 188, 405) with a searching radius of 0.050713508 deg corresponding to 99.9% level of confidence.

Our search found a single source with IR colors typical of gamma-ray blazars within the Fermi positional uncertainty. The source is WISE J052145.96+211251.4 and its coordinates are:
R.A.(J2000) = 05h21m45.96s (80.4415151 deg)
Dec. (J2000) = +21d12m51.48s (+21.2142989 deg).

According to the WISE All-sky catalog (available at http://wise2.ipac.caltech.edu/docs/release/allsky/ ), WISE J052145.96+211251.4 is detected in all four energy bands with the following Vega magnitudes:
m(3.4 micron) = 10.631 +/- 0.023
m(4.6 micron) = 9.799 +/- 0.022
m(12 micron) = 7.556 +/- 0.019
m(22 micron) = 5.609 +/- 0.037

The WISE gamma-ray blazar candidate (WISE J052145.96+211251.4) is positionally consistent (0.086 arcsec offset) with the VLBA position of the radio source RGB J0521+212 (associated to the 1RXS J052147.2+211246 X-ray source) (R.A. (J2000): 05h21m45.9658s, Dec. (J2000): +21d12m51.451s, Beasley et al. 2002, ApJS, 141, 38), which was indicated as a possible low energy counterpart of the VERITAS source (ATel# 2260 and ATel# 2309). Optical spectroscopic observations permitted the identification of the RGB J0521+212 as a BL Lac source, revealing a continuum-dominated spectrum with either no absorption or emission lines, as reported in Errando et al. 2011 Fermi Symposium PROC. (http://arxiv.org/abs/1111.1209) Then, with our WISE association we confirm that the new VHE source is most probably a gamma-ray blazar with the IR colors similar to those of the other TeV BL Lac objects.

This analysis makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration.

Additional details concerning the association procedure can be found in D'Abrusco, R. et al. 2012 ApJ, 748, 68; Massaro, F. et al. 2012 ApJ, 750, 138; Massaro, F. et al. 2012 ApJ in press (http://arxiv.org/abs/1203.3801); Massaro, F., et al. 2012 ApJ in press (http://arxiv.org/abs/1203.4230).