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XMM-Newton and Swift-XRT see declining X-ray flux from the nova XMMSL1 J070542.7-381442 (V598 Pup)

ATel #1301; A M Read (Leicester), R D Saxton (ESAC), J P Osborne (Leicester), K L Page (Leicester), P Esquej (MPE), J-U Ness (Arizona State), A Pollock (ESAC), A Ibarra (ESAC), E Kuulkers (ESAC)
on 23 Nov 2007; 15:52 UT
Credential Certification: Dr. Andy Read (amr30@star.le.ac.uk)

Subjects: Nova, Transient

Referred to by ATel #: 1305

The bright new X-ray transient, XMMSL1 J070542.7-381442 (= V598 Pup [IAUC # 8898 ]), discovered by ESA's XMM-Newton in its Slew Survey mode [ATel #1282] and identified as a classical nova [ATel #1285], has been observed by XMM-Newton in its normal pointed mode and by Swift.

The observed 0.2-2 keV EPIC-pn count rate has declined from 53.7 ct/s (XMM-Slew, rev.1434, obsid 9143400002, full-frame, medium filter: 2007-10-08 [ATel #1282]) to 3.7 ct/s (XMM-pointed, rev.1445, obsid 0510010901, 5 ksec exposure, small window, medium filter, 2007-10-30).

The slew data were severely affected by pile-up, thus the observed count rate for the discovery observation is due to a significantly higher flux than would normally be associated with this count rate.

The pointed XMM-EPIC-pn X-ray light curve shows non-monotonic variations by a factor ~ 4 during the 5 ksec observation. The soft flux (E=0.2-0.5 keV) is seen to be variable, while the harder flux (E=0.5-1.0 keV) is not. No periodic variability is seen.

The XMM-EPIC-pn spectrum at this time can be fit by a 2-component model incorporating a white dwarf atmosphere and emission from optically thin gas (T <~ 2 MK). The observed 0.2-2 keV flux is 6.7e-12 ergs/s/cm^2.

In this observation the XMM-RGS X-ray spectrum appears to be dominated by emission lines due to CVI, the NVI triplet and NVII, indicating optically thin gas at a temperature of order 1 MK. This spectrum is reasonably consistent with the model fit to the EPIC-pn.

On 2007-11-21 a 3.8 ks Swift-XRT PC-mode observation shows the source to have a 0.2-2 keV count rate of 0.025 c/s and a flux (derived with the XMM-EPIC-pn model fit) of 1.3e-12 ergs/s/cm^2. No significant flux variation is seen between the 6 Swift orbits, which were taken over ~8 hours.

The X-ray source reduced in flux by a factor of at least 15 in the 3 weeks from discovery to the pointed XMM observation, and a further factor of 5 in the following 3 weeks to the Swift observation.

On the basis of the X-ray spectrum and short-term soft X-ray flux variation, this nova appears to have been in a super-soft state, at least during the XMM observations, and probably also during the Swift observations. Such a state is due to nuclear burning on the white dwarf. Measurement of its intensity, duration and temperature can constrain the distance to the nova and the mass of the white dwarf (e.g. Balman, Krautter & Oegelmann 1998 - ApJ 499, 395; Lanz et al. 2005 - ApJ 619, 517L). Although post-discovery observations show reduced fluxes, further observations are required to demonstrate that this phase has truly ended as the super-soft phase of RS Oph showed extreme soft X-ray variability during this phase (Osborne et al ATel #770). The emission lines detected by the XMM-RGS may indicate that V598 Pup is a CO nova.

We would like to thank the XMM-Newton and Swift teams for their rapid scheduling of these follow-up observations.