A&A 482, L1-L4 (2008)
DOI: 10.1051/0004-6361:200809456
LETTER TO THE EDITOR
A. M. Read1 - R. D. Saxton2 - M. A. P. Torres3 - P. Esquej4 - E. Kuulkers2 - P. G. Jonker5,3 - J. P. Osborne1 - M. J. Freyberg4 - P. Challis3
1 - Dept. of Physics and Astronomy, Leicester University, Leicester LE1 7RH, UK
2 - ESA/ESAC, Apartado 78, 28691 Villanueva de la Cañada, Madrid, Spain
3 - Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
4 - Max-Planck-Institut für extraterrestrische Physik, 85748 Garching, Germany
5 - SRON, Netherlands Institute for Space Research, 3584 CA, Utrecht, The Netherlands
Received 25 January 2008 / Accepted 2 March 2008
Abstract
Aims. In an attempt to catch new X-ray transients while they are still bright, the data taken by XMM-Newton as it slews between targets are being processed and cross-correlated with other X-ray observations as soon as the slew data appear in the XMM-Newton archive.
Methods. A bright source, XMMSL1 J070542.7-381442, was detected on 9 Oct. 2007 at a position where no previous X-ray source had been seen. The XMM slew data and optical data acquired with the Magellan Clay 6.5 m telescope were used to classify the new object.
Results. No XMM slew X-ray counts are detected above 1 keV and the source is seen to be 750 times brighter than the ROSAT All-Sky Survey upper limit at that position. The normally
16 star, USNO-A2.0 0450-03360039, which lies 3.5'' from the X-ray position, was seen in our Magellan data to be very much enhanced in brightness. Our optical spectrum showed emission lines that identified the source as a nova in the auroral phase; hence, this optical source is undoubtedly the progenitor of the X-ray source - a new nova (now also known as V598 Pup). The X-ray spectrum indicates that the nova was in a super-soft state (with
35 eV). We estimate the distance to the nova to be
3 kpc. Analysis of archival robotic optical survey data shows a rapid-decline light curve consistent with what is expected for a very fast nova.
Conclusions. The XMM-Newton slew data present a powerful opportunity to find new X-ray transient objects while they are still bright. Here we present the first such source discovered by the analysis of near real-time slew data.
Key words: stars: novae, cataclysmic variables - stars: individual: V598 Puppis - surveys - X-rays: general
To date, the publicly available XMM-Newton slew data cover over 25%
of the sky, while the soft band (0.2-2 keV) slew sensitivity limit
(6
10-13 erg cm-2 s-1) is close to that of the
RASS. (The hard-band [2-12 keV] limit is 4
10-12 erg cm-2 s-1.) For details of the slew data and catalogue and
the first science results, see Saxton et al. (2008) and Read et al. (2006). The near real-time comparison of XMM-Newton slew data with
ROSAT data is now giving, for the first time, the opportunity of finding
all manner of high-variability X-ray objects, e.g. tidal disruption
candidates (Esquej et al. 2007), AGN, blazars, and also Galactic
sources such as novae, flare stars, cataclysmic variables, and
eclipsing X-ray binaries. It is only with a large-area survey, such as
the XMM-Newton Slew Survey, that such rare events have a chance of
being caught - within the first slew catalogue (XMMSL1, covering
14% of the sky; Saxton et al. 2008),
40 individual slew sources are seen at fluxes >20 times greater than their corresponding ROSAT counterparts or 2
upper limits (assuming a 70 eV black body model with Galactic
). 55% of these are believed to be new X-ray
transients.
In an effort to find transient X-ray sources while they remain active,
we are now attempting to perform the slew data acquisition, analysis,
and source-searching as quickly as possible. Catalogue
cross-correlations with RASS and ROSAT pointed data fluxes and upper
limits are swiftly performed to identify highly variable X-ray
candidates. Slew datasets appear in the XMM science
archive (XSA) typically 10 days after the slew has been
performed. This systematic processing of the most recent data has been
going on since October 2006.
One such rare event, XMMSL1 J070542.7-381442, was discovered in an XMM slew from 2007 Oct. 09. Here we describe the XMM-Newton slew observations, the identification of the optical counterpart, and a spectral confirmation of the source as a nova in the auroral phase. The optical lightcurve of the source up to six months after outburst is also presented. Since the discovery, follow-up observations have been made with XMM-Newton and with Swift (Read et al. 2007b), and the X-ray flux is observed to have declined continually. The discussion of these observations is deferred to a later paper.
