Issue |
A&A
Volume 506, Number 3, November II 2009
|
|
---|---|---|
Page(s) | 1309 - 1317 | |
Section | Stellar structure and evolution | |
DOI | https://doi.org/10.1051/0004-6361/200912082 | |
Published online | 27 August 2009 |
A&A 506, 1309-1317 (2009)
XMMSL1 J060636.2-694933: an XMM-Newton slew discovery and Swift/Magellan follow up of a new classical nova in the LMC
A. M. Read1 - R. D. Saxton2 - P. G. Jonker3,4 - E. Kuulkers2 - P. Esquej1 - G. Pojmanski5 - M. A. P. Torres4 - M. R. Goad1 - M. J. Freyberg6 - M. Modjaz7
1 - Dept. of Physics and Astronomy, Leicester University, Leicester LE1 7RH, UK
2 -
XMM-Newton SOC, ESAC, Apartado 78, 28691 Villanueva de la Cañada, Madrid, Spain
3 -
SRON, Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA, Utrecht, The Netherlands
4 -
Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
5 -
Warsaw University Observatory, A1. Ujazdowskie 4, 00-478, Warsaw, Poland
6 -
Max-Planck-Institut für extraterrestrische Physik, 85748 Garching, Germany
7 -
University of California, 601 Campbell Hall, Berkeley, CA 94720, USA
Received 16 March 2009 / Accepted 24 August 2009
Abstract
Aims. In order to discover new X-ray transients, the data taken
by XMM-Newton as it slews between targets are being processed and
cross-correlated with other X-ray observations.
Methods. A bright source, XMMSL1 J060636.2-694933, was
detected on 18 July 2006 at a position where no previous X-ray source
had been seen. The XMM-Newton slew data, plus follow-up dedicated
XMM-Newton and Swift observations, plus optical data acquired with the
Magellan Clay telescope, and archival All-Sky Automated Survey (ASAS)
data were used to classify the new object, and to investigate its
properties.
Results. No XMM-Newton slew X-ray counts are detected above
1 keV and the source is seen to be over five hundred times
brighter than the ROSAT All-Sky Survey upper limit at that position.
The line-rich optical spectrum acquired with the Magellan telescope
allows the object to be classified as an A0 auroral phase
nova, and the soft X-ray spectrum indicates that the nova was in a
super-soft source state in the X-ray decline seen in the follow-up
X-ray observations. The archival ASAS data suggests that the nova at
onset (Oct 2005) was a ``very fast'' nova, and an estimate of its
distance is consistent with the nova being situated within the LMC.
Conclusions. With the discovery presented here of a new
classical nova in the LMC, it is clear that XMM-Newton slew data are
continuing to offer a powerful opportunity to find new X-ray transient
objects.
Key words: stars: novae, cataclysmic variables - stars: individual: XMMSL1 J060636.2-694933 - surveys - X-rays: general
1 Introduction
The publicly available XMM-Newton slew data covers to date around 35%
of the sky. The soft band (0.2-2 keV) sensitivity limit of the slews
(
erg cm-2 s-1) is close to that of the
ROSAT All-Sky Survey (RASS; Voges et al. 1999), and in the medium
(2-12 keV) band, the slew data goes significantly deeper
(
erg cm-2 s-1) than all other previous
large area surveys. Over 7700 individual sources have so far been
detected to a positional accuracy of 8
.
For details on
the construction and
characteristics of the first released XMM-Newton slew survey
catalogue, see Saxton et al. (2008). For details of the initial
science results from the slew survey, see Read et al. (2006).
The comparison of XMM-Newton slew data with the RASS is now giving, for the first time, the opportunity to find exotic, extreme high-variability X-ray bursting objects, e.g. tidal disruption candidates (Esquej et al. 2007), and also Galactic novae, flare stars, and flaring white dwarfs, plus eclipsing binaries, AGN and blazars. It is only with such a large-area survey as the XMM-Newton Slew Survey, that transient events as these have a chance of being caught.
One such rare event, XMMSL1 J060636.2-694933, which we here show to be a new Classical Nova, was discovered in an XMM-Newton slew from 18th July 2006 at a very high count rate of 23.3 ct s-1 (EPIC-pn: 0.2-2 keV).
Table 1: Details of the four XMM-Newton slew observations and the single (Rev. 1378) dedicated XMM-Newton pointed observation.
Classical novae (see Bode & Evans 2008, for a review) occur in interacting binary systems consisting of a white dwarf primary star and a lower-mass secondary star. The nova itself is a cataclysmic nuclear explosion caused by the accretion of material (via Roche Lobe overflow or wind accretion) from the secondary star onto the surface of the white dwarf; here the pressure and temperature at the base of the accreted material becomes sufficient to trigger a thermonuclear runaway. A recent review of the thermonuclear processes powering classical novae can be found in Starrfield et al. (2008). The accreted material is partially expelled, obscuring the X-ray emission from the surface of the white dwarf. At later stages, the ejected material expands further and becomes optically thin, revealing the nuclear burning on the surface of the white dwarf. This emission peaks in the soft X-ray regime and it is known as the super-soft source (SSS) state (Krautter 2008). Models of the classical nova SSS state can be found in Tuchman & Truran (1998) and Sala & Hernanz (2005).
Though many classical novae have been observed in X-rays in their SSS states (Ness et al. (2007) for example discuss several examples observed with Swift), it is in the optical band, early in their outbursts, that classical novae are almost always discovered. This is because they are intrinsically optically bright and easily found in inexpensive wide-area shallow surveys. XMMSL1 J060636.2-694933 is very unusual therefore in that it has been discovered, as we shall see, later in its evolution, in the SSS X-ray state.
