The X-ray and extreme-ultraviolet flux evolution of SS Cygni throughout outburst

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.     

The boundary layer of VW Hydri in quiescence

In this short paper, we suggest that the missing boundary layer luminosity of dwarf novae in quiescence is released mainly in the ultraviolet (UV) as the second component commonly identified in the far-UV as the 'accretion belt'. We present the well-studied SU UMa-type system VW Hyi in detail as a prototype for such a scenario. We consider detailed multiwavelength observations and in particular the recent Far Ultraviolet Spectroscopic Explorer ( FUSE ) observations of VW Hyi in quiescence, which confirm the presence of a second component (the 'accretion belt') in the far-UV spectrum. The temperature  (≈ 50 000 K)  and rotational velocity  (≈ 3000 km s ⁻¹)  of this second component are entirely consistent with the optically thick region  (τ≈ 1)  located just at the outer edge of optically thin boundary layer in the simulations of Popham. This second component contributes about 20 per cent of the far-UV flux. Using geometrical assumptions and taking into account the X-ray luminosity, we find that the total boundary layer luminosity sums up to   L _BL= (0.53 ± 0.25) L _disc  , while the theory (Kluźniak) predicts, for the rotation rate of VW Hyi's white dwarf,   L _BL≈ (0.76 ± 0.03) L _disc  . About one-fifth of the boundary layer energy is emitted in the X-ray and the remaining is emitted in the UV. This scenario is consistent with the recent simultaneous X-ray and UV observations of VW Hyi by Pandel, Córdova & Howell, from which we deduce here that the viscosity in the boundary layer region must be of the order of  ν≈ 10¹³–10¹⁴ cm² s ⁻¹  , depending on the white dwarf mass and the size of the boundary layer.     

Indirect imaging of the accretion stream in eclipsing polars – II. HU Aquarii

We apply our technique for indirect imaging of the accretion stream to the polar HU Aquarii, using eclipse profiles observed when the system was in a high accretion state. The accretion stream is relatively luminous, contributing as much as the accretion region on the white dwarf, or more, to the overall system brightness. We model the eclipse profiles using a model stream consisting of a ballistic trajectory from the L1 point followed by a magnetically channelled trajectory that follows a dipole field line out of the orbital plane. We perform model fits using two geometries: a stream that accretes on to both footpoints of the field line, and a stream that accretes only on to the footpoint of the field line above the orbital plane. The stream images indicate that the distribution of emission along the stream is not a simple function of the radial distance from the white dwarf. The stream is redirected by the magnetic field of the white dwarf at a distance 1.0–1.3×10¹⁰ cm from the white dwarf; this implies a mass transfer rate in the range 8–76×10¹⁶ g s⁻¹. The absorption dips in the light curve indicate that the magnetically entrained part of the stream moves from 42° to 48° from the line of centres over the three nights of observation. This is in close agreement with the results of the one-footpoint models, suggesting that this is the more appropriate geometry for these data. The stream images show that, in almost all sections of the stream, the flux peaks in B and is successively fainter in U , V and R .     

An extremely slow nova?

The binary companion to the peculiar F supergiant HD 172481 is shown to be a Mira variable with a pulsation period of 312 d. Its characteristics are within the normal range found for solitary Miras of that period, although its pulsation amplitude and mass-loss rate ̇ ∼3×10⁻⁶ M_⊙ yr⁻¹ are higher than average. Reasons are given for suspecting that the F supergiant, which has L ∼10⁴ L_⊙, is a white dwarf burning hydrogen accreted from its companion.     

Inflow and outflow from the accretion disc of the microquasar SS 433: UKIRT spectroscopy

A succession of near-infrared (near-IR) spectroscopic observations, taken nightly throughout an entire cycle of SS 433's orbit, reveal (i) the persistent signature of SS 433's accretion disc, having a rotation speed of  ∼500 km s⁻¹  , (ii) the presence of circumbinary disc recently discovered at optical wavelengths by Blundell, Bowler & Schmidtobreick (2008) and (iii) a much faster outflow than has previously been measured for the disc wind, with a terminal velocity of  ∼1500 km s⁻¹  . The increased wind terminal velocity results in a mass-loss rate of  ∼10⁻⁴ M_⊙ yr⁻¹  . These, together with the newly (upwardly) determined masses for the components of the SS 433 system, result in an accurate diagnosis of the extent to which SS 433 has super-Eddington flows. Our observations imply that the size of the companion star is comparable with the semiminor axis of the orbit which is given by     , where e is the eccentricity. Our relatively spectral resolution at these near-IR wavelengths has enabled us to deconstruct the different components that comprise the Brackett-γ (Brγ) line in this binary system, and their physical origins. With this line being dominated throughout our series of observations by the disc wind, and the accretion disc itself being only a minority (∼15 per cent) contribution, we caution against use of the unresolved Brγ line intensity as an 'accretion signature' in X-ray binaries or microquasars in any quantitative way.     

