[vsnet-alert 18937] V4641 Sgr active (ATEL)

[Taichi Kato]

V4641 Sgr active (ATEL)

   Most recent observation by Andrew Pearce:

SGRV4641        20150804.567    12.8    PEA

===

ATEL #7874                                                           ATEL #7874

Title:  Swift follow-up observations of the new outburst of the black
                hole candidate V4641 Sgr
Author:  D. Altamirano (Southampton), A. Bahramian, G. Sivakoff (Alberta),
                M. Middleton (Cambridge), C. Knigge, P. Gandhi (Southampton), R. Hynes,
                C. Johnson (Louisiana State University), P. Casella (INAF-Roma), S.
                Motta (Oxford), J. Miller-Jones (Curtin), J. Neilsen (MIT)
Queries:        d.altamirano at soton.ac.uk
Posted: 4 Aug 2015; 10:38 UT
Subjects:X-ray, Black Hole, Transient



The MAXI team has recently reported on the outburst onset of the black

hole candidate V4641 Sgr as detected with MAXI/GSC (ATEL #7858). In 
order to confirm the outburst and characterize its current accretion 
state, a Swift/XRT PC-mode pointed observation was performed on UT 
07:31:00 02/08/2015 for a total of 435 seconds. V4641 Sgr is clearly 
detected at an average of ~2 cts/s (0.3-10 keV), confirming that the 
source is in outburst. We did not find any evidence of flaring 
activity during this observation. 

We extracted a spectrum using the online tool at 
<http://www.swift.ac.uk/user_objects> 
http://www.swift.ac.uk/user_objects (Evans et al. 2009, MNRAS, 397, 
1177), which corrects for pile-up effects. The data are satisfactorily

fit with an absorbed [column density N_H = (5+/-2)*1E21 cm^-2)] 
power-law of index 2.1+/-0.4 (reduced-chi2 of 1.3 for 9 dof). We note 
that due to the low quality of the current data, other models can also

satisfactorily fit the data.  The unabsorbed 0.5-10 keV flux of the 
power-law fit is ~3.5E-10 erg/cm^2/s, corresponding to an X-ray 
luminosity of ~1.5E36 erg/s at a distance of 6.2 kpc (MacDonald et 
al. 2014, ApJ, 784, 2). Our choice of spectral model is based on the 
assumption that at this luminosity, V4641 Sgr is likely in the hard 
state. 

We warmly thank the Swift team for the rapid scheduling of the Swift 
observation of V4641 Sgr. Swift will monitor the evolution of the 
current outburst with daily observations for at least the next 10 
days.  Observations at other wavelengths are encouraged.