In this paper we describe the XMM-Newton slew observations (Sect. 2), and the follow-up X-ray observations by the Swift XRT (Sect. 3) and XMM-Newton (Sect. 4). Multiwavelength observations with Swift-UVOT, Magellan and ASAS are described in Sect. 5. We then present a discussion of the results (Sect. 6), and conclusions.
2 XMM-Newton slew observations
XMMSL1 J060636.2-694933 was discovered in XMM-Newton slew 9121000003 from revolution 1210 on 18th July 2006. Details of the standard XMM-Newton slew data reduction and analysis used, plus the source-searching and catalogue cross-correlation etc., are presented in Saxton et al. (2008).
The source passed through the EPIC-pn detector in 14 s, at a small
off-axis angle, such that an effective vignetting-corrected soft band
(0.2-2 keV) exposure time of 9.8 s was achieved. A total of 229
source counts lie within a radius of 20
,
yielding a (EPIC-pn:
0.2-2 keV) count rate of 23.4 ct s-1.
The source is seen to have no cross-correlation identifications in the
RASS, and no other multiwavelength candidates within 30
in
Simbad
,
NED
, and
HEASARC
. The position of the
source in the sky is such that it lies apparently at the outer eastern
edge of the LMC.
XMM-Newton has slewed over this region of sky a number of times, and
though nothing was detected in previous slews from 7th November 2001
and 12th January 2004, the source was seen again on 28th September
2006 (rev. 1246, 72 days after the rev. 1210 discovery), at the same
position, but at a reduced flux level (3.8 ct s-1; EPIC-pn:
0.2-2 keV). i.e. it had reduced in flux by a factor of 6
in 72 days. XMM-Newton has not slewed over this area of sky since
rev. 1246. Details of the relevant XMM-Newton slews, together with
the (0.2-2 keV) EPIC-pn source position, detected source counts,
count rate and detection likelihood are given in
Table 1.
The fact that XMMSL1 J060636.2-694933 is detected in the total-band (0.2-12 keV) and the soft-band (0.2-2 keV), whilst effectively zero counts are seen in the hard-band (2-12 keV), is immediately indicative of the source being very soft.
The moderately high count rate indicates that the spectrum is affected
by pile-up (the on-axis limit is 6 ct s-1 for EPIC-pn full-frame
mode). This distorts the spectrum and makes
quantitative spectral analysis of the slew data difficult. We
minimized these effects by following the standard procedure, i.e.
ignoring the central part of the Point Spread Function (PSF), and
extracted an event spectrum (containing single and double events) of
the source from within an annulus of 5
-30
radius,
centred on the source position. Unresolved problems associated with
the motion of sources across the detector still exist within slew
data, and approximations currently have to be made when calculating
the associated effective area and detector response matrix files. In
order to perform qualitative spectral analysis, an effective area file
was generated by averaging the individual core-removed effective area
files at 9 different positions along the detector track made by the
source. This accounts for the removal of the piled-up core, and takes
the vignetting and PSF variations 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.
For the response matrix, we used the equivalent canned detector
response matrix for the vignetting-weighted average source position,
for single plus double events and for full-frame mode:
epn_ff20_sdY6_v6.9.rmf. A background spectrum was extracted from a
much larger circular region close to the source and at a similar
off-axis angle.
To fit the slew spectral data, and indeed all the high-energy spectra
in the present paper, the
XSPEC
spectral fitting package has been used. As
minimization is
not valid when fitting spectra of low statistical quality, for the
fitting of the slew spectrum (and all the spectral fitting in the
present paper), C-statistics have been used. To take into account the
absorbing column along the line of sight, the wabs model with
the wilm cosmic abundance table (Wilms et al. 2000) has been
used throughout the paper. All the errors quoted in the present paper
are 90% confidence intervals, unless otherwise stated.
The rev. 1210 slew spectrum shows that the source is very soft, and
appears consistent with a 63
-10+12 eV black body, absorbed by
a hydrogen column density of
cm-2. The fit is good, with a
P-statistic value of 0.11, obtained via the XSPEC goodness
command for this fit, based on 5000 random simulations. The best-fit
hydrogen column is equal to the full Galactic hydrogen column in the
direction of the source (
cm-2; Dickey
& Lockman 1990, calculated via the FTOOL nh
).
The slew spectrum, plus the best fit simple black body model and the
deviations from the model, are shown in Fig. 1. The
observed count rate corresponds to a (0.2-2 keV) flux, corrected
for the removal of the saturated PSF core, of
erg cm-2 s-1 (an
increase in flux over the RASS upper limit, assuming the same spectral
model, by a factor of more than 500).
Simple power-law, thermal Bremmstrahlung, and other optically thin hot
plasma models are unable to fit the spectrum adequately well. Given
that we later are able to identify the source as a nova (Sect. 5.2),
then the black-body model will likely be a good approximation.
Furthermore, as we have obtained here a moderate number of slew
counts, the more physically realistic, though more complex atmosphere
model for CO white dwarfs of MacDonald & Vennes (1991), provided by
K. Page (private communication), was attempted. This model, used
e.g. to model the nova V1974 Cyg (Balman et al. 1998), yielded a
marginal fit (and not formally a more statistically significant fit;
P-statistic = 0.03, based on 5000 random simulations), with an
effective temperature of 70
+8-6 eV, an
of
cm-2, and a PSF-corrected
(0.2-2 keV) flux of 4.5
erg cm-2 s-1. Note that a smaller
(though perhaps
still consistent with the full Galactic hydrogen column) is now
obtained using the white dwarf atmosphere model. (Note that the
MacDonald & Vennes 1991 ONe white dwarf atmosphere model was also
attempted, but yielded a marginally worse fit than the CO white dwarf
atmosphere model; only the CO atmosphere model has been used in the
subsequent analysis.)