XMMSL1 J070542.7-381442 was discovered at an X-ray position of 07:05:42.7-38:14:42 (J2000; error radius: 8'') in slew 9143400002 from XMM revolution 1434, made in the EPIC-pn full-frame mode using the medium filter (Read et al. 2007a). Contours of the XMM-Newton slew data are shown superimposed on an optical DSS image in Fig. 1. This sky position has not yet been observed during any other XMM-Newton slew.
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Figure 1:
Contours (16.8, 33.6, 50.4 etc. cts arcsec-2) of
(4'' FWHM) Gaussian-smoothed XMM-Newton slew data (EPIC-pn; 0.2-2 keV) from
XMMSL1 J070542.7-381442 superimposed on an optical DSS image. A
slight elongation of the X-ray emission along the slew direction
(due partly to the source slew drift during an individual EPIC-pn
cycle time (73 ms) and also to the PSF shape at this off-axis
position) is evident. The progenitor, the ![]() ![]() |
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The source passed through the centres of CCDs 3 and 12 of the EPIC-pn
detector in 14 s, at a large off-axis angle (minimum
14
), such that an effective vignetting-corrected soft
band (0.2-2 keV) exposure time of 3.9 s was achieved. A total of
210 source counts lie within a radius of 20'', yielding a (EPIC-pn:
0.2-2 keV) count rate of 54.5 ct s-1, after correcting for
the encircled energy function. The high count rate indicates that the
spectrum is affected by pile-up, though the effect here, far off-axis,
is less than on-axis (the on-axis limit is 6 ct s-1 for EPIC-pn
full-frame mode
). Some X-ray loading (Smith 2004), where events
below the cut-off threshold sum together to produce artificial
accepted events, is also present. These effects work to distort the
spectrum and make quantitative spectral analysis difficult. One can
minimize these effects by ignoring the central part of the PSF, and we therefore extracted a spectrum of the source
from within an annulus of 5''-60'' radius, centred on the
source position. Just single events were selected, and these were
spectrally grouped to give a minimum of 20 counts per bin. Slew data
also still has unresolved problems associated with the motion of the
source across the detector, so that approximations currently have to be made
when calculating the associated effective area and detector response
matrix files. To perform a qualitative spectral analysis, an effective
area file, accounting for the removal of the piled-up core, was
generated by averaging the individual core-removed effective area
files at 9 different positions along the detector track made by the
source. This takes the variations in the vignetting and the PSF into
account to a good approximation. Individual BACKSCAL values have been
set by hand, as have the EXPOSURE values, estimated by calculating the
distance travelled by the source in detector coordinates and finding
the time taken to do this given a 90 deg h-1 slew speed, then
subtracting the appropriate fractions for chip gaps and bad pixels
(calculating the exposure time from the source lightcurve, gives the
same value to within a few tenths of a second). For the response
matrix, we used the equivalent canned detector response matrix for the
vignetting-weighted average source position, for single events and for
Full Frame mode: epn_ff20_sY6_v6.9.rmf. A background spectrum was
extracted from a much larger region close to the source and at a
similar off-axis angle.
Simple power-law, blackbody, thermal Bremmstrahlung, and optically thin
hot plasma models are unable to fit the spectrum adequately (all have
a
12 for 6 degrees of freedom). Given that the source
is identified as a nova (Sect. 3), a more physically realistic white
dwarf atmosphere model of the type used to model the nova V1974 Cyg
(Balman et al. 1998) was used, yielding an acceptable fit (reduced
1.4 for 6 degrees of freedom), an effective
temperature of 35
+2-1 eV, and an
of
4.8
+5.7-2.7
1020 cm-2 (errors 90% for one
interesting parameter). The spectrum and model fit are shown in
Fig. 2. The model normalization, corrected for the
removal of the saturated PSF core, can be used to derive a distance to
the source (assuming the emission to be sub-Eddington, and a typical
emitting region of spherical radius 109 cm) of
4.3
2.3 kpc. A PIMMS
v3.9d conversion of the RASS 0.0076 ct s-1 2
upper limit
corresponds to an EPIC-pn limit
750 times less than what is observed of
0.073 ct s-1, assuming the same spectral model.
