Joachim Krautter

Swift observations of the 2006 outburst of the recurrent nova RS Ophiuchi: I. Early X-ray emission from the shocked ejecta and red giant wind

RS Ophiuchi began its latest outburst on 2006 February 12. Previous outbursts have indicated that high-velocity ejecta interact with a preexisting red giant wind, setting up shock systems analogous to those seen in supernova remnants. However, in the previous outburst in 1985, X-ray observations did not commence until 55 days after the initial explosion. Here we report on Swift observations covering the first month of the 2006 outburst with the Burst Alert Telescope (BAT) and X-Ray Telescope (XRT) instruments. RS Oph was clearly detected in the BAT 14-25 keV band from t = 0 to t ~ 6 days. XRT observations from 0.3 to 10 keV started 3.17 days after outburst. The rapidly evolving XRT spectra clearly show the presence of both line and continuum emission, which can be fitted by thermal emission from hot gas whose characteristic temperature, overlying absorbing column (NH)W, and resulting unabsorbed total flux decline monotonically after the first few days. Derived shock velocities are in good agreement with those found from observations at other wavelengths. Similarly, (NH)W is in accord with that expected from the red giant wind ahead of the forward shock. We confirm the basic models of the 1985 outburst and conclude that standard phase I remnant evolution terminated by t ~ 6 days and the remnant then rapidly evolved to display behavior characteristic of phase III. Around t = 26 days, however, a new, luminous, and highly variable soft X-ray source began to appear, whose origin will be explored in a subsequent paper.

A Chandra low energy transmission grating spectrometer observation of V4743 Sagittarii : a supersoft X-ray source and a violently variable light curve

V4743 Sagittarii (Nova Sgr 2002 No. 3) was discovered on 2002 September 20. We obtained a 5 ks ACIS-S spectrum in 2002 November and found that the nova was faint in X-rays. We then obtained a 25 ks Chandra Low Energy Transmission Grating Spectrometer (LETGS) observation on 2003 March 19. By this time, it had evolved into the supersoft X-ray phase exhibiting a continuous spectrum with deep absorption features. The light curve from the observation showed large-amplitude oscillations with a period of 1325 s (22 minutes) followed by a decline in the total count rate after ~13 ks of observations. The count rate dropped from ~40 counts s-1 to practically zero within ~6 ks and stayed low for the rest of the observation (~6 ks). The spectral hardness ratio changed from maxima to minima in correlation with the oscillations and then became significantly softer during the decay. Strong H-like and He-like lines of oxygen, nitrogen, and carbon were found in absorption during the bright phase, indicating temperatures between 1 and 2 MK, but they were shifted in wavelength corresponding to a Doppler velocity of -2400 km s-1. The spectrum obtained after the decline in count rate showed emission lines of C VI, N VI, and N VII, suggesting that we were seeing expanding gas ejected during the outburst, probably originating from CNO-cycled material. An XMM-Newton Target of Opportunity observation, obtained on 2002 April 4 and a later LETGS observation from 2003 July 18 also showed oscillations, but with smaller amplitudes.

Identification of a Complete Sample of Northern ROSAT All-Sky Survey X-Ray Sources. III. The Catalog

We present a catalog of optical identifications of a representative sample of northern (δ > -9°) ROSAT All-Sky Survey (RASS) sources. A full identification has been carried out for a count-rate- and area-limited complete RASS subsample comprising 674 sources. All sources are within six study areas outside the Galactic plane (| b | > 196), one area being near the north Galactic pole and one near the north ecliptic pole.

The soft X-ray turnoff of Nova Muscae 1983

Nova GQ Muscae 1983 was detected by ROSAT as a luminous supersoft X-ray source in 1992, nearly a decade after outburst. Further, this is the only classical postnova known to have maintained constant luminosity on a timescale predicted by theoretical models. Follow-up observations were made with the ROSAT position-sensitive proportional counter in 1993 January and September, and complemented with B-band photometry taken in 1993 January. By 1993 January, the X-ray count rate had declined by a factor of 17, while there was neither an appreciable decrease in the optical magnitude nor a change in the amplitude of modulation. In 1993 September the soft X-ray flux was below the ROSAT threshold limit, implying a decrease of a factor greater than or equal to 30 in the count rate. This decline can be interpreted by the turnoff of nuclear processes due to the complete consumption of the residual hydrogen-rich envelope. However, the optical luminosity of the system is not simply coupled to the X-ray luminosity (e.g., through reprocessing).

An X-ray emission-line spectrum of Nova V382 Velorum 1999

We report on the analysis of an X-ray grating spectrum of the Classical Nova V382 Vel (1999), obtained with the Low Energy Transmission Grating (LETG)+HRC-S instrument onboard Chandra, which shows emission lines dominating over any continuum. Lines of Si, Mg, Ne, O, N and C are identified, but no Fe lines are detected. The total luminosity in the lines is ∼4 x 10 27 erg s -1 (corrected for N H = 1.2 x 10 21 cm -2 ). The lines have broad profiles with full width at half-maximum corresponding to a velocity ∼2900 ± 200 km s -1 . Some structure is identified in the profiles, but for different elements we find different profile structures. While lines of O show a broadened Gaussian profile, those of Ne are double-peaked, suggesting a fragmented emitting plasma. Using the emission measure distribution, we derive relative element abundances and find abundances of Ne and N that are significantly enhanced relative to that of O, while Fe is not overabundant. The lack of any source emission longwards of 50 A and the O viii Ly α /Ly β line ratio supports previous values of the hydrogen column density. We find weak continuum emission from the white dwarf, consistent with a blackbody spectrum with an upper limit to the temperature of T = 3 × 10 5 K, assuming a source radius of 6000 km. The upper limit for the integrated blackbody luminosity is 2 x 10 36 erg s -1 . The BeppoSAX and Chandra ACTS observations of V382 Vel show that the nova was bright and in the Super-Soft phase as late as 1999 December 30. Our LETG observation obtained 6 weeks later, as well as all subsequent X-ray observations, showed a remarkable fading to a nearly pure emission line phase which suggests that nuclear burning on the white dwarf had turned off by February. In the absence of a photoionizing source, the emission lines were formed in a collisionally ionized and excited expanding shell.