![]() |
Figure 1: XMM-Newton Slew spectrum of XMMSL1 J060636.2-694933 from XMM-Newton revolution 1210. The data points (crosses; adjacent data bins having been grouped together for the plot to have a significance of at least 3) have been fitted with a black body model (kT=63 eV; see text). The solid line shows the best fit to the spectrum. The ratio of the data to the best fit model is shown in the lower panel. |
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It is well known (e.g. Krautter et al. 1996) that, because of the
energy-dependent opacity in the white dwarf atmosphere, fits to super
soft source novae spectra with black body models give larger fluxes
and lower temperatures than atmosphere models fit to the same spectra,
and this is seen in the present case. Thus the black body model
requires a larger
to fit the same data than the atmosphere
model, as is seen.
The model normalizations, corrected for the removal
of the saturated PSF core, can be used to derive an approximate
distance to the source. If we assume a typical emitting region for
the white dwarf atmosphere to be of spherical radius 109 cm,
then, for the black body model, this distance turns out to be
20
+31-10 kpc. The effects discussed above however can lead to
usage of the black body model giving rise to an underestimation of the
distance. For the white dwarf atmosphere model, a larger distance of
71
+27-23 kpc is obtained. Both estimates are consistent with
the distance to the LMC (50 kpc, see Sect. 6), and assuming a
distance of 50 kpc, the black body derived flux corresponds to a
(pile-up corrected) 0.2-2 keV X-ray luminosity of
1.4
erg s-1.
3 Swift XRT X-ray observations
We requested and received a prompt observation with Swift of this
source before it moved out of the Swift visibility window in April
2007. We received over 14 ks of Swift-XRT time in 7
separate observations and the details of these observations are listed
in Table 2. All of the observations were in photon
counting mode and none of the observations showed any times of
significant high-BG flux. In none of the observations did the source
position coincide with any of the dead (micrometeorite-induced)
detector columns. The analysis has been performed using HEASOFT
v6.1.2. The individual XRT observations were astrometrically-corrected
and then stacked to ascertain a best Swift-XRT position - this was
found to be 06 06 37.00 -69 49 33.9 (with a 90% error radius of
4.0
). Source counts were then extracted from each observation
from a circle of radius of 40
at this position. Background
counts were extracted from each observation from large-radius
off-source circles close to the source position. Source counts and
count rates for the individual XRT observations are given in
Table 2.
Table 2: Details of the Swift-XRT observations (observation ID, observation date and cleaned exposure time) are tabulated, together with the total (0.2-2.0 keV) background-subtracted counts and count rate from XMMSL1 J060636.2-694933 (see text).
The observation naturally fell into three time-separated groups, those
of obs. 1, obs. 2-5 and obs. 6-7. A similar analysis applied to
these groups (where the statistics are improved) gives rise to source
counts and count rates of
counts and
ct s-1 (for obs. 2-5), and
counts and
ct s-1 (for
obs. 6-7). (Analysis of all the data together yields
counts and
ct s-1).
A spectrum was extracted from all the Swift-XRT data from a 40
radius circle, using grades 0-12, centred on the Swift-XRT position.
A background spectrum was extracted again from all the Swift-XRT data,
from large-radius off-source circles close to the source position. An
ARF file was created using xrtmkarf and the appropriate RMF
(swxpc0to12_20010101v008.rmf) from the Swift-XRT Calibration Database
was obtained.
Standard spectral models were again fit to the spectral data using
XSPEC. Again, C-statistics were used, as was the wabs absorption
model with the wilm cosmic abundance table. It was again
obvious that only a very soft spectrum would be appropriate for the
data, and the only simple model that was able to fit the data
adequately was a black-body model of temperature
kT=59+14-10 eV, with an absorbing hydrogen column of
cm-2. No sufficiently constrained parameters could
be obtained using the CO white dwarf atmosphere model (MacDonald &
Vennes 1991). The Swift-XRT spectrum, together with the best-fit black
body model is shown in Fig. 2. The corresponding
(0.2-2.0 keV) flux is 2.7
erg cm-2 s-1 (i.e. a reduction by more than a factor 100 from
the XMM-Newton slew discovery flux), and the X-ray luminosity, for the
assumed distance of 50 kpc, is
erg s-1.
![]() |
Figure 2: Swift-XRT spectrum from XMMSL1 J060636.2-694933. The data points (crosses; adjacent data bins having been grouped together for the plot to have a significance of at least 3) have been fitted with a black body model (kT=59 eV; see text). The source has faded by a factor of >100 since the XMM-Newton revolution 1210 slew discovery. The solid line show the best fit to the spectrum. The ratio of the data to the best fit model is shown in the lower panel. |
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A cautious estimate of the size of the emitting region can be obtained
from the model normalization; the assumed distance of 50 kpc yields a
maximum radius of
cm (the fit normalization is
essentially unconstrained at the lower bound). Though great care
should be taken in interpreting this result, as the black body model
is possibly overestimating the luminosity, this obtained radius is
still consistent with that of moderately massive (>1.1
)
white dwarfs (Hamada & Salpeter 1961), i.e. the whole white dwarf
surface may still be emitting at 59 eV.
4 Dedicated XMM-Newton observations
We were granted an XMM-Newton Target of Opportunity (ToO) observation,
once the source became again visible to XMM-Newton, and a 10 ks
XMM-Newton EPIC observation was made on 19th June 2007 (see
Table 1). All the XMM-Newton EPIC data, i.e. the data
from the two MOS cameras and the single pn camera, were taken in
full-frame mode with the thin filter in place. These data from the
three EPIC instruments have been reprocessed using the standard
procedures in XMM-Newton SAS (Science Analysis System) - v.7.1.0.