Hot accretion discs with thermal Comptonization and advection in luminous black hole sources

We solve for the structure of a hot accretion disc with unsaturated thermal Comptonization of soft photons and with advection, generalizing the classical model of Shapiro et al. The upper limit on the accretion rate due to advection constrains the luminosity to ≲ 0.15 y^3/5 α^7/5 of the Eddington limit, where y and α are the Compton and viscosity parameters, respectively. The characteristic electron temperature and Thomson optical depth of the inner flow at accretion rates within an order of magnitude of that upper limit are ∼ 10⁹ K and ∼ 1, respectively. The resulting spectra are then in close agreement with the X-ray and soft γ-ray spectra from black hole binaries in the hard state and Seyferts. At low accretion rates, bremsstrahlung becomes the dominant radiative process.     

Episodic accretion in magnetically layered protoplanetary discs

We study protoplanetary disc evolution assuming that angular momentum transport is driven by gravitational instability at large radii, and magnetohydrodynamic (MHD) turbulence in the hot inner regions. At radii of the order of 1 au such discs develop a magnetically layered structure, with accretion occurring in an ionized surface layer overlying quiescent gas that is too cool to sustain MHD turbulence. We show that layered discs are subject to a limit cycle instability, in which accretion on to the protostar occurs in ∼10⁴-yr bursts with ̇ ∼10⁻⁵ M_⊙ yr⁻¹, separated by quiescent intervals lasting ∼10⁵ yr where ̇ ≈10⁻⁸ M_⊙ yr⁻¹. Such bursts could lead to repeated episodes of strong mass outflow in young stellar objects. The transition to this episodic mode of accretion occurs at an early epoch ( t ≪1 Myr), and the model therefore predicts that many young pre-main-sequence stars should have low rates of accretion through the inner disc. At ages of a few Myr, the discs are up to an order of magnitude more massive than the minimum-mass solar nebula, with most of the mass locked up in the quiescent layer of the disc at r ∼1 au. The predicted rate of low-mass planetary migration is reduced at the outer edge of the layered disc, which could lead to an enhanced probability of giant planet formation at radii of 1–3 au.     

The 'off' state of GX 339–4

We report BeppoSAX and optical observations of the black hole candidate GX 339–4 during its X-ray 'off' state in 1999. The broad-band (0.8–50 keV) X-ray emission can be fitted by a single power law with spectral index, α ∼1.6. The observed luminosity is 6.6×10³³ erg s⁻¹ in the 0.5–10 keV band, which is at the higher end of the flux distribution of black hole soft X-ray transients in quiescence, comparable to that seen in GS 2023+338 and 4U 1630–47. An optical observation just before the BeppoSAX observation shows the source to be very faint at these wavelengths as well ( B =20.1, V =19.2). By comparing with previously reported 'off' and low states (LS), we conclude that the 'off' state is actually an extension of the LS, i.e. an LS at lower intensities. We propose that accretion models such as the advection-dominated accretion flows are able to explain the observed properties in such a state.     

BF Eridani: a cataclysmic variable with a massive white dwarf and an evolved secondary

We present high- and medium-resolution spectroscopic observations of the cataclysmic variable BF Eridani (BF Eri) during its low and bright states. The orbital period of this system was found to be 0.270881(3) d. The secondary star is clearly visible in the spectra through the absorption lines of the neutral metals Mg  i , Fe  i and Ca  i . Its spectral type was found to be K3±0.5. A radial velocity study of the secondary yielded a semi-amplitude of   K ₂= 182.5 ± 0.9 km s⁻¹  . The radial velocity semi-amplitude of the white dwarf was found to be   K ₁= 74 ± 3 km s⁻¹  from the motion of the wings of the Hα and Hβ emission lines. From these parameters, we have obtained that the secondary in BF Eri is an evolved star with a mass of  0.50–0.59 M_⊙  , whose size is about 30 per cent larger than a zero-age main-sequence single star of the same mass. We also show that BF Eri contains a massive white dwarf  ( M ₁≥ 1.2 M_⊙)  , which allows us to consider the system as a Type Ia supernova progenitor. BF Eri also shows a high γ-velocity  (γ=−94 km s⁻¹)  and substantial proper motion. With our estimation of the distance to the system  ( d ≈ 700 ± 200 pc)  , this corresponds to a space velocity of ∼350 km s⁻¹ with respect to the dynamical local standard of rest. The cumulative effect of repeated nova eruptions with asymmetric envelope ejection might explain the high space velocity of the system. We analyse the outburst behaviour of BF Eri and question the current classification of the system as a dwarf nova. We propose that BF Eri might be an old nova exhibiting 'stunted' outbursts.     

