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 (firstname.lastname@example.org)
Subjects: Optical, Ultra-Violet, X-ray, AGN
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
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