Periods of high-background, of which there were very few, were
filtered out of each dataset by creating a high-energy 10-15 keV
lightcurve of single events over the entire field of view, and
selecting times when this lightcurve peaked above 0.75 ct s-1(for pn) or 0.25 ct s-1 (for MOS). This resulted in
9.4(8.0) ks of low-background MOS(pn) data. Details of this dedicated
XMM-Newton observation, together with source position, and
(0.2-2 keV) all-EPIC combined (pn, MOS1, MOS2) detected source
counts, count rate and detection likelihood are given in
Table 1.
Source spectra, containing single and double events, were extracted
from the datasets from circles (none of the data were now piled up)
centred on the source position. An extraction radius, estimated from
where the radial surface brightness profile was seen to fall to the
surrounding background level, was set to 30
.
Background spectra
were extracted from each cleaned dataset from a 40
-80
annulus centred on the source position. Point sources seen to
contaminate these larger-area background spectra were removed from the
background spectra to a radius of 60
.
ARF files were created
for the source spectra, and were checked to confirm that the correct
extraction area calculations had been performed. Finally RMF response
files were generated.
Standard spectral models were again fit to the spectral data using
XSPEC. Once again it was obvious that only a very soft model would fit the data; the only
simple model that was able to fit the data well (a P-statistic = 0.17,
based on 5000 random simulations) was a black-body model of
temperature
kT=70+3-4 eV, with an absorbing hydrogen column
of 6.9
cm-2. The spectrum, together with this best-fit
model are shown in Fig. 3. The corresponding
(0.2-2.0 keV) flux is only marginally less than the Swift-XRT value
at 2.2
erg cm-2 s-1 and the
X-ray luminosity (for the assumed distance of 50 kpc) is
6.7
erg s-1.
![]() |
Figure 3: XMM-Newton ToO spectrum from XMMSL1 J060636.2-694933. The data points (crosses; adjacent data bins having been grouped together for the plot to have a significance of at least 3)) have been fitted again with a black body model (kT=70 eV) (see text). EPIC-pn data is shown in black, with EPIC-MOS1 in red and EPIC-MOS2 in green. The solid lines show the best fit to the spectra. The ratios of the data to the best fit model are shown in the lower panel. |
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Given that, in this XMM-Newton ToO observation, we had obtained a
larger number of counts (
1500 over the 3 EPIC cameras), the
physically more realistic CO white dwarf atmosphere model (MacDonald &
Vennes 1991) was also attempted. This yielded a marginal fit (and formally
a no more statistically significant fit; P-statistic = 0.04, based on
5000 random simulations), with an effective temperature of
73
+3-2 eV, and an
of
3.4
cm-2. Again, usage of the black body model results
in a larger fitted
and a lower fitted temperature than
with the atmosphere model.
As before, the model normalization can be used to obtain a cautious
estimate of the size of the emitting region. For the assumed distance
of 50 kpc, then the black body model returns an emitting region
radius of only
cm. Again care should be
taken, as this may be an overestimation, the black body model having
perhaps overestimated the luminosity. For the white dwarf atmosphere
model, a smaller radius of
cm is
obtained. Note further that the assumption of a larger distance (see
Sect. 6) would result in a proportionally larger emitting radius.
The range in allowed radius therefore is quite large, and it is not
impossible for for the whole of the white dwarf surface to be emitting
at 70 eV. If this is the case, then the white dwarf would have to be
at the high end of the mass range (>1.2
;
Hamada &
Salpeter 1961). It may be the case then that we are at this point at,
or close to the end of the SSS phase, where the effective temperature
has reached a maximum (Sala & Hernanz 2005), as is tentatively seen
in the spectral fitting results, and where the photospheric radius has
reached a minimum, close to the white dwarf radius.
4.1 X-ray variability
The full (XMM-Newton slew plus Swift-XRT plus XMM-Newton ToO) X-ray
lightcurve of XMMSL1 J060636.2-694933 is shown in
Fig. 4. The calculated (0.2-2.0 keV) flux values
are shown plotted against the number of days since the rev. 1210
XMM-Newton Slew discovery. The first two data points are the
rev. 1210 and the rev. 1246 XMM-Newton Slew observations. Then the
three nested Swift-XRT points are shown and finally the XMM-Newton ToO
observation. The level of RASS upper limit is shown to the bottom
left. The (0.2-2.0 keV) X-ray flux is seen to have dropped by more
than two orders of magnitude in 230 days since the discovery, but is
then seen to have levelled off for the next 120 days, at a level still
3 times that of the RASS. Finally, no evidence for any
short-term variability (using time bins down to 100 s) is seen in the
highest statistic continuous X-ray lightcurve (the
8.0 ks
background-filtered EPIC-pn lightcurve) obtained from the 19/06/07
XMM-Newton observation.
![]() |
Figure 4: The full X-ray lightcurve of XMMSL1 J060636.2-694933. Plotted are the calculated (0.2-2.0 keV) flux values versus time. The first point is the rev. 1210 XMM-Newton Slew observation, then the rev. 1246 XMM-Newton Slew observation. The three nested Swift-XRT points are shown next and finally the XMM-Newton ToO observation. The RASS upper limit is shown bottom left. |
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5 Multi-wavelength follow-up
5.1 Swift UVOT
For the Feb/Mar 2007 Swift observations, we arranged for both the Swift UVOT-B filter and the UVOT-UVW2 filters to be used in an approximate exposure time ratio of 1:5, thus ensuring roughly equal numbers of counts in the two bands (though there is a spectral type dependency here). Swift UVOT images in these two filters of the area of sky around XMMSL1 J060636.2-694933 are shown in Fig. 5.