Evidence of a high-velocity ionized outflow in a second narrow-line quasar PG 0844+349

Following the discovery of X-ray absorption in a high-velocity outflow from the bright quasar PG 1211 + 143 we have searched for similar features in XMM–Newton archival data of a second (high accretion rate) quasar PG 0844+349. Evidence is found for several faint absorption lines in both the EPIC and RGS spectra, whose most likely identification with resonance transitions in H-like Fe, S and Ne implies an origin in highly ionized matter with an outflow velocity of order ∼0.2c. The line equivalent widths require a line-of-sight column density of   N _H∼ 4 × 10²³ cm⁻²  , at an ionization parameter of log  ξ∼ 3.7  . Assuming a radial outflow being driven by radiation pressure from the inner accretion disc, as suggested previously for PG 1211 + 143, the flow in PG 0844+349 is also likely to be optically thick, in this case within ∼25 Schwarzschild radii. Our analysis suggests that a high-velocity, highly ionized outflow is likely to be a significant component in the mass and energy budgets of active galactic nuclei accreting at or above the Eddington rate.     

A spectroscopic study of V603 Aquilae: stellar parameters and continuumline variations

A time-resolved spectroscopic study of V603 Aql (Nova Aquilae 1918) is presented. An orbital period of P _orb=01385±00002, consistent with previous results, and a radial velocity semi-amplitude of K =20±3 km s¹ are obtained from the radial velocity variations of the H emission line. Similar K values are also found in H , H , and He  i emission lines. Using the measured FWHM of the H line and assuming that the derived semi-amplitude is that of the white dwarf, we deduce a most likely mass ratio of q =0.24±0.05 and stellar masses of M ₂=0.29±0.04 M and M ₁=1.2±0.2 M for the secondary and primary (the white dwarf) star, respectively. The dynamical solution also indicates a very low orbital inclination, i =13°±2°. We find that the continuum and line variations are modulated with both the positive and the negative superhump periods, indicating that they arise from similar regions of the accretion disc. Moreover, we find, for the first time from spectroscopy, evidence of negative superhumps in addition to the positive superhumps. Positive superhumps are explained within the disc instability model as caused by an eccentric disc surrounding the white dwarf, which is precessing (apsidal advance) because of tidal instabilities, causing the observed positive superhumps. A nodal precession in the accretion disc is currently believed to be the cause of the observed negative superhumps. The low value of q is consistent with the expected value for systems that show superhumps, in accordance with the eccentric disc model. We find no evidence of periodicity associated with the spin period.     

Ultraluminous X-ray source 1E 0953.8+6918 (M81 X-9): an intermediate-mass black hole candidate and its environs

We present a ROSAT and ASCA study of the Einstein source X-9 and its relation to a shock-heated shell-like optical nebula in a tidal arm of the M81 group of interacting galaxies. Our ASCA observation of the source shows a flat and featureless X-ray spectrum well described by a multicolour disc blackbody model. The source most likely represents an optically thick accretion disc around an intermediate-mass black hole  ( M ∼10² M_⊙)  in its high/soft state, similar to other variable ultraluminous X-ray sources observed in nearby disc galaxies. Using constraints derived from both the innermost stable orbit around a black hole and the Eddington luminosity, we find that the black hole is fast-rotating and that its mass is between ∼80 M_⊙–1.5×10² M_⊙. The inferred bolometric luminosity of the accretion disc is ∼(1.1×10⁴⁰ erg s⁻¹)/(cos  i ). Furthermore, we find that the optical nebula is very energetic and may contain large amounts of hot gas, accounting for a soft X-ray component as indicated by archival ROSAT PSPC data. The nebula is apparently associated with X-9; the latter may be powering the former and/or they could be formed in the same event (e.g. a hypernova). Such a connection, if confirmed, could have strong implications for understanding both the birth of intermediate-mass black holes and the formation of energetic interstellar structures.     

