Halpha emission from HLX-1 in outburst
ATel #4344; Roberto Soria (ICRAR-Curtin), George Hau (ESO-Santiago)
on 29 Aug 2012; 04:34 UT
Credential Certification: Roberto Soria (rsoria@physics.usyd.edu.au)
Subjects: Optical, X-ray, Black Hole, Transient
Following the current, periodic re-brightening of the intermediate-mass
black hole candidate HLX-1 (Godet et al. 2012, ATel #4327; Kong 2012,
ATel #4332), we observed its optical counterpart with the FORS2 spectrograph
on the Very Large Telescope, on 2012-08-27 (between 08:05 and 09:35 UT),
and on 2012-08-28 (between 06:17 and 07:48 UT). The total exposure time
was ~4500s on each night. The grism was GRIS_300I+11, with a 1" slit.
The DIMM seeing was ~ 0".8 - 0".9; conditions were photometric; the Moon was up.
The wavelength range was ~ 6000 - 10000 Ang.
After subtracting the diffuse stellar emission coming from the unresolved halo
of ESO243-49, we find a residual Halpha emission line at the location
of HLX-1, comparable in flux and full-width-half-maximum (FWHM) with the line
found by Wiersema et al. (2010, ApJ, 721, L102) in the early stages
of the 2010 outburst. A coadded 2-D spectrum from the two nights is shown here: http://tinyurl.com/9hf3veg
Our very preliminary data analysis gives a central wavelength of 6719.4 Ang,
corresponding to a systemic velocity ~ 7160 km/s. The average recession velocity
of the nearby, candidate host galaxy ESO243-49 is only ~ 6700 km/s (from NED, and
from Wiersema et al. 2010). This velocity offset is a factor of 2 higher than
the maximum rotational velocity expected in the halo of a galaxy such as ESO243-49.
Hence, we suggest that HLX-1 may not be bound to or located inside ESO243-49.
For HLX-1, we estimate an Halpha FWHM ~ 12 Ang ~ 550 km/s. This is a surprisingly
narrow width, by comparison with the Balmer emission line widths typically measured
in the high state of Galactic stellar-mass black holes. In the latter class of systems,
Halpha emission comes from the outer annuli of the illuminated accretion disk,
and has a typical Keplerian width ~ 2*sqrt(GM/R_out)*(sin i). For an outer disk
radius R_out <~ 1E13 cm (Farrell et al. 2012, ApJ, 747, L13; Soria et al. 2012a,
MNRAS, 420, 3599; Soria et al 2012b, submitted to MNRAS) and a black hole mass
>~ a few 10^3 Msun (Davis et al. 2011, ApJ, 734, 111), we would expect Keplerian
line widths one order of magnitude higher, unless the disk is seen almost
face on (i <~ 5 deg) or the Halpha emission is not from a thin Keplerian disk.
We also recovered the strong Halpha emission from an extended far-UV-bright region
a few arcsec to the west of HLX-1, already noted by Wiersema et al. (2010):
the line centre is at ~ 6767 Ang, corresponding to a recession velocity
~ 9330 km/s. The physical relation of this structure with HLX-1 or
with the host cluster Abell 2877 remains unclear.
The Swift X-Ray Telescope monitoring datapoint nearest to our VLT observations
was from 2012-08-28 at 14:14:37 UT, and showed a count rate of ~ 0.25 ct/s.
Further observations at all bands in the next few weeks are strongly encouraged.
We thank Dr Ivo Saviane and Dr David Jones for their outstanding technical
support during the two nights.
Coadded red spectrum from 2012 Aug 27-28