Prior to the Swift UVOT observations, a `best-guess' to the possible
candidate optical/IR counterpart would have been the USNO-A2.0 source
0150-04066298 (B mag: 17.4, R mag: 16.1), seen 4
south of the
XMM-Newton slew position. The UVOT images however immediately showed
that the optically fainter source at position RA, Dec (J2000) = 06 06
36.4, -69 49 34.3 (error radius: 0.5
)
was a very strong UVW2
source and very blue, and was very likely the true counterpart to
XMMSL1 J060636.2-694933. (The UVW2 filter spans approximately
800 Å, centred at
1900 Å)
![]() |
Figure 5:
Swift UVOT images of the field around XMMSL1 J060636.2-694933 from observation
00030895002. Left shows the UVOT B-filter and right shows the
UVOT UVW2-filter. The large circle is a 20
|
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The Swift UVOT pipeline processed data were analysed using the UVOT
photometry package uvotsource released with
FTOOLs.
This package performs aperture photometry on pre-specified source and
background regions, accounting for photometric- (via PSF fitting) and
coincidence loss- effects using the UVOT calibration files. Source
counts were extracted using a 5
radius aperture centred on the
source, while for the background we used a 10
radius aperture
located in a nearby source-free region. We used a larger background
aperture to effectively smooth over the modulo-8 fixed pattern noise
present in UVOT observations and to improve the statistics of the
background counts. Source counts were converted to UVOT UV-magnitudes
using the UVW2 zero-point calibration released with version 2.8 (Build
22) of the CALDB. The source is seen (see Fig. 6) to be
roughly constant over the short duration of the Swift observations,
with a suggestion of a decline towards the end. This is in keeping
with the general form of the X-ray lightcurve (Fig. 4)
at this time.
![]() |
Figure 6:
Variation of the UVW2 magnitude of the bright UV source
during the Swift observations. The same time axis as
Fig. 4 has been used to aid comparison, and a zoom
is also shown. The UVW2 filter was only employed during observations
00030895002, 00030895004, 00030895005, 00030895006 & 00030895007
(hence the points span the dates 07/03/07 to 22/03/07). The errors here are 1- |
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It is possible to include the UVOT-detected flux with the XRT spectrum
described in Sect. 3. UVOT files, created using uvot2pha for
the five observations (00030895002, 00030895004, 00030895005,
00030895006 & 00030895007) where the UVW2 filter was employed, were
incorporated into xspec, along with the appropriate response
file (swuw2_20041120v104.rsp) from the Swift-XRT Calibration
Database. We attempted to fit a single black-body spectrum to the
Swift-XRT+UV data (again using C-statistics, the wabs absorption
model and the wilm cosmic abundance table, plus the inclusion of
the xspec-redden component to model the absorption in the UV
band). The best fit however, with a much lower temperature of
kT = 36+3-4 eV, is a very poor fit to the data; we obtain a
goodness P-statistic value of 0.00, based on 5000 random
simulations. This notwithstanding, a flux in the UVW2
(1.57-7.77 eV) band of
erg cm-2s-1 can be obtained, corresponding to a UVW2 luminosity, for the
assumed distance of 50 kpc, of
erg s-1.
The very poor single black-body fit above, plus the large change in fitted temperature is strongly suggestive that a model other than, or in addition to the XRT-derived kT=59 eV black body model (Sect. 3) should be used to describe the UVW2 data. As we have no UV data other than in the UVW2 filter, all that can be done is to apply the XRT-derived black body model to the UVW2+XRT data, and in doing this, a large flux excess with respect to the XRT-derived black body model is seen in the UVW2 band. This is shown in Fig. 7. This excess in UV emission (most of the 1035 erg s-1 discussed above) is likely due to a combination of residual post-nova nuclear burning on the surface of the white dwarf, plus accretion in the disk, including from emission lines. The situation is likely to be rather complex, depending on the structure of both the ejecta and the accretion disk, and is beyond the scope of the present work, where we only have sparse UV data. For a review of the UV emission from classical novae, see Shore (2008).
![]() |
Figure 7: Swift-XRT spectrum (black) from XMMSL1 J060636.2-694933, plus the best-fit black-body model to this spectrum (Sect. 3; Fig. 2), but extending into the UV to the Swift-UVOT UVW2 flux points (coloured) (see text). The data points are plotted such that adjacent data bins have been grouped together to have a significance of at least 3. The solid line show the best fit to the Swift-XRT spectrum. The ratio of the data to the best fit model is shown in the lower panel. |
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5.2 Magellan optical observations
On Nov. 13-15, 2007, XMMSL1 J060636.2-694933 was observed
with the Low-Dispersion Survey Spectrograph 3 (LDSS3) mounted on the
Magellan Clay telescope. Images were obtained through the Sloan
,
and
filters. On Nov. 15, 2007
conditions were photometric and the Landolt field RU 149A was observed
to flux calibrate the data in the
,
and
-bands. The Landolt (1992) magnitudes of the standards
were converted to Sloan magnitudes using the transformations presented
in Smith et al. (2002). All the images were debiased and flatfielded
using dome flatfield frames. We applied aperture photometry on each of
the images using DAOPHOT in IRAF
to
compute the instrumental magnitudes of the stars. Differential
photometry of the optical counterpart to XMMSL1 J060636.2-694933
(marked by an arrow in Fig. 8) was performed with respect
to the field star (marked with a ``c'' in Fig. 8). This was the
brightest isolated and unsaturated star common to all frames. The
calibrated brightness of this comparison star is
,
and
.