ORFEUS II Far-Ultraviolet Observations of 3C 273: Interstellar and Intergalactic Absorption Lines*

We present the first intermediate-resolution (λ/Δλ = 3000) spectrum of the bright quasi-stellar object 3C 273 at wavelengths between 900 and 1200 A. Observations were performed with the Berkeley spectrograph aboard the ORFEUS II mission. We detect Lyβ counterparts to intergalactic Lyα features identified by Morris and coworkers at cz = 19,900, 1600, and 1000 km s−1; counterparts to other putative Lyα clouds along the sight line are below our detection limit. The strengths of the two very low redshift Lyβ features, which are believed to arise in Virgo intracluster gas, exceed preflight expectations (Weymann and coworkers), suggesting that the previous determination of the cloud parameters may underestimate the true column densities. A curve-of-growth analysis sets a minimum H I column density of 4 × 1014 cm−2 for the 1600 km s−1 cloud. We find marginally significant evidence for Galactic H2 along the sight line, with a total column density of about 1015 cm−2. We detect the stronger interstellar O VI doublet member unambiguously; the weaker member is blended with other features. If the Doppler b-value for O VI is comparable to that determined for N V by Sembach and collaborators, then the O VI column density is (7 ± 2) × 1014 cm−2, significantly above the only previous estimate, by Davidsen. The O VI/N V ratio is about 10, consistent with the low end of the range observed in the disk, as shown in the compilation by Hurwitz & Bowyer. Additional interstellar species detected for the first time toward 3C 273 (at modest statistical significance) include P II, Fe III, Ar I, and S III.

Infrared space observatory and ground-based infrared observations of the classical Nova V723 Cassiopeiae

We present observations of the classical nova V723 Cassiopeiae (Nova Cas 1995), obtained both with the Infrared Space Observatory (ISO) and from the ground. The infrared spectrum was dominated in the first year by H and He recombination lines, and at later times by coronal lines. The H recombination lines imply a reddening of E(B-V) = 0.78, an electron temperature of 7000 K, and an electron density of 2 × 108 cm-3 on day 250. We argue that the high-ionization species in the infrared are most likely the result of collisional ionization rather than photoionization and are therefore truly coronal; we estimate a temperature of 3.2 × 105 K in the coronal region and abundance ratios of S/Si 2.1, Ca/Si 1.6, and Al/Si 1.5. The ejected mass as determined from the Brα line was 2.6 × 10-5 M⊙ for a distance of 4 kpc; however, the mass deduced from the free-free emission, which we conclude arises primarily in the coronal zone, is 4.3 × 10-4 M⊙. V723 Cas did not display the [O IV] 25.89 μm fine-structure line, which was typically seen in the spectra of novae observed with ISO. There was no evidence of dust emission in V723 Cas.

The detection of X-rays from nova Muscae 1983 with the Exosat satellite

Le 20 avril 1984, le satellite Exosat a observe la nova classique rapide Muscae 1983, environ 4.10 7 s apres le maximum. Les detecteurs RX de moyenne energie nont observe aucun flux significatif, et on a etabli une limite superieure de 3.10 −12 ergs cm −2 s −1 dans le domaine de 2 a 6 keV. Le telescope a faible energie (0,04-2 keV) a permis de detecter la source. Une enveloppe de gaz circumstellaire emettant un rayonnement de freinage thermique a 10 35 ergs −1 de luminosite ou un reste de naine blanche emettant un rayonnement de corps noir de 3,5×10 5 K a 10 37 erg s −1 de luminosite sont tous deux compatibles avec les mesures

Observations and predictions of EUV emission from classical novae

Theoretical modeling of novae in outburst predicts that they should be active emitters of radiation both in the EUV and soft X-ray wavelengths twice during the outburst. The first time is very early in the outburst when only an all sky survey can detect them. This period lasts only a few hours. They again become bright EUV and soft X-ray emitters late in the outburst when the remnant object becomes very hot and is still luminous. The predictions imply both that a nova can remain very hot for months to years and that the peak temperature at this time strongly depends upon the mass of the white dwarf. It is important to observe novae at these late times because a measurement of both the flux and temperature can provide information about the mass of the white dwarf, the tun-off time scale, and the energy budget of the outburst. We review the existing observations of novae in late stages of their outburst and present some newly obtained data for GQ Mus 1983. We then provide results of new hydrodynamic simulations of novae in outburst and compare the predictions to the observations. 43 refs., 6 figs.

Chandra ACIS‐I and LETGS X‐ray observations of Nova 1999 Velorum (V382 Vel)

The prelimenary analysis of the Chandra ACIS‐I and LETGS observations carried out over a period of 8 months starting 7 months after the outburst of Nova Velorum 1999 (=V382 Vel) is presented here. We find that within a period of less than 6 weeks the flux of the soft X‐ray component (.0.4–0.8 keV) decreases nearly 200 fold. In addition, we find that the soft component seems to become fainter following a steeper power law than the hard component. Most lines detected with the Chandra LETGS are broadened with a corresponding FWHM ∼ 2000 km/s that is compatible with the velocity of the expanding shell. The He‐like triplets of OVII and NVI allow us to put some constraints on the plasma temperature of 4.5–5.0 105 K.