[vsnet-alert 19015] V4641 Sgr (ATEL)

[Taichi Kato]

   The state has changed.  Any variations now?

ATEL #7966							     ATEL #7966

Title:	V4641 Sgr appears to have transitioned to a highly radio-quiet
		non-thermal accretion state
Author:	G. R. Sivakoff, A. Bahramian (Alberta), J. Miller-Jones (ICRAR
		- Curtin), D. Altamirano (Southampton), C. Heinke, A. Tetarenko (Alberta),
		M. Middleton (Cambridge) on behalf of a larger XRB collaboration
Queries:	sivakoff at ualberta.ca
Posted:	29 Aug 2015; 02:54 UT
Subjects:Radio, X-ray, Black Hole, Transient

Since 2015 July 29, the black hole X-ray binary V4641 Sgr has been in outburst
(ATel #7858, #7874). Surprisingly, the source was found to be in a thermal
accretion state at a low X-ray luminosity corresponding to an Eddington
luminosity fraction below 0.6% (ATel #7904). In such a state, no compact
radio jet is expected, which was consistent with a measured radio upper
limit (ATel #7908). Here we report that Swift X-ray Telescope (XRT) observations
indicate that between August 14 and August 18 the source transitioned to
a non-thermal accretion state and its X-ray luminosity has since dropped,
reaching ~1e35 erg/s by August 22 (0.5 - 10 keV); assuming a distance of
6.2 kpc (MacDonald et al. 2014, ApJ, 784, 2). 

Swift XRT observations in either Windowed Timing (WT) or Photon Counting
(PC) mode have been occurring approximately daily between 2015 August 2
and August 13, and approximately every two days since the latter. Spectra
from these observations have been fit with both an absorbed power-law spectral
model (non-thermal accretion state) and an absorbed disk blackbody (thermal
accretion state), allowing the absorption column density, the power-law
photon index or disk blackbody temperature, and the model normalization
to vary. On and prior to August 14, the thermal model was always a better
statistical fit, with a reduced chi^2 value of 1.14 (compared to a reduced
chi^2 value of 2.4 for the non-thermal model, both with 815 degrees of
freedom over a total of 17 datasets), absorption column densities of 1.3
- 3.0 x 1e21 cm^-2, and disk temperatures of ~1.4 - 2.2 keV. By August
18, we see a transition to the non-thermal state has likely occurred, as
indicated by a reversal of how well the models fit the spectra (a reduced
chi^2 value of 1.1 for the non-thermal model and a reduced chi^2 value
of 1.7 for the thermal model, both with 81 degrees of freedom over 4 data
sets). In addition, the absorption column density fit by the thermal model
begins to drop below the expected Galactic absorption, while the absorption
column density for the non-thermal model is consistent with 2.8 - 5.0 x
1e21 cm^-2; the power-law photon index here is ~1.9 - 2.2. While the X-ray
luminosity (assuming a distance of 6.2 kpc) on August 14 was ~ 1.5e36 erg/s
(0.5 - 10 keV), it was 5e35 erg/s on August 18 and monotonically decreased
to 1.0e35erg/s on August 22; its luminosity on August 27 was 1.1e35 erg/s.

Since the non-thermal accretion state is associated with a compact radio
jet, we obtained quasi-simultaneous X-ray and radio observations on August
22 with Swift XRT (09:43 - 10:03 UT; MJD = 57256.40 - 57256.42) and the
Karl G. Jansky Very Large Array (VLA; 05:54 - 05:58 UT; MJD = 57256.246
- 57256.249). During the VLA observation, the array was in its most extended
A-configuration and we observed in the 4-8 GHz band, using the full 4 GHz
of bandwidth and achieving 4.5 minutes of on-source time. We used 3C48
to set the amplitude scale, and J1820-2528 to determine the complex gains
of the instrument. Surprisingly, V4641 Sgr was not detected, to a 3-sigma
upper limit of 42 microJy/beam. For a distance of 6.2 kpc, this corresponds
to a 3-sigma upper limit to the radio luminosity ( nu L_nu at 5 GHz) of
9.7e27 erg/s. The Swift X-ray luminosity at 1-10 keV is 7.6e34 erg/s. Based
on Figure 10 from Deller et al. 2015 (ApJ, 809, 13), V4641 Sgr is a factor
of at least 3 times lower in radio luminosity than any other known black
hole X-ray binary observed at this X-ray luminosity in the non-thermal
accretion state. 

We note that given the relatively steep power-law we fit, the non-thermal
accretion state is actually softer than the thermal accretion state in
the ~0.5 - 10 keV Band. 

We thank the NRAO and Swift teams for their rapid response and support
of these observations.