![]() |
Figure 8: Magellan Clay LDSS3 finder chart. The counterpart to XMMSL1 J060636.2-694933 (and the bright Swift-UVOT UVW2-filter source; Figs. 5 and 6) is marked with an arrow. The comparison star is shown marked with a ``c''. |
Open with DEXTER |
In addition to the imaging observations described above, we have
obtained spectroscopic observations on Nov. 13-15, 2007 using
the VPH All grism, which has 660 lines per mm, and employing a
1
wide slit. This set-up provides a mean dispersion of 2 Å
per pixel. For a slit width of 1 arcsecond and a mean seeing close to
1
,
the mean spectral resolution is
10 Å. On Nov. 13, 2007
we took 4 exposures of 450 s each, on Nov. 14, 2007 we took 2
exposures of 900 s each, and on Nov. 15, 2007 we took one 1200 s
exposure with the slit at the parallactic angle. The spectra were bias
and flatfield corrected, and extracted in IRAF. The
instrumental response was corrected using the spectrophotometric flux
calibrators LTT 3218 (Nov. 13), H600 (Nov. 14) and LTT 9293 (Nov. 15).
Significant differences in the flux around H
are apparent with
the flux being 50% higher during the Nov. 15, 2007 with respect to
the Nov. 13, 2007 observations. Since there is no evidence for
brightening in the
images we attribute the difference to
the fact that the source was not observed at the parallactic angle on
Nov. 13 and 14, 2007. We exported the one dimensional spectra to the
spectral analysis software package MOLLY for further
analysis.
![]() |
Figure 9: Magellan Clay averaged optical spectrum of the optical source associated with XMMSL1 J060636.2-694933. The flux scaling is approximate. The prominent strong emission lines are marked (see text). |
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We have averaged all spectra (see Fig. 9). We find several
strong emission lines. The strongest of these emission lines are best
interpreted as due to [OIII] 4958.9 Å and 5006.9 Å, He II at
4685.8 Å and a blend of the H
plus the [NII] at 6548.1 Å
and 6583.4 Å, lines found often in novae (Williams 1992). In this
case the main [OIII] lines appear redshifted by approximately 2000 km s-1. We interprete this as due to clumpy outflows in the nova
shell. The integrated light from different outflowing parts can also
explain the substructure that is present in the [OIII] lines. The
outflow velocities that we obtain for the H
and H
lines
is
350 km s-1, hence less than that for the [OIII]
lines. Note that, if XMMSL1 J060636.2-694933 does reside within the
LMC, then the systematic line-of-sight recession velocity of the LMC,
km s-1 (van der Marel et al. 2002), should be taken
into account; i.e. a good fraction of the observed H
and H
recession would then be due to the recession of the LMC itself.
5.3 Long-term optical light curve
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 (ASAS; Pojmanski 2002) show that the visual magnitude
of this source rose from
to
between
Sep. 18, 2005 and Sep. 30, 2005, and then declined rapidly thereafter (see
Fig.10). ASAS did not detect any significant emission from
the source after around November 2005, the source having dimmed below
the limiting magnitude of ASAS.
The decline from the brightest data point (2.2 mag in
10 days, then a further
1.3 mag in 46 days) suggests that
this is a nova of the ``very fast'' speed class (Warner 1995, Downes
et al. 2001). We estimate that the time that the light curve takes to
decline 2 magnitudes below maximum observed brightness is
days (see Sect. 6).
![]() |
Figure 10: All Sky Automated Survey V-band magnitudes of the optical counterpart to XMMSL1 J060636.2-694933, during outburst (late September 2005) and afterwards. |
Open with DEXTER |
6 Discussion
The optical spectrum, showing lines of [OIII] 4958.9 Å and
5006.9 Å, He II at 4685.8 Å and a blend of the H
plus
[NII] at 6548.1 Å and 6583.4 Å suggests that
XMMSL1 J060636.2-694933 was a nova, observed (in Nov 2007) in the late
A0 auroral phase. The fact that the observed [OIII] lines are not
in the more usual, optically thin 3:1 ratio, can be explained in terms
of a clumpy outflow scenario, whereby individual clumps of both
rest-frame and redward-shifted material are observed, and the
superposition of these account for the observed [OIII] ratio (note
further that density enhancements can change observed [OIII] ratios to
more like
1:1). Clumps of material are often seen in nova ejecta
(e.g. Shara et al. 1997), and outflows of speeds around 2000 km s-1 are not uncommon in novae (e.g. in nova LMC 1991; Schwartz et al. 2001).
XMMSL1 J060636.2-694933 was likely at its onset (in Oct. 2005) a very
fast, Fe II nova (Sect. 3 and Williams et al. 1991; Williams
et al. 1994). An accurate classification now however is not possible,
so late after maximum brightness. The soft (
-70 eV) X-ray spectrum indicates that the nova was
in a super-soft source (SSS) state (Krautter 2008) during its
discovery (in July 2006), and throughout its X-ray decline (by more
than two orders of magnitude) in the observations of Sept. 2006, March
2007 and June 2007. Such a state originates from nuclear burning on
the surface of the white dwarf, and measurements of the intensity,
duration, and temperature can be used to estimate the distance to the
nova and the mass of the white dwarf (e.g. Balman et al. 1998; Lanz
et al. 2005). Indeed, we believe (Sect. 4) that the white dwarf
within XMMSL1 J060636.2-694933 may be quite massive
(>1.2
).
As discussed earlier, classical novae are almost always discovered
optically in the early phases of their outbursts.
XMMSL1 J060636.2-694933 is very unusual therefore in that it has been
discovered first in X-rays. As such, it is useful to compare it with
XMMSL1 J070542.7-381442 (also known as V598 Pup; Read et al. 2008),
another nova recently discovered (in X-rays) in the XMM-Newton slew
survey. With a peak mV of
,
XMMSL1 J060636.2-694933 is
not a particularly bright nova (c.f. V598 Pup, which reached an
mV of
4), and so it is not surprising that it went
unnoticed, only being discovered in X-rays during the later (here
291 days after the outburst), optically thin nebular phase, when
classical novae are typically observed as soft X-ray sources. Though
this delay should be taken as a upper limit, it is long when compared
to V598 Pup (
127 days), but may instead be more similar to the
delays of
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). In their X-ray monitoring of optical
novae in M31, Pietsch et al. (2007) detect 11 out of 34 novae in
X-rays within a year after their optical outbursts. Seven novae are
seen to be X-ray bright, several (3-9) years after outburst, and
three novae showed very short X-ray outbursts, starting within
50 days of outburst, but lasting only two to three months.
