[vsnet-outburst 13427] MAXI J0158-744/XRF 111111A (ATEL)

[Taichi Kato]

ATEL #3758							     ATEL #3758

Title:	MAXI J0158-744/XRF 111111A: Swift X-ray and U band detection
Author:	J. A. Kennea, E. A. Hoversten (PSU), P. Curran (CEA-Saclay),
		P. A. Evans (U Leicester). H. A. Krimm (CRESST/GSFC/USRA), P. Romano,
		V. Mangano (INAF-IASFPA), K. Yamaoka (AGU) and H. Negoro (Nihon U.),
		M. Kimura (Osaka U.), H. Tomida (JAXA), T. Sootome, M. Serino, T.
		Mihara, S. Nakahira (RIKEN), M. Morii, N. Kawai (Tokyo Tech)
Queries:	kennea at astro.psu.edu
Posted:	12 Nov 2011; 00:48 UT
Subjects:Optical, X-ray, Transient

At 15:33:21UT on November 11th, 2011 Swift began a target of opportunity
observation of the error circle of the new transient MAXI J0158-744/XRF
111111A (Kimura et al., ATEL #3756), approximately 12 hours after its initial
detection by MAXI. We find an uncatalogued moderately bright (1.5 XRT c/s)
X-ray source at the following location: RA/Dec (J2000) = 29.85647, -74.25795,
which is equivalent to: 

RA(J2000) = 01h 59m 25.55s 

Dec(J2000) = -74d 15m 28.6s 

with an estimated uncertainty of 2.4 arc-seconds radius (90% confidence).
This lies 8.9 arc-minutes from the center of the refined MAXI error ellipse
(Morii et al., GCN #12555), consistent with the ellipse when considering
both statistical and systematic errors. 

Within the XRT error circle lies a catalogued optical source, reported
by Nishiyama et al, 2007 to be a candidate Herbig Ae/Be star in the Magellanic
Bridge. This optical counterpart is detected by UVOT, which finds a position
of: RA/Dec(J2000) = 29.85762, -74.25775, which is equivalent to: 

RA (J2000) = 01h 59m 25.83s 

Dec (J2000) = -74h 15m 27.9s 

with an estimated uncertainty of 0.5 arcsec (radius, 90% confidence, statistical
+ systematic). UVOT measures a brightness of U = 13.06 +/- 0.02 (not including
a 0.02 systematic error). In comparison the USNO-A2.0 catalog reports a
brightness of B=15.4 for the optical counterpart, which lies inside the
UVOT error circle. Since such a large U-B value is unlikely and suggests
that the optical counterpart has brightened, implying that this is likely
the counterpart of the X-ray transient MAXI J0158-744. 

The X-ray source appears soft, with no emission above 2 keV. However, further
longer observations will be needed to determine of the spectral parameters,
due to pile-up issues and the relatively short exposure time. A search
of archival images from the BAT transient monitor for the past month show
no sign of emission in the 15-50 keV range with a an average 1-sigma limit
of 0.001 ct/s/cm^2 (~5 mCrab). This is consistent with the soft spectrum
of the source. 

Further observations of this target with Swift have been requested.

==============================================================================
ATEL #3759							     ATEL #3759

Title:	Swift observations of the supersoft X-ray transient XRF 111111A
		/ MAXI J0158-744
Author:	K.L. Li, A.K.H. Kong, P.H.T. Tam, J.H.K. Wu (NTHU, Taiwan)
Queries:	lilirayhk at gmail.com
Posted:	12 Nov 2011; 02:36 UT
Subjects:Optical, X-ray, Transient

We report Swift follow-up observations of the soft X-ray transient MAXI
J0158-744 (ATel #3756). The observations was performed with the XRT on
2011-11-11 15:33:38 UTC for a duration of 320s in the Photon Counting (PC)
mode. 

We found a bright X-ray source within the refined error region (including
the 0.1 deg systematic uncertainty, GCN #12555) of MAXI J0158-744 in the
XRT observations. Using the ``Build Swift-XRT products'' tool (Evans et
al. 2009, MNRAS 397, 1177) in the UK Swift Science Data Centre at the University
of Leicester, we got an UVOT enhanced position of 

(R.A., Dec) = (29.85647 deg, -74.25795 deg) = (01 59 25.55 -74 15 28.6)
(J2000) 

with 2.3 arcsec uncertainty (90% error radius). We analyzed the energy
spectrum of the observation and fitted with an absorbed blackbody model.
Using a fixed column density nH=1.36e21 cm^-2 obtained from the weighted
LAB map (Kalberla et al. 2005, A&A, 440, 775), we get a best fit temperature
of 106+/-10 eV which is corresponding to an unabsorbed flux of 7.7e-11
(+0.4/-0.8) ergs/cm^2/s in the 0.2-1.0 keV band. Assuming a distance of
61.3kpc (distance of the SMC), we estimated the X-ray luminosity to be
3.5e37 ergs/s. It is worth noting the X-ray source is currently in a decaying
phase from a peak count rate of 2.4 cts/s to < 0.2 cts/s in about 7 minutes.

Swift UVOT has also detected a bright UV source with U=13.08 +/- 0.04 (stat)
+/- 0.02(sys) (in Vega system) at the X-ray position. In addition, we found
a 16.16 mag B-band counterpart of this UV source in the USNO B1.0 catalog
in the epoch 1989 with a 1.3 arcsec offset which suggested that the optical
source could be a variable in the optical band. The bright UV source is
also detected with the 2MASS with J=14.8, H=14.8, and K=14.4. 

All the results are consistent with Kennea et al. (ATel #3758). 

>From the positional coincidence, similar hardness and luminosity, and the
X-ray and optical variability, we believed that this X-ray source is indeed
the soft X-ray transient MAXI J0158-744. By comparing it with the four
other supersoft sources (SSSs) in the SMC, all five sources have similar
features (i.e. low temperatures of tens eV and ultra high X-ray luminosity
~10^37-38 erg/s), which indicated that MAXI J0158-744 is a SSS.  

We would like to thank the Swift team for their prompt response of our
ToO request.