MAXI J1305-704: bright candidate counterpart from Swift and GROND

ATel #4030; Jochen Greiner, Arne Rau, Patricia Schady (all MPE) report on behalf of the GROND team:
on 11 Apr 2012; 10:48 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Arne Rau (arau@mpe.mpg.de)

Tweet

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.