[vsnet-alert 18938] Re: V4641 Sgr active (ATEL)

[Josch Hambsch]

Hi,

I have observed V4641 Sgr remotely in time series with CCD and V filter.
Using the AAVSO sequence comp stars 116 (ref) and 124 (check), I find a
magnitude range of 13.5 V to 13.8 V.

Data have been sent to T. Kato for further analysis. I will continue
monitoring this star. 
Regards,

Josch Hambsch

-----Original Message-----
From: vsnet-alert [mailto:vsnet-alert-bounces at ooruri.kusastro.kyoto-u.ac.jp]
On Behalf Of Taichi Kato
Sent: Mittwoch, 5. August 2015 02:26
To: variable_star_forum at yahoogroups.com;
vsnet-alert at ooruri.kusastro.kyoto-u.ac.jp; vsnet-alert at yahoogroups.com;
vsnet-campaign-xray at ooruri.kusastro.kyoto-u.ac.jp
Subject: [vsnet-alert 18937] V4641 Sgr active (ATEL)

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. 


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