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1.
Detection of X‐rays from classical novae, both in outburst and post‐outburst, provides unique and crucial information about the explosion mechanism. Soft X‐rays reveal the hot white dwarf photosphere, whenever hydrogen (H) nuclear burning is still on and expanding envelope is transparent enough, whereas harder X‐rays give information about the ejecta and/or the accretion flow in the reborn cataclysmic variable. The duration of the supersoft X‐ray emission phase is related to the turn‐off of the classical nova, i.e., of the H‐burning on top of the white dwarf core. A review of X‐ray observations is presented, with a special emphasis on the implications for the duration of post‐outburst steady H‐burning and its theoretical explanation. The particular case of recurrent novae (both the “standard” objects and the recently discovered ones) will also be reviewed, in terms of theoretical feasibility of short recurrence periods, as well as regarding implications for scenarios of type Ia supernovae (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

2.
We present the complete set of 34 ASCA observations of non-magnetic cataclysmic variables. Timing analysis reveals large X-ray flux variations in dwarf novae in outburst (Z Cam, SS Cyg and SU UMa) and orbital modulation in high inclination systems (including OY Car, HT Cas, U Gem, T Leo). We also found episodes of unusually low accretion rate during quiescence (VW Hyi and SS Cyg). Spectral analysis reveals broad temperature distributions in individual systems, with emission weighted to lower temperatures in dwarf novae in outburst. Absorption in excess of interstellar values is required in dwarf novae in outburst, but not in quiescence. We also find evidence for subsolar abundances and X-ray reflection in the brightest systems.
LS Peg, V426 Oph and EI UMa have X-ray spectra that are distinct from the rest of the sample and all three exhibit candidate X-ray periodicities. We argue that they should be reclassified as intermediate polars.
In the case of V345 Pav we found that the X-ray source had been previously misidentified.  相似文献   

3.
We present extensive, high-density Swift observations of V2491 Cyg (Nova Cyg 2008 No. 2). Observing the X-ray emission from only one day after the nova discovery, the source is followed through the initial brightening, the super-soft source phase and back to the pre-outburst flux level. The evolution of the spectrum throughout the outburst is demonstrated. The UV and X-ray light curves follow very different paths, although changes occur in them around the same times, indicating a link between the bands. Flickering in the late-time X-ray data indicates the resumption of accretion. We show that if the white dwarf (WD) is magnetic, it would be among the most magnetic known; the lack of a periodic signal in our later data argues against a magnetic WD, however. We also discuss the possibility that V2491 Cyg is a recurrent nova, providing recurrence time-scale estimates.  相似文献   

4.
We present the most complete multiwavelength coverage of any dwarf nova outburst: simultaneous optical, Extreme Ultraviolet Explorer and Rossi X-ray Timing Explorer observations of SS Cygni throughout a narrow asymmetric outburst. Our data show that the high-energy outburst begins in the X-ray waveband 0.9–1.4 d after the beginning of the optical rise and 0.6 d before the extreme-ultraviolet rise. The X-ray flux drops suddenly, immediately before the extreme-ultraviolet flux rise, supporting the view that both components arise in the boundary layer between the accretion disc and white dwarf surface. The early rise of the X-ray flux shows that the propagation time of the outburst heating wave may have been previously overestimated.
The transitions between X-ray and extreme-ultraviolet dominated emission are accompanied by intense variability in the X-ray flux, with time-scales of minutes. As detailed by Mauche & Robinson, dwarf nova oscillations are detected throughout the extreme-ultraviolet outburst, but we find they are absent from the X-ray light curve.
X-ray and extreme-ultraviolet luminosities imply accretion rates of  3 × 1015 g s−1  in quiescence,  1 × 1016 g s−1  when the boundary layer becomes optically thick, and  ∼1018 g s−1  at the peak of the outburst. The quiescent accretion rate is two and a half orders of magnitude higher than predicted by the standard disc instability model, and we suggest this may be because the inner accretion disc in SS Cyg is in a permanent outburst state.  相似文献   