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Figure 2: The XMM-Newton slew spectrum of XMMSL1 J070542.7-381442, fit with a white dwarf atmosphere model of effective temperature 35 eV. The data and model are shown in the upper panel and the deviations of the data from the model are shown in the lower panel. |
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The field containing XMMSL1 J070542.7-381442 was observed on 16 Nov. 2007 with the Magellan Clay 6.5 m telescope at Las Campanas Observatory. A bright, saturated optical counterpart was found with a position consistent with the
16 point-like source
USNO-A2.0 0450-03360039 (07:05:42.5
-38:14:39; J2000), 3.5'' from the slew position. On the basis of the positional coincidences, the USNO object was proposed as a possible progenitor of the nova (Read et al. 2007a; Torres et al. 2007). This claim was confirmed by the detection of the radio counterpart to the slew source, which has a high-precision position consistent with the USNO object (Rupen et al. 2007).
Spectroscopic observations of the optical counterpart were obtained
with the Low Dispersion Survey Spectrograph (LDSS-3), equipped with
the 400 line mm-1 VPH ALL grism and a mask with a 1'' width
long-slit cut near the centre of the field of view. The detector was
the STA0500A 4k
4k unbinned CCD. This setup allowed us to cover
the spectral range 3510-10 620 Å with a dispersion of
2.0 Å pix-1 and a resolution of
8 Å FWHM. Several
exposures (2
1 s, 1
2 s, 1
5 s, and
1
60 s) were obtained. These spectra were affected by
2nd-order light contamination beyond
7000 Å. To
obtain useful coverage in the red, we also acquired 1 s, 5 s, and
60 s spectra with the OG590 order-blocking filter, giving useful
5800-10 620 Å wavelength coverage. The frames were reduced using
standard routines in IRAF. The spectra were then extracted and
wavelength-calibrated with the help of HeNeAr calibration lamp
spectra. The instrumental response was corrected using spectroscopic
standard stars observed with and without the blocking filter. The
shape of the continuum is reliable except at the blue and red ends.
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Figure 3:
Blue ( top) and red ( bottom) spectra of the optical counterpart to
XMMSL1 J070542.7-381442 acquired on 16/11/07. The spectra have been
normalized to unity at ![]() ![]() |
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Blue and red spectra, obtained 163 days after the maximum optical brightness reported
by Pojmanski et al. (2007) (see Sect. 4), are shown in Fig. 3. The data show emission lines characteristic of a late post-outburst nova spectrum. The
strongest lines in the blue spectrum are [O III]
followed by H
blended with [N II]
,
[O III]
(blended with H
), [N
II]
,
and H
.
The strongest emission feature in
the red spectrum is due to [O II]
-7330auroral transitions. The emission lines have an average FWHM of 2070
50 km s-1, as derived from isolated line profiles and
correcting for the instrumental broadening. The H
line is stronger than
He II
.
This characteristic, together with the
presence of strong forbidden [O III], [N II] and [O II] auroral lines, and weak Ne lines, suggests that this source was an Fe II nova (see e.g. Williams 1992). However, the data were acquired too long after maximum light to make an accurate
classification possible. The nova was observed in the A0 auroral
phase according to the Tololo classification (Williams et al. 1991, 1994) - any forbidden auroral transition at wavelengths
-7600 has a larger flux than
the strongest non-Balmer permitted lines, and [O III]
is the strongest auroral transition. The coronal [Fe X]
line (if present) is weaker than [Fe
VII]
,
excluding the possibility of a coronal stage.
Analysis of archival robotic optical survey data from 3-min CCD exposures (pixel size 14
8), obtained with a 70 mm (200 mm focal length) f/2.8 telephoto lens in the course of the All Sky
Automated Survey (Pojmanski 2002) show that the visual magnitude of
this source rose from
14 to
4.1 between 2 Jun. (23:27 UT) and 5 Jun. (23:13 UT) 2007 and has declined since (see Fig. 4). The source was seen to be saturated in the June 5 observation, and it is thought that the source may have been
0.1-0.5 mag brighter. The decline from outburst (
2.4 mag in 12 days, then a further 2.8 mag in 62 days) indicates that this is a nova of the very fast speed class (Warner
1995). We estimate t2, the time that the light curve takes to
decline 2 mag below maximum brightness as 9
1 days. The later decay rate (mid-August onwards) is 1.36
0.04 mag per 100 days (including errors to obtain a reduced
of
1
for the fit).
