[vsnet-alert 14473] (fwd) MAXI J1305-704

[Taichi Kato]

   A bit delayed forwarding, but try obseration!  The object appears
to have a high inclination and short period.  Photometric waves
(orbital or superhump) are expected.

===

The Astronomer's Telegram                   http://www.astronomerstelegram.org

==============================================================================
ATEL #4030                                                           ATEL #4030

Title:  MAXI J1305-704: bright candidate counterpart from Swift and
                GROND 
Author: Jochen Greiner, Arne Rau, Patricia Schady (all MPE) report
                on behalf of the GROND team:
Queries:        arau at mpe.mpg.de
Posted: 11 Apr 2012; 10:48 UT
Subjects:Infra-Red, Optical, Ultra-Violet, X-ray, Binary, Black Hole,
                Transient

Following the discovery by MAXI (Sato et al. 2012, ATEL #4024), a Swift

target of opportunity observation was triggered (PI: Nakahira) with five

tiled pointings to cover the full MAXI error box. In three of these  
pointings an X-ray source was detected at

RA(J2000) = 13:06:56.44 
Dec(J2000) = -70:27:04.91 

with an uncertainty of 5". This position was not covered in the remaining
two pointings.

The source is bright and variable, with a count rate between 5-10 cts/s.

The X-ray spectrum is well described (reduced chi^2 of 1.0) with a power
law  
of photon index -1.7+-0.1, and an absorbing column of NH=(1.6+-0.3)E21
cm^-2.  
A disk blackbody model fit is also acceptable, but has a worse reduced

chi^2 of 1.2). The unabsorbed 0.2-10 keV luminosity is about  
5E34 (D/1 kpc)^2 erg/s.

On April 11th 07:06 UT we obtained a single  observation simultaneously
in g'r'i'zJHK with GROND (Greiner et al. 2008, PASP, 120, 405) mounted
at the 2.2m MPI/ESO telescope in La Silla. Within the XRT error circle
we find a new bright point source at 

RA(J2000) = 13:06:55.36  (196.73065 deg)

Dec(J2000) = -70:27:05.0  (-70.45139 deg) 

with an astrometric solution accuracy of 0"3 in RA and 0"2 in Dec.

This source is not visible in archival DSS or 2MASS images. We measure
the following AB magnitudes

g' = 16.51 +/- 0.05

r' = 16.50 +/- 0.05

i' = 16.61 +/- 0.05

z' = 16.65 +/- 0.05

J = 16.52 +/- 0.05

H = 16.59 +/- 0.05

calibrated against GROND zeropoints in g'r'i'z' and 2MASS field stars in
JH.

The GROND source is also detected in a Swift/UVOT observation with the
uvw1 filter obtained on April 10th 16:26 UT with an AB magnitude of uvw1
= 17.23 +/- 0.04.

The combined GROND+UVOT Galactic foreground extinction (E(B-V)=0.34mag;
Schlegel et al. 1998) corrected spectral energy distribution is best fit
by a simple power law following F_Lambda propto lambda^-2.8, reminiscent
of an accretion disk spectrum. 

The high amplitudes in the optical (>3mag compared to DSS) and at X-rays
(>500 compared to ROSAT) together with the blue optical SED and the relatively
hard X-ray spectrum suggest this source to be a new low-mass black hole
X-ray transient (similar in several aspects to XTE J1118+480). Follow-up
observations, in particular optical spectroscopy and radio observations,
are strongly encouraged.

We are grateful to J. Alarcon (ESO/La Silla) for support in the GROND observation.