Investigating the structure of the accretion disc in WZ Sge from multiwaveband time-resolved spectroscopic observations – I

We present the first of two papers describing an in-depth study of multiwaveband phase-resolved spectroscopy of the unusual dwarf nova WZ Sge. In this paper we present an extensive set of Doppler maps of WZ Sge covering optical and infrared emission lines, and describe a new technique for studying the accretion discs of cataclysmic variables using ratioed Doppler maps. Applying the ratioed Doppler map technique to our WZ Sge data shows that the radial temperature profile of the disc is unlike that predicted for a steady state α disc. Time-averaged spectra of the accretion disc line flux (with the bright spot contribution removed) show evidence in the shapes of the line profiles for the presence of shear broadening in a quiescent non-turbulent accretion disc. From the positions of the bright spots in the Doppler maps of different lines, we conclude that the bright spot region is elongated along the ballistic stream, and that the density of the outer disc is low. The velocity of the outer edge of the accretion disc measured from the H α line is found to be 723±23 km s⁻¹. Assuming that the accretion disc reaches to the 3:1 tidal resonance radius, we derive a value for the primary star mass of 0.82 M_⊙. We discuss the implications of our results on the present theories of WZ Sge type dwarf nova outbursts.     

Modelling the formation of galaxy clusters in MOND

We present time-resolved spectroscopy and photometry of the cataclysmic variable (CV) SDSS J133941.11+484727.5 (SDSS 1339) which has been discovered in the Sloan Digital Sky Survey (SDSS) Data Release 4. The orbital period determined from radial velocity studies is 82.524(24) min, close to the observed period minimum. The optical spectrum of SDSS 1339 is dominated to 90 per cent by emission from the white dwarf (WD). The spectrum can be successfully reproduced by a three-component model (white dwarf, disc, secondary) with   T _WD=12 500 K  for a fixed  log   g = 8.0, d = 170 pc  , and a spectral type of the secondary later than M8. The mass-transfer rate corresponding to the optical luminosity of the accretion disc is very low,  ≃ 1.7 × 10⁻¹³ M_⊙ yr⁻¹  . Optical photometry reveals a coherent variability at 641 s with an amplitude of 0.025 mag, which we interpret as non-radial pulsations of the white dwarf. In addition, a long-period photometric variation with a period of either 320 or 344 min and an amplitude of 0.025 mag is detected, which bears no apparent relation with the orbital period of the system. Similar long-period photometric signals have been found in the CVs SDSS J123813.73−033933.0, SDSS J204817.85−061044.8, GW Lib and FS Aur, but so far no working model for this behaviour is available.     

Swift observations of GW Lib: a unique insight into a rare outburst

The second known outburst of the WZ Sge type dwarf nova GW Lib was observed in 2007 April. We have obtained unique multiwavelength data of this outburst which lasted ∼26 days. The American Association of Variable Star Observers ( AAVSO ) recorded the outburst in the optical, which was also monitored by Wide Angle Search for Planets , with a peak V magnitude of ∼8. The outburst was followed in the ultraviolet and X-ray wavelengths by the Swift ultraviolet/optical and X-ray telescopes. The X-ray flux at optical maximum was found to be three orders of magnitude above the pre-outburst quiescent level, whereas X-rays are normally suppressed during dwarf nova outbursts. A distinct supersoft X-ray component was also detected at optical maximum, which probably arises from an optically thick boundary layer. Follow-up Swift observations taken 1 and 2 years after the outburst show that the post-outburst quiescent X-ray flux remains an order of magnitude higher than the pre-outburst flux. The long interoutburst time-scale of GW Lib with no observed normal outbursts support the idea that the inner disc in GW Lib is evacuated or the disc viscosity is very low.     

Discovery of type I X-ray bursts from the low-mass X-ray binary 4U 1708 – 40

We report the discovery of type I X-ray bursts from the low-mass X-ray binary  4U 1708 − 40  during the 100-ks observation performed by BeppoSAX on 1999 August 15–16. Six X-ray bursts have been observed. The unabsorbed 2–10 keV fluxes of the bursts range from ∼3 to  9 × 10⁻¹⁰ erg cm⁻² s⁻¹  . A correlation between peak flux and fluence of the bursts is found, in agreement with the behaviour observed in other similar sources. There is a trend of the burst flux to increase with the time interval from the previous burst. From the value of the persistent flux we infer a mass accretion rate     , which may correspond to the mixed hydrogen/helium burning regime triggered by thermally unstable hydrogen. We have also analysed a BeppoSAX observation performed on 2001 August 22 and previous RXTE observations of  4U 1708 − 40  , where no bursts have been observed; we find persistent fluxes of more than a factor of 7 higher than the persistent flux observed during the BeppoSAX observation showing X-ray bursts.     