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Figure 4: V-band magnitudes from Pojmanski et al. (2007) of the optical counterpart to XMMSL1 J070542.7-381442. The 2007 dates of the XMM-Newton slew and Magellan observations are marked. |
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As a confirmed classical nova, XMMSL1 J070542.7-381442 has been given
the name V598 Puppis (Samus 2007). With a peak
4, it is one of the brightest optical novae seen for many years. An
4 nova has been discovered in eruption every
8 years or so (Warner 1995), and the only novae discovered in recent decades of comparable peak optical magnitude are V382 Vel (
)
and V1494 Aql (
), both discovered in 1999, and V1280 Sco (
),
discovered in Feb. 2007.
Classical novae are usually discovered optically in the early phases
of the outburst, since they are intrinsically optically bright and
easily found in inexpensive wide-area shallow surveys. Unusually the
optical outburst went unnoticed in this case, and V598 Pup was only
discovered in X-rays during the later (100 days after outburst),
optically thin nebular phase, when classical novae are typically
observed as soft X-ray sources.
On the basis of the optical spectrum, this nova was observed in the
A0 auroral phase, and was likely a very fast, Fe II nova
(Sect. 3 and Williams et al. 1991, 1994), though an
accurate classification is no longer possible, this late after maximum brightness.
The soft (
= 35 eV) X-ray spectrum
indicates that the nova was in a super-soft state (Pietsch et al. 2007). Such a state stems from nuclear burning on the white dwarf (Ness
et al. 2007). Measurement of its intensity, duration, and temperature
can constrain the distance to the nova and the mass of the white dwarf
(e.g. Balman et al. 1998; Lanz et al. 2005). From the slew measurement
of V598 Pup, we see that the delay from the outburst (2-5 Jun. 2007) to the onset of the X-ray super-soft state is
127 days. This is short when compared with the
200 days
seen in V1974 Cyg (Krautter et al. 1996),
6 months of V382 Vel
(Orio et al. 2002), and 6-8 months of V1494 Aql (Drake et al. 2003).
From the Galactic latitude and the fact that the Galactic scale height
of white dwarfs is <500 pc (conservatively, Nelson et al. 2002), an
upper limit to the distance of V598 Pup of 2.1 kpc can be
derived, consistent with the 4.3
2.3 kpc estimated from the
X-ray spectral fitting. Another way to estimate the distance is to use
the relation between absolute magnitude at maximum brightness and t2(see e.g. Eq. (5.2) in Warner 1995).
Using t2 = 9
1 days, we estimate
= -8.4
0.4. With
= 0.27
+0.31-0.15 (90% error), as derived (Predehl & Schmitt 1995)
from
= 4.8
+5.7-2.7
1020 cm-2,
= -8.4
0.4, and peak
= 4.1, we derive a distance of
2.8
+0.8-0.5 kpc. An absolute magnitude of
= -8.4 would imply a
peak luminosity
7 times the Eddington luminosity for a 1
white dwarf. This is quite typical of novae. The source had, at the time of the slew detection, an absorbed (0.2-2 keV) X-ray flux of 1.54
+0.08-0.23
10-10 erg cm-2 s-1, corresponding to a (0.2-2 keV) X-ray luminosity of 1.6
+0.1-0.2
1035 erg s-1, assuming a distance of 3 kpc, and a bolometric
luminosity of 2.4
+6.4-1.7
1036 erg s-1 (errors
calculated at the boundary of the 90% region for two interesting
parameters;
and
). This is at the lower end of
the luminosities discussed e.g. in Orio et al. (2002) and Ness et al. (2007).
Acknowledgements
This research made use of the VIZIER database, operated at the CDS, Strasbourg, France. The XMM-Newton project is an ESA Science Mission with instruments and contributions directly funded by ESA Member States and the USA (NASA). We thank K. L. Page & M. Modjaz for useful discussions, and the referee for useful comments that have improved the paper. A.M.R. & J.P.O. acknowledge the funding support of PPARC/STFC, PE of MPE, and PGJ of the Netherlands Organisation for Scientific Research.