5.
The ROSAT Wide Field Camera (WFC) survey of the extreme ultraviolet (EUV) has provided us with evidence for the existence of a previously unidentified sample of hot white dwarfs in unresolved, detached binary systems. These stars are invisible at optical wavelengths due to the close proximity of their much more luminous companions (spectral type K or earlier). However, for companions of spectral type ∼A5 or later the white dwarfs are easily visible at far-ultraviolet wavelengths, and can be identified in spectra taken by IUE . 16 such systems have been discovered in this way through ROSAT EUVE IUE observations, including four identified by us in Paper I. In the present paper we report the results of our continuing search during the final year of IUE operations. One new system, RE J0500−364 (DA+F6/7V), has been identified. This star appears to lie at a distance of ∼500−1000 pc, making it one of the most distant white dwarfs, if not the most distant, to be detected in the EUV surveys. The very low line-of-sight neutral hydrogen volume density to this object could place a lower limit on the length of the β CMa interstellar tunnel of diffuse gas, which stretches away from the Local Bubble in a similar direction to RE J0500−364. In this paper we also analyse a number of the stars observed where no white dwarf companion was found. Some of these objects show evidence for chromospheric and coronal activity. Finally, we present an analysis of the previously known WD+active F6V binary HD 27483 (Bo¨hm-Vitense 1993), and show that, at T  ≈ 22 000 K, the white dwarf may be contributing significantly to the observed EUV flux. If so, it is one of the coolest such stars to be detected in the EUV surveys.  相似文献   

6.
In this paper, we discuss the early phases of the ongoing outburst that CI Cyg, a prototype symbiotic star, is currently undergoing after 30-year quiescence. We have tightly monitored CI Cyg in   B V R C I C  bands, starting a whole year before the onset of the outburst, and in addition we obtained numerous Echelle high- and low-resolution absolutely flux-calibrated spectra. The outburst started while the accreting white dwarf (WD) was being eclipsed by the Roche lobe filling M giant companion, and it was discovered during the egress phase on the second half of 2008 August. The system reached peak V -band brightness in early 2008 October and has been characterized by amplitudes  Δ B = 1.9, Δ V = 1.5, Δ R C= 0.9, Δ I C= 0.4  mag. At maximum V -band brightness, the outbursting WD had expanded to closely resemble an F3 II/Ib star, with   MV =−3.5, T eff∼ 6900 K  and   R = 28 R⊙  . The high-ionization emission lines ([Ne  v ], [Fe  vii ], He  ii ) disappeared and only lower ionization lines were visible. Balmer and He  i emission lines declined in equivalent width but increased in absolute flux. The output radiated by the hot component during the outburst corresponds to nuclear burning proceeding at a  2 × 10−8 M⊙ yr−1  rate.  相似文献   

7.
We report on multi-epoch X-ray observations of the Type IIn (narrow emission line) Supernova (SN) 1995N with the ROSAT and ASCA satellites. The 1998 January ASCA X-ray spectrum is well fitted by a thermal bremsstrahlung         or power-law         model. The X-ray light curve shows evidence for significant flux evolution between 1996 August and 1998 January: the count rate from the source decreased by 30 per cent between our 1996 August and 1997 August ROSAT observations, and the X-ray luminosity most likely increased by a factor of ∼2 between our 1997 August ROSAT and 1998 January ASCA observations, although evolution of the spectral shape over this interval is not ruled out. The high X-ray luminosity,     places SN 1995N in a small group of Type IIn supernovae with strong circumstellar interaction, and the evolving X-ray luminosity suggests that the circumstellar medium is distributed inhomogeneously.  相似文献   