The component masses of the cataclysmic variable V347 Puppis

We present time-resolved spectroscopy and photometry of the double-lined eclipsing cataclysmic variable V347 Pup (=LB 1800). There is evidence of irradiation on the inner hemisphere of the secondary star, which we correct for using a model to give a secondary-star radial velocity of   K _R= 198 ± 5 km s⁻¹  . The rotational velocity of the secondary star in V347 Pup is found to be   v sin  i = 131 ± 5 km s⁻¹  and the system inclination is   i = 840 ± 23  . From these parameters we obtain masses of   M ₁= 0.63 ± 0.04 M_⊙  for the white dwarf primary and   M ₂= 0.52 ± 0.06 M_⊙  for the M0.5V secondary star, giving a mass ratio of   q = 0.83 ± 0.05  . On the basis of the component masses, and the spectral type and radius of the secondary star in V347 Pup, we find tentative evidence for an evolved companion. V347 Pup shows many of the characteristics of the SW Sex stars, exhibiting single-peaked emission lines, high-velocity S-wave components and phase-offsets in the radial velocity curve. We find spiral arms in the accretion disc of V347 Pup and measure the disc radius to be close to the maximum allowed in a pressureless disc.     

A ROSAT observation of the dipping, flaring and eclipsing polar MN Hya (RX J0929.1–2404)

We present the results of a 22.5 ks pointed ROSAT PSPC observation of the 3.4-h period eclipsing polar MN Hya (RX J0929.1−2404). The X-ray light curve exhibits a 'double-humped' shape, with a secondary minimum occuring at φ∼ 0.45, a morphology consistent with two-pole accretion. Strong aperiodic flaring activity, with flux enhancements of ∼ 6 × the quiescent level, is also observed. A pre-eclipse 'dip' occurs in the phase interval φ= 0.87–0.95 with the X-rays becoming harder, indicative of photoelectric absorption by the pre-shock flow. There is also evidence of a secondary spectrally hard 'dip' near φ = 0.45–0.55, which might be associated with a second accretion stream flowing to the other magnetic pole.   The X-ray spectrum is best represented by a combination of a ∼50 eV blackbody and a thermal bremsstrahlung component of kT 1.6 keV, with a total absorption column of N _H  = 2.9 × 10²⁰ cm⁻².   The primary maximum (φ∼ 0.65) has a slightly larger column and normalization compared to the secondary maximum. Although there are few photons, the dip spectrum is very flat in comparison to other phases, and is best represented by a single bremsstrahlung component. This is indicative of the spectral hardening seen in the light curves attributed to photoabsorption. The ratio of unabsorbed bremsstrahlung and blackbody luminosities is ∼ 0.1 for the best-fitting average spectral models. This implies a magnetic field strength  30 MG on the basis of the empirical L _hard/ L _soft −  B relationships, although consideration of the cyclotron flux and aspect effects could allow for an even higher field (55 MG).     

PSR J1012+5307: younger than it looks?

Lorimer et al. have recently reported that the spin-down age (∼7 × 10⁹ yr) of the low-mass binary pulsar PSR J1012+5307 is much higher than the cooling age (3 × 10⁸ yr) of its white dwarf companion. The proposed solutions for this discrepancy are outlined and discussed. In particular, the revised cooling age estimate proposed by Alberts et al. agrees with data from other low-mass binary pulsar systems if a transition to the 'classical' cooling regime occurs between ∼0.14 and ∼0.28 M_⊙. If this transition is excluded, PSR J1012+5307 seems to have finished its accretion phase far from the spin-up line.     

Boundary layer on the surface of a neutron star

In an attempt to model the accretion on to a neutron star in low-mass X-ray binaries, we present 2D hydrodynamical models of the gas flow in close vicinity of the stellar surface. First, we consider a gas pressure-dominated case, assuming that the star is non-rotating. For the stellar mass we take   M _star= 1.4 × 10⁻² M_⊙  and for the gas temperature   T = 5 × 10⁶ K  . Our results are qualitatively different in the case of a realistic neutron star mass and a realistic gas temperature of T ≃ 10⁸ K, when the radiation pressure dominates. We show that to get the stationary solution in a latter case, the star most probably has to rotate with the considerable velocity.