[vsnet-alert 10071] V2491 Cyg (ATEL) prior X-ray detections and discussion

[Taichi Kato]

ATEL #1478							     ATEL #1478

Title:		The persistent, spectral variable pre-nova X-ray source in
		V2491 Cyg: Swift J194302.1+321913
Author:	A. Ibarra, E. Kuulkers (ESA/ESAC, Spain), A. Beardmore, P.
		Evans (U. Leicester, UK), K. Mukai (NASA/GSFC & UMBC, USA), J.-U.
		Ness (Arizona State U., USA), M. Orio (INAF-Padova, Italy & U. of
		Wisconsin, USA), J.P. Osborne, K.L. Page (U. Leicester, UK), R. Saxton
		(ESA/ESAC, Spain), S. Starrfield (Arizona State U., USA), J. Tueller
		(NASA/GSFC, USA)
Queries:	Erik.Kuulkers at sciops.esa.int
Posted:	15 Apr 2008;  19:43  UT
Subjects:	X-ray, Gamma Ray, Binaries, Cataclysmic Variables, Novae,
		Transients, Variables

We provide further details on the V2491 Cyg pre-nova X-ray source (ATel
#1473).   The full designation of the pre-nova XRT source is Swift J194302.1+321913.

Below we list all Swift/XRT pre-nova observations, and provide the 
background corrected count rates in two bands, and the 68% confidence
Baysian rate errors   (Kraft et al. 1991, ApJ 374, 344) because of the
low numbers of counts.   The source is variable, and changes dramatically
in spectral shape between   observations on a time scale down to 4 days.
Especially noticeable is   the soft spectrum on 2007 July 21.      

<TABLE width='400'>   <TR><TD>Date</TD><TD>exposure</TD><TD>c/ks</TD><TD>c/ks</TD></TR>
<TR><TD></TD><TD>time (ks)</TD><TD>0.3-1 keV</TD><TD>1-8 keV</TD></TR>
<TR><TD>2007-05-25</TD><TD>1.06</TD><TD>1.9 +1.9/-1.3</TD><TD>20 +5/-4</TD></TR>
<TR><TD>2007-06-10</TD><TD>0.97</TD><TD>11.8 +4.2/-3.6</TD><TD>31 +7/-6</TD></TR>
<TR><TD>2007-07-17</TD><TD>3.82</TD><TD>3.5 +1.3/-1.1</TD><TD>19 +3/-3</TD>
</TR>   <TR><TD>2007-07-21</TD><TD>4.87</TD><TD>27.9 +3.2/-3.2</TD><TD>9
+2/-2</TD></TR>   <TR><TD>2008-01-02</TD><TD>5.65</TD><TD>3.9 +1.0/-1.0</TD><TD>20
+2/-2</TD></TR>   </TABLE>      

The last 3 observations provided sufficient counts to fit the spectra.
We adopted   N_H=1.5e21 atoms/cm^2 derived   from E(B-V)=0.3 (IAUC #8935)
and obtained satisfactory fits with simple power laws   (except in the
case of the softest spectrum, which appears to be more complex than a power
law).   We find photon indices of -1.4+/-0.3, -4.8+/-0.5, and -1.2+/-0.3
for the   observations performed on 2007 Jul 17, Jul 21 and 2008 Jan 2,
respectively (90% errors are quoted).   The 0.3-8 keV unabsorbed fluxes
at the 3 epochs are (1.4, 4.4, 1.5)e-12 erg/cm^2/s, respectively.   Except
for the softest spectrum, the spectra are quite hard and more like   those
seen from magnetic rather than non-magnetic cataclysmic variables in quiescence.

Swift J194302.1+321913 has a position consistent with that of the  
faint (1.71+/-0.43 e-11 erg/cm^2/s, 14-195 keV) unidentified Swift/BAT
source   Swift J1942.8+3220. However, the BAT source has a position error
(~6') which   also includes the X-ray source Swift J194245.9+322411 at
(J2000)    RA=19h42m45.9s, Dec=+32d24'10.7",   with 90% confidence error
radius of 3.6". While the latter source has an absorbed   AGN-like X-ray
spectrum, it is not bright enough to account for the BAT detection.   

The XMM-Newton slew-survey source XMMSL1 J194301.9+321911 (see ATel
#1473) has a position error   radius of 8" including systematics. This
position is consistent with the   pre-nova XRT source, but not with the
AGN-like XRT source. The 1.2+/-0.6 c/s XMM-Newton slew source   count rate
would give ~170 c/ks in the XRT assuming the mean XRT spectrum   (power-law
index=-1.4, N_H=1.5e21 atoms/cm^2), somewhat higher than the observed XRT
count rates.   We note that the XMM-Newton slew source detection significance
was less than 4 sigma.      

The ROSAT All-Sky Survey Faint Source 1RXS J194259.9+321940 (see ATel
#1473) had a PSPC count rate of   0.028+/-0.011 c/s. Assuming the mean
XRT spectrum, this corresponds to ~27 XRT c/ks, consistent with the   XRT
observations. The position of this source is also not consistent with that
of the AGN-like XRT source   reported above.      

Summarising, a variable and sometimes very soft X-ray source has been
present   at the position of V2491 Cyg from the ROSAT survey era (1990/91)
to 3 months before   the nova outburst. A harder spectral component would
be suggested by the Swift/BAT detection,   but the association with the
pre-nova is not secure.   This is only the 2nd nova (after V2487 Oph, Hernanz
& Sala 2002, Science   298, 393) to have been seen in X-rays before eruption.
Together with the optical spectral peculiarity (IAUC #8935) and the very
fast   decline (ATel #1475), these observations mark out V2491 Cyg as an
unusual nova,   worthy of further observations.    

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Password Certification:  Erik Kuulkers (ekuulker at rssd.esa.int)
http://www.astronomerstelegram.org/?read=1478