Swift follow-up observations of the new outburst of the black hole candidate V4641 Sgr

ATel #7874; 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)
on 4 Aug 2015; 10:38 UT
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Credential Certification: Diego Altamirano (d.altamirano@soton.ac.uk)

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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 (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.