Swift X-ray detection and VLT/FORS2 spectroscopy of the nuclear transient Gaia17cmd/AT2017har

ATel #10901; K. Sokolovsky (IAASARS NOA/ASC Lebedev/SAI MSU), M. Gromadzki, L. Wyrzykowski (Warsaw Observatory), N. Ihanec (University of Nova Gorica)
on 27 Oct 2017; 19:56 UT
Credential Certification: Kirill Sokolovsky (kirx@scan.sai.msu.ru)

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The optical transient Gaia17cmd was discovered on 2017-09-30 coinciding with the nucleus of a barred spiral galaxy 2MASX J07414558+1613063. The galaxy is suggested to be an AGN based on WISE colors by Secrest et al (2015 ApJS, 221, 12), however no X-ray or radio detection has been published. The Palomar 1.5m telescope SED-Machine spectroscopy on 2017-10-03 reported by N. Blagorodnvoa in TNS Classification Report No. 1265 suggested this is an AGN at z=0.11.

We obtained the spectrum of Gaia17cmd on 2017-10-05 using the FOcal Reducer/low dispersion Spectrograph 2 (FORS2) on the Very Large Telescope (VLT). The spectrum reveals blue continuum with superimposed emission lines of the Balmer series, [N II], SII, [O II] and [O III] at redshift z=0.099. Gaia17cmd is very similar to a composite of broad-line AGN (quasars) taken from the Sloan Digital Sky Survey (Vanden Berk et al. 2001, AJ, 122, 549), but the iron complex seams to be slightly stronger.

We observed Gaia17cmd with Swift for 2.1ks on 2017-10-18. Swift/XRT detected an X-ray source with the net count rate of 0.0207 +/-0.0031 cts/s at the position of the transient. Fixing the absorbing column density to the Galactic value in the direction of the source, n_H1 = 3.51x10^20 cm^-2 (Kalberla et al. 2005 A&A, 440, 775), the spectrum can be fit with the absorbed power law with the photon index of 2.04 +/-0.32 and the unabsorbed 0.3-10 keV flux of 8.1x10^-13 ergs/cm^2/s.

An ultraviolet source is detected at the position of the transient with the following UVOT magnitudes (Vega system, no host light subtracted):

 
   JD       Band  Mag. Err. 
 2458045.29 UVW2 17.39 0.06 
 2458045.30 UVM2 17.47 0.07 
 2458045.30 UVW1 17.51 0.07 

Taking into account the Galactic reddening of E(B-V)=0.029 (Schlafly et al. 2011 ApJ, 737, 103) these magnitudes suggest a black-body emission with the temperature ~20000 K. The high color temperature together with the X-ray detection, optical spectrum properties and archival WISE colors are consistent with this event being an AGN flare.

We thank the Swift team for rapid execution of this observation. We acknowledge ESA Gaia, DPAC and the Photometric Science Alerts Team (Rixon et al, 2014, ATel #6593). The VLT classification spectrum will be available on the IAU's Transient Name Server (TNS). The observations were carried under ToO programme 0100.B-0503(A), PI: M. Gromadzki. We thank ESO/VLT staff for their support.

Gaia17cmd/AT2017har at the Transient Name Server