XMMSL1 J060636.2-694933 therefore is not particularly unusual.
A method to estimate the distance to the nova is to use the relation
between the absolute magnitude at maximum brightness and the time that
the light curve takes to decline 2 mag below maximum
brightness, t2 (Della Valle & Livio 1995). We have no
information over the 12 days between the data point of maximum
brightness and the lower limit prior to this (Fig. 10), and
therefore we have no exact outburst date, nor exact apparent
magnitude at outburst. Assuming for the moment though that we have
caught the outburst exactly in the Sep. 30, 2005 observation, then we
can estimate (Sect. 5.3) t2 to be days, and using this,
we can estimate (Della Valle & Livio 1995) the absolute magnitude at
maximum brightness MV to be
.
An absolute magnitude
of
MV=-8.7 implies a peak luminosity
7 times the Eddington
luminosity for a 1
white dwarf. This is quite typical of novae.
With
AV=0.39+0.05-0.09 (90% error), as derived (Predehl
& Schmitt 1995) from
cm-2 (from the highest
statistic spectral fit; the XMM-Newton ToO observation), and with
,
and a peak mV of 12.0, we can derive a
distance to XMMSL1 J060636.2-694933 of 115
+43-30 kpc. As
discussed above however, we are unsure as to the exact outburst date
and the maximum brightness at outburst. Our assumed peak mV of
12.0 is almost certainly an underestimation. Although we have no
information in the 12 days prior to Sep. 30, 2005, a simple linear
extrapolation of the early October lightcurve back prior to Sep. 30,
2005 suggests that the actual peak mV was somewhere between 9 and
12. The corresponding distance estimates are then between 29 and
115 kpc (with a mid-point
mV=10.5 value yielding a distance
estimate of 58 kpc). Many methods have been used to estimate the
distance to the LMC (e.g. Kovacs 2000; Nelson et al. 2000), but a
value of around 50 kpc appears to be quite robust. Our distance
estimate is certainly consistent with that of the LMC, though the
errors are quite large. It does appear to be the case however, that
our distance estimate places the source far outside of our own Galaxy.
This, together with the source's position on the sky (at the eastern
edge of the LMC) and the sizable (
Galactic) X-ray hydrogen
column densities obtained from the spectral fits, suggest strongly
that XMMSL1 J060636.2-694933 lies within the LMC itself. Note further
that the (pile-up corrected) spectral model normalizations to the
initial Slew discovery data (Sect. 2) also imply an approximate
distance to XMMSL1 J060636.2-694933 of
50 kpc.
The source had, at the time of the slew detection, an absorbed
(0.2-2 keV) X-ray flux of 4.8
erg
cm-2 s-1, corresponding to a 0.2-2 keV X-ray luminosity
(at 50 kpc) of 1.4
erg s-1.
Assuming instead for the moment a distance more like 100 kpc (though
this is thought to be well beyond the LMC, e.g. Kovacs 2000), then the
(0.2-2 keV) X-ray luminosity of
5.7
erg s-1 obtained is at the high end of the X-ray luminosities of classical SSS-phase novae discussed e.g. in Orio et al. (2002) and
Ness et al. (2007). As discussed though, we have very likely missed
the outburst peak, and as such, our more probable assumed distance of
50 kpc gives rise to a more typical SSS-phase X-ray luminosity. The
luminosities of 7-
erg s-1, obtained during
the Swift and pointed XMM-Newton observations, are more typical of
novae at later times, when the emission can also sometimes be
described by a thermal plasma, rather than a black-body type spectrum,
or a more mixed spectrum, due to the complex structure of the ejecta
and the accretion disk (Krautter 2008; Shore 2008).
7 Conclusions
A bright X-ray source, XMMSL1 J060636.2-694933, was detected in an XMM-Newton slew on 18 July 2006 at a position where no previous X-ray source had been seen. The XMM-Newton slew data, plus follow-up dedicated XMM-Newton and Swift observations, plus optical imaging and spectroscopic data acquired with the Magellan Clay telescope and All-Sky Automated Survey (ASAS) data were used to classify the new object as a nova, and to examine its properties. The primary conclusions are as follows:
- The soft X-ray spectrum indicates that the nova was in a
super-soft source (SSS) state at its discovery in July 2007
(XMM-Newton slew) and through its X-ray decline (by over two
orders of magnitude) in September 2006 (XMM-Newton slew), March
2007 (Swift) and June 2007 (XMM-Newton).
- The Magellan optical spectrum (Nov. 2007) of the source
indicates that it was very likely then a nova in the late
A0 auroral phase.
- The very fast optical decline (ASAS) during the nova's onset
(Oct 2005), indicates that the initial nova was likely of speed class
``very fast''.
- The very fast speed, together with the absolute magnitude at
maximum brightness and the X-ray absorption, give rise to a
distance to the source far beyond our own Galaxy. The large
distance, together with the source's position in the sky, at the
eastern edge of the LMC, and the spectral information from the
X-ray data, are very suggestive that the nova is situated within
the LMC itself.
- Analysis of XMM-Newton slew data is continuing to provide a
powerful means of finding new X-ray transient objects.