8.
I present pointed ROSAT PSPC observations of the pre-cataclysmic binary V471 Tauri. The hard X-ray emission (>0.4 keV) is not eclipsed by the K star, demonstrating conclusively that this component cannot be emitted by the white dwarf. Instead I show that its spectrum and luminosity are consistent with coronal emission from the tidally spun-up K star. The star is more active than other K stars in the Hyades, but equally active as K stars in the Pleiades with the same rotation periods, demonstrating that rotation — and not age — is the key parameter in determining the level of stellar activity.   The soft X-ray emission (<0.4 keV) is emitted predominately by the white dwarf and is modulated on its spin period. I find that the pulse profile is stable on time-scales of hours and years, supporting the idea that it is caused by the opacity of accreted material. The profile itself shows that the magnetic field configuration of the white dwarf is dipolar and that the magnetic axis passes through the centre of the star.   There is an absorption feature in the light curve of the white dwarf, which occurs at a time when our line of sight passes within a stellar radius of the K star. The column density and duration of this feature imply a volume and mass for the absorber that are similar to those of coronal mass ejections of the Sun.   Finally I suggest that the spin–orbit beat period detected in the optical by Clemens et al. may be the result of the interaction of the K-star wind with the magnetic field of the white dwarf.  相似文献   

9.
Up to now, only a very small number of dwarf novae have been studied during their outburst state (∼30 per cent in the Northern hemisphere). In this paper we present the first comprehensive atlas of outburst spectra of dwarf novae. We study possible correlations between the emission and absorption lines seen in the spectra and some fundamental parameters of the binaries. We find that out of the 48 spectra presented, 12 systems apart from IP Peg show strong He  ii in emission: SS Aur, HL CMa, TU Crt, EM Cyg, SS Cyg, EX Dra, U Gem, HX Peg, GK Per, KT Per, V893 Sco, IY UMa, and seven others less prominently: FO And, V542 Cyg, B  i Ori, TY Psc, VZ Pyx, ER UMa and SS UMi. We conclude that these systems are good targets for finding spiral structure in their accretion discs during outburst if the models of Smak and Ogilvie are correct. This is confirmed by the fact that hints of spiral asymmetries have already been found in the discs of SS Cyg, EX Dra and U Gem.  相似文献   

10.
We present K -band spectra of the short-period dwarf novae YZ Cnc, LY Hya, BK Lyn, T Leo, SW UMa and WZ Sge, the nova-like variables DW UMa, V1315 Aql, RW Tri, VY Scl, UU Aqr and GP Com, and a series of field dwarf stars with spectral types ranging from K2 to M6.
The spectra of the dwarf novae are dominated by emission lines of H  i and He  i . The large velocity and equivalent widths of these lines, in conjunction with the fact that the lines are double-peaked in the highest inclination systems, indicate an accretion disc origin. In the case of YZ Cnc and T Leo, for which we obtained time-resolved data covering a complete orbital cycle, the emission lines show modulations in their equivalent widths that are most probably associated with the bright spot (the region where the gas stream collides with the accretion disc). There are no clear detections of the secondary star in any of the dwarf novae below the period gap, yielding upper limits of 10–30 per cent for the contribution of the secondary star to the observed K -band flux. In conjunction with the K -band magnitudes of the dwarf novae, we use the derived secondary star contributions to calculate lower limits to the distances to these systems.
The spectra of the nova-like variables are dominated by broad, single-peaked emission lines of H  i and He  i – even the eclipsing systems we observed do not show the double-peaked profiles predicted by standard accretion disc theory. With the exception of RW Tri, which exhibits Na  i , Ca  i and 12CO absorption features consistent with a M0V secondary contributing 65 per cent of the observed K -band flux, we find no evidence for the secondary star in any of the nova-like variables. The implications of this result are discussed.  相似文献   

11.
We present a phase-resolved ROSAT HRI X-ray light curve of the dwarf nova OY Car in quiescence. The X-ray flux is eclipsed at the same time as the optical eclipse of the primary, and the region of X-ray emission is comparable in size to the white dwarf. We use subsequent optical observations to update the orbital ephemeris of the system.  相似文献   

