GCN Circular 20795
Subject
LIGO/Virgo G275697: optical spectroscopy of Swift/XRT source 1
Date
2017-03-02T18:39:28Z (8 years ago)
From
Daniele Malesani at DARK/NBI <malesani@dark-cosmology.dk>
Johan P.U. Fynbo, Daniele Malesani, Lise Christensen (DARK/NBI), Giorgos
Leloudas (WIS), and Elena Pian (SNS; INAF/IASF Bo) report on behalf of a
larger collaboration:
We observed the location of the X-ray "source 1" within the LVC error
region for the GW trigger G275697 (LVC Circ. 20763) reported by Swift
(Evans et al., LVC Circ. 20773) using the Nordic Optical Telescope (NOT)
equipped with the ALFOSC camera and spectrograph. Observations were
carried out in the r band, and started on 2017 Mar 1.25 UT (in twilight,
at high airmass).
We clearly detect the 2MPZ galaxy mentioned by Evans et al. (LVC GCN
20773) and coincident with the X-ray source. We measure a magnitude r =
16.67 +- 0.01 AB (5" aperture radius, calibrated against nearby stars
from the Pan-STARRS catalog). This value is consistent with the (Kron)
magnitude r = 16.68 listed in the Pan-STARRS catalog, indicating no
variability compared with the archival value.
A 900-s spectrum was obtained covering the wavelength range 3300-9500
AA, under non-photometric conditions. Several emission lines are
detected, corresponding to Halpha, Hbeta, the [O III] doublet, [N II]
and [S II], all at a common redshift z = 0.0385, corresponding to a
(luminosity) distance of 170 Mpc.
The width of the Halpha and Hbeta lines is around 1700 km s^-1,
indicating the presence of a broad-line region typical of an AGN. Also,
the location of this object in the extinction-insensitive BPT diagram
(e.g. Kewley et al. 2013, ApJ, 774, L10) is consistent with LINERs and
AGN. We caution that the [N II] lines, used for the BPT diagram, falls
at this redshift close to the telluric B band, but correcting for the
flux loss should, if anything, move our target even more towards the AGN
locus. Finally, we note that the Halpha/Hbeta ratio indicates
significant extinction, even after correcting for the non-negligible
Galactic reddening (A_V ~ 1 mag).
Our data thus indicate that the XRT source 1 is an AGN, which would
naturally explain the detected X-ray emission (at a level of ~10^43 erg
s^-1).
We acknowledge excellent support from the NOT staff, in particular Pere
Blay and Amanda Djupvik.