The XMM-Newton project is an ESA Science Mission with instruments and contributions directly funded by ESA Member States and the USA (NASA). The XMM-Newton project is supported by the Bundesministerium für Wirtschaft und Technologie/Deutsches Zentrum für Luft- und Raumfahrt (BMWI/DLR, FKZ 50 OX 0001), the Max-Planck Society and the Heidenhain-Stiftung. AMR and PE acknowledge the support of STFC funding, and PGJ of the Netherlands Organisation for Scientific Research. The ASAS project is supported by the N2030731/1328 grant from the MNiSzW. We thank the referee (G. Sala) for very useful comments and several references that have improved the paper notably. We thank Kim Page for providing the white dwarf atmosphere model, and we thank her and Graham Wynn for useful discussions. The use of the spectral analysis software package MOLLY written by Tom Marsh is also acknowledged. M.M. acknowledges support by a Miller Institute Research Fellowship during the time in which part of the work was completed.
References
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Footnotes
- ...
Simbad
- http://simbad.u-strasbg.fr/simbad/
- ...
NED
- http://nedwww.ipac.caltech.edu/index.html
- ...
HEASARC
- http://heasarc.gsfc.nasa.gov/
- ...
mode
- http://xmm.esac.esa.int/external/xmm_user_support/ documentation /uhb_2.5/index.html
- ...
XSPEC
- http://heasarc.gsfc.nasa.gov/docs/xanadu/xspec/
- ... nh
- http://heasarc.gsfc.nasa.gov/lheasoft/ftools/fhelp/ nh.txt
- ...
FTOOLs
- http://heasarc.nasa.gov/lheasoft/ftools/ ftools_menu.html
- ... IRAF
- IRAF is distributed by the National Optical Astronomy Observatories
All Tables
Table 1: Details of the four XMM-Newton slew observations and the single (Rev. 1378) dedicated XMM-Newton pointed observation.
Table 2: Details of the Swift-XRT observations (observation ID, observation date and cleaned exposure time) are tabulated, together with the total (0.2-2.0 keV) background-subtracted counts and count rate from XMMSL1 J060636.2-694933 (see text).
All Figures
![]() |
Figure 1: XMM-Newton Slew spectrum of XMMSL1 J060636.2-694933 from XMM-Newton revolution 1210. The data points (crosses; adjacent data bins having been grouped together for the plot to have a significance of at least 3) have been fitted with a black body model (kT=63 eV; see text). The solid line shows the best fit to the spectrum. The ratio of the data to the best fit model is shown in the lower panel. |
Open with DEXTER | |
In the text |
![]() |
Figure 2: Swift-XRT spectrum from XMMSL1 J060636.2-694933. The data points (crosses; adjacent data bins having been grouped together for the plot to have a significance of at least 3) have been fitted with a black body model (kT=59 eV; see text). The source has faded by a factor of >100 since the XMM-Newton revolution 1210 slew discovery. The solid line show the best fit to the spectrum. The ratio of the data to the best fit model is shown in the lower panel. |
Open with DEXTER | |
In the text |
![]() |
Figure 3: XMM-Newton ToO spectrum from XMMSL1 J060636.2-694933. The data points (crosses; adjacent data bins having been grouped together for the plot to have a significance of at least 3)) have been fitted again with a black body model (kT=70 eV) (see text). EPIC-pn data is shown in black, with EPIC-MOS1 in red and EPIC-MOS2 in green. The solid lines show the best fit to the spectra. The ratios of the data to the best fit model are shown in the lower panel. |
Open with DEXTER | |
In the text |
![]() |
Figure 4: The full X-ray lightcurve of XMMSL1 J060636.2-694933. Plotted are the calculated (0.2-2.0 keV) flux values versus time. The first point is the rev. 1210 XMM-Newton Slew observation, then the rev. 1246 XMM-Newton Slew observation. The three nested Swift-XRT points are shown next and finally the XMM-Newton ToO observation. The RASS upper limit is shown bottom left. |
Open with DEXTER | |
In the text |
![]() |
Figure 5:
Swift UVOT images of the field around XMMSL1 J060636.2-694933 from observation
00030895002. Left shows the UVOT B-filter and right shows the
UVOT UVW2-filter. The large circle is a 20
|
Open with DEXTER | |
In the text |
![]() |
Figure 6:
Variation of the UVW2 magnitude of the bright UV source
during the Swift observations. The same time axis as
Fig. 4 has been used to aid comparison, and a zoom
is also shown. The UVW2 filter was only employed during observations
00030895002, 00030895004, 00030895005, 00030895006 & 00030895007
(hence the points span the dates 07/03/07 to 22/03/07). The errors here are 1- |
Open with DEXTER | |
In the text |
![]() |
Figure 7: Swift-XRT spectrum (black) from XMMSL1 J060636.2-694933, plus the best-fit black-body model to this spectrum (Sect. 3; Fig. 2), but extending into the UV to the Swift-UVOT UVW2 flux points (coloured) (see text). The data points are plotted such that adjacent data bins have been grouped together to have a significance of at least 3. The solid line show the best fit to the Swift-XRT spectrum. The ratio of the data to the best fit model is shown in the lower panel. |
Open with DEXTER | |
In the text |
![]() |
Figure 8: Magellan Clay LDSS3 finder chart. The counterpart to XMMSL1 J060636.2-694933 (and the bright Swift-UVOT UVW2-filter source; Figs. 5 and 6) is marked with an arrow. The comparison star is shown marked with a ``c''. |
Open with DEXTER | |
In the text |
![]() |
Figure 9: Magellan Clay averaged optical spectrum of the optical source associated with XMMSL1 J060636.2-694933. The flux scaling is approximate. The prominent strong emission lines are marked (see text). |
Open with DEXTER | |
In the text |
![]() |
Figure 10: All Sky Automated Survey V-band magnitudes of the optical counterpart to XMMSL1 J060636.2-694933, during outburst (late September 2005) and afterwards. |
Open with DEXTER | |
In the text |
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