12.
We study the absorption lines present in the spectra of the long-period cataclysmic variable GK Per during its quiescent state, which are associated with the secondary star. By comparing quiescent data with outburst spectra we infer that the donor star appears identical during the two states and the inner face of the secondary star is not noticeably irradiated by flux from the accreting regions. We obtain new values for the radial velocity semi-amplitude of the secondary star,     , a projected rotational velocity,     and consequently a measurement of the stellar mass ratio of GK Per,     . The inferred white dwarf radial velocities are greater than those measured traditionally using the wings of Doppler-broadened emission lines suspected to originate in an accretion disc, highlighting the unsuitability of emission lines for mass determinations in cataclysmic variables. We determine mass limits for both components in the binary,     and     .  相似文献   

13.
The first supersoft source (SSS) identification with an optical nova in M 31 was based on ROSAT observations. Twenty additional X‐ray counterparts (mostly identified as SSS by their hardness ratios) were detected using archival ROSAT, XMM‐Newton and Chandra observations obtained before July 2002. Based on these results optical novae seem to constitute the major class of SSS in M 31. An analysis of archival Chandra HRC‐I and ACIS‐I observations obtained from July 2004 to February 2005 demonstrated that M 31 nova SSS states lasted from months to about 10 years. Several novae showed short X‐ray outbursts starting within 50 d after the optical outburst and lasting only two to three months. The fraction of novae detected in soft X‐rays within a year after the optical outburst was more than 30%. Ongoing optical nova monitoring programs, optical spectral follow‐up and an up‐to‐date nova catalogue are essential for the X‐ray work. Re‐analysis of archival nova data to improve positions and find additional nova candidates are urgently needed for secure recurrent nova identifications. Dedicated XMM‐Newton/Chandra monitoring programs for X‐ray emission from optical novae covering the centre area of M 31 continue to provide interesting new results (e.g. coherent 1105 s pulsations in the SSS counterpart of nova M31N 2007‐12b). The SSS light curves of novae allow us – together with optical information – to estimate the mass of the white dwarf, of the ejecta and the burned mass in the outburst. Observations of the central area of M 31 allow us – in contrast to observations in the Galaxy – to monitor many novae simultaneously and proved to be prone to find many interesting SSS and nova types (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

14.
We present optical spectroscopy and optical and infrared photometry of the neutron star soft X-ray transient Aql X–1 during its X-ray outburst of 1997 August. By modelling the X-ray, optical and IR light curves, we find a 3-d delay between the IR and X-ray rise times, analogous to the UV–optical delay seen in dwarf novae outbursts and black hole X-ray transients. We interpret this delay as the signature of an 'outside-in' outburst, in which a thermal instability in the outer disc propagates inward. This outburst is the first of this type definitively identified in a neutron star X-ray transient.  相似文献   

15.
We re-analyse the ASCA Ginga X-ray data from BY Cam, a slightly asynchronous magnetic accreting white dwarf. The spectra are strongly affected by complex absorption, which we model as a continuous (power-law) distribution of covering fraction and column of neutral material. This absorption causes a smooth hardening of the spectrum below ∼ 3 keV, and is probably produced by material in the pre-shock column which overlies the X-ray emission region. The ASCA data show that the intrinsic emission from the shock is not consistent with a single-temperature plasma. Significant iron L emission coexisting with iron K shell lines from H- and He-like iron clearly shows that there is a wide range of temperatures present, as expected from a cooling shock structure. The Ginga data provide the best constraints on the maximum temperature emission in the shocked plasma, with kT max = 21+18−4 keV. Cyclotron cooling should also be important; it suppresses the highest temperature bremsstrahlung components, so the X-ray data provide only a lower limit on the mass of the white dwarf of M  ≥ 0.5 M⊙. Reflection of the multitemperature bremsstrahlung emission from the white dwarf surface is also significantly detected.   We stress the importance of modelling all these effects in order to gain a physically self-consistent picture of the X-ray spectra from polars in general and BY Cam in particular.  相似文献   

