Variable strong X-ray emission lines in U Sco

ATel #2469; J.-U. Ness (ESA/ESAC), B. Schaefer (Louisiana State University), J. J. Drake (CfA), J. P. Osborne, K. L. Page, and A. Beardmore (Leicester), G. Schwarz (AAS), R. Gonzalez and E. Kuulkers (ESA/ESAC), M. F. Bode (Liverpool JMU), S. Starrfield (ASU), J. Krautter (Heidelberg), N. Gehrels (NASA-GSFC), C. E. Woodward, R. D. Gehrz (University of Minnesota), S. P. S. Eyres (UCLan), A. Evans (Keele), M. Hernanz (CSIC-IEEC), S. Balman (METU
on 10 Mar 2010; 19:32 UT
Distributed as an Instant Email Notice Novae
Credential Certification: Jan-Uwe Ness (Jan-Uwe.Ness@asu.edu)

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An XMM-Newton observation of the eclipsing, recurrent nova U Sco was carried out, yielding a high-resolution X-ray spectrum. The observation with the Reflection Grating Spectrometer (RGS) started 2010-02-19T15:41:09 (=22.23 days after discovery) and ended 2010-02-20T09:24:41. During the exposure time of 63360s, an average count rate of 1 count per second (cps) was observed with variations between 0.5 and 1.5 cps. No periodic oscillations were detected.

The observed flux, obtained by direct integration over the wavelength range 16-38A (0.32-0.77keV) of the RGS spectrum is 2.9E-11 erg/cm^2/s. A Chandra observation starting at 2010-02-14T11:39:27 (18.05 days after discovery, ATel#2451) yields an observed flux of 1.8E-11 erg/cm^2/s over the same wavelength range, and the average X-ray flux has thus increased by a factor 1.6 from day 18.1 to 22.2. The flux given here is different from that given in ATel#2451 because a flux derived from a fitted model was given. The XMM-Newton observation includes the eclipse, covering 0.84-0.44 in phase, but no indication in the spectra or light curve for a significant eclipse-induced reduction of X-ray emission can be seen. The Chandra observation occurred between phases 0.64 and 0.87, and was thus outside of eclipse.

The spectrum consists of photospheric continuum emission with strong emission lines superimposed. Between days 18.1 and 22.1, the continuum has hardly changed, while the already strong nitrogen emission lines reported in ATel#2451 have become significantly stronger. Additional emission lines of oxygen and carbon are also detected in the new spectrum.

Listed below are some ions with observed wavelength, rest wavelength, full width at half maximum (FWHM), and line flux:

ion & lam (A) & lam_0 (A) & FWHM (A) & flux (10^-12 erg/cm^2/s)
day 18.1
NVII 1s-2p & 25.00 & 24.78 & 0.28 & 0.93
NVI 1s-3p & 25.00 & 24.90 & 0.28 & 0.93
NVI 1s-2p & 28.90 & 28.78 & 0.19 & 0.95
CVI 1s-2p & 33.80 & 33.74 & 0.44 & 0.29
day 22.2
NVII 1s-2p & 24.93 & 24.78 & 0.32 & 3.74
NVI 1s-3p & 24.93 & 24.90 & 0.32 & 3.74
NVI 1s-2p & 28.90 & 28.78 & 0.27 & 1.70
NVI 1s-2p(i)& 29.13 & 29.10 & 0.16 & 0.50
NVI 1s-2p(f)& 29.42 & 29.54 & 0.48 & 1.38
NVII 1s-3p & 20.96 & 20.90 & 0.25 & 0.16
OVIII 1s-2p & 19.01 & 18.97 & 0.30 & 0.29
OVII 1s-2p & 21.70 & 21.60 & 0.49 & 0.51
CVI 1s-2p & 33.76 & 33.74 & 0.46 & 0.26

The strongest emission line arises at 24.9A and originates from either NVII 1s-2p (lam_0=24.78A) or NVI 1s-3p (lam_0=24.9A) or a combination of both. From day 18.1 to 22.2, the line flux has increased by a factor 4. Higher order NVII Ly series lines up to 1s-5p as well as the oxygen lines are only detected in the XMM-Newton spectrum. The CVI line can also be identified in the Chandra spectrum, indicating that the C/N abundance could be higher than in RS Oph, where the CVI line was not detected Ness et al. (2007).
The line widths correspond to 3000-4000 km/s, and all emission lines appear slightly red shifted. For some lines, also blue-shifted absorption lines can be identified, and the red shift may be a result from reduced emission in the blue wing of the emission line profile.
The NVII absorption line is much broader than that of NVI. The interstellar OI line can also be seen at 23.5A, but the NI line (lam_0=31.28A) can not be seen. This line has been seen in V2491 Cyg (Ness et al. in prep) and its absence could indicate a lower N abundance along the line of sight of USco.

We thank the XMM-Newton Science Operations Centre for rapid scheduling.