16.
For several novae, a bright X‐ray source with a spectrum resembling the class of Super Soft X‐ray Sources (SSS) has been observed a few weeks to months after outburst. Novae are powered by explosive nuclear burning on the surface of a white dwarf, and enough energy is produced to power a radiatively driven wind. Owing to the evolution of the opacity of the ejecta, the observable spectrum gradually shifts from optical to soft X‐rays (SSS phase). It has sometimes been assumed that at the beginning of the SSS phase no more mass loss occurs. However, high‐resolution X‐ray spectra of some novae have shown highly blue‐shifted absorption lines, indicating a significant expansion. In this paper, I show that all novae that have been observed with X‐ray gratings during their SSS phase show significant blue shifts. I argue that all models that attempt to explain the X‐ray bright SSS phase have to accommodate the continued expansion of the ejecta (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

17.
With extensive monitoring data spanning over 30 years from Vela 5B , Ariel 5 , Ginga , Compton Gamma Ray Observatory , Rossi X-ray Timing Explorer and BeppoSAX , we find evidence for long-term X-ray variability on time-scales     from the black hole low-mass X-ray binary system     . Such variability resembles the outburst cycle of Z Cam-type dwarf novae, in which the standard disc instability model plays a crucial role. If such a model is applicable to     , then the observed variability might be due to the irradiation of an unstable accretion disc. We show that within the framework of the X-ray irradiation model, when the accretion rate exceeds a critical value,     enters a 'flat-topped' high/soft state, such as seen in 1998, which we suggest corresponds to the 'standstill' state of Z Cam systems.  相似文献   

18.
We present spectroscopy and photometry of GD 448, a detached white dwarf – M dwarf binary with a period of 2.47 h. We find that the Na  I  8200-Å feature is composed of narrow emission lines, owing to irradiation of the M dwarf by the white dwarf, within broad absorption lines that are essentially unaffected by heating. Combined with an improved spectroscopic orbit and gravitational redshift measurement from spectra of the Hα line, we are able to derive masses for the white dwarf and M dwarf directly (0.41 ± 0.01 and 0.096 ± 0.004 M⊙, respectively). We use a simple model of the Ca II emission lines to establish the radius of the M dwarf assuming the emission from its surface to be proportional to the incident flux per unit area from the white dwarf. The radius derived is 0.125 ± 0.020 R⊙. The M dwarf appears to be a normal main-sequence star in terms of its mass and radius, and is less than half the size of its Roche lobe. The thermal time-scale of the M dwarf is much longer than the cooling age of the white dwarf, so we conclude that the M dwarf was unaffected by the common-envelope phase. The anomalous width of the Hα emission from the M dwarf remains to be explained, but the strength of the line may be due to X-ray heating of the M dwarf owing to accretion on to the white dwarf from the M dwarf wind.  相似文献   

19.
We argue that the quiescent value of the viscosity parameter of the accretion disc in WZ Sge may be  αcold∼ 0.01  , in agreement with estimates of αcold for other dwarf novae. Assuming the white dwarf in WZ Sge to be magnetic, we show that, in quiescence, material close to the white dwarf can be propelled to larger radii, depleting the inner accretion disc. The propeller therefore has the effect of stabilizing the inner disc and allowing the outer disc to accumulate mass. The outbursts of WZ Sge are then regulated by the (magnetically determined) evolution of the surface density of the outer disc at a radius close to the tidal limit. Numerical models confirm that the recurrence time can be significantly extended in this way. The outbursts are expected to be superoutbursts since the outer disc radius is forced to exceed the tidal (3:1 resonance) radius. The large, quiescent disc is expected to be massive, and to be able to supply the observed mass accretion rate during outburst. We predict that the long-term spin evolution of the white dwarf spin will involve a long cycle of spin-up and spin-down phases.  相似文献   

20.
We present the spectra of accretion discs around white dwarfs calculated with an improved and updated version of Shaviv and Wehrse [Shaviv, G., Wehrse, R., 1991. A&A 251, 117] model. The new version includes line opacities and convective energy transport and can be used to calculate the spectra of hot discs in bright systems (nova-like variables or dwarf novae in outburst) as well as the spectra of cold accretion discs in quiescent dwarf novae.  相似文献   

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