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We study a sample of six X-ray-selected broad absorption line (BAL) quasi-stellar objects (QSOs) from the XMM-Newton Wide Angle Survey. All six objects are classified as BALQSOs using the classic balnicity index, and together they form the largest sample of X-ray-selected BALQSOs. We find evidence for absorption in the X-ray spectra of all six objects. An ionized absorption model applied to an X-ray spectral shape that would be typical for non-BAL QSOs (a power law with energy index ¿ = 0.98) provides acceptable fits to the X-ray spectra of all six objects. The optical to X-ray spectral indices, ¿OX, of the X-ray-selected BALQSOs, have a mean value of = 1.69 ± 0.05, which is similar to that found for X-ray-selected and optically selected non-BAL QSOs of a similar ultraviolet luminosity. In contrast, optically selected BALQSOs typically have much larger ¿OX and so are characterized as being X-ray weak. The results imply that X-ray selection yields intrinsically X-ray bright BALQSOs, but their X-ray spectra are absorbed by a similar degree to that seen in optically selected BALQSO samples; X-ray absorption appears to be ubiquitous in BALQSOs, but X-ray weakness is not. We argue that BALQSOs sit at one end of a spectrum of X-ray absorption properties in QSOs related to the degree of ultraviolet absorption in C IV 1550 Å., MJP acknowledges financial support from the UK Science and Technology Facilities Council. FJC and SM acknowledge financial support through grant AYA2015-64346-C2-1-P (MINECO/FEDER). MTC acknowledges support by the Spanish Programma Nacional de Astronomia y Astrofisica under grant AYA2009-08059. MK acknowledges support by DFG grant KR 3338/3-1.

The probability of photon measurement in some photon counting instrumentation, such as the Optical Monitor on the XMM-Newton satellite, and the UVOT on the Swift satellite, does not follow a Poisson distribution due to the detector characteristics, but a Binomial distribution. For a single-pixel approximation, an expression was derived for the incident countrate as a function of the measured count rate by Fordham, Moorhead and Galbraith (2000). We show that the measured countrate error is binomial, and extend their formalism to derive the error in the incident count rate. The error on the incident count rate at large count rates is larger than the Poisson-error of the incident count rate.

We present the results of X-ray and optical observations of GRB050712 performed by Swift. The X-ray lightcurve of this burst exhibits episodes of flares in the first 1000s, the same epoch at which the UVOT detected an optical counterpart. A shallow X-ray decay, with a decay slope of -0.73, followed and lasted ~70ks. This behaviour can be explained in terms of activity of GRB 'inner engine', with the possibility of that the last flare is caused by the interaction of the ejecta with the surrounding medium. We also find interesting spectral parameters for the X-ray emission. In particular, data suggest the presence of an intrinsic absorption in the first 1000s, which can be explained if circumbust medium clouds lie along the line of sight.

Multiwavelength monitoring observations of the AM Her system, QS Tel, are presented, the centrepiece being a coordinated campaign with the ASCA, EUVE and HST satellites and optical observations on 1996 September 28–29. The star was initially observed in an intermediate brightness, single pole state in 1996 April by ROSAT. However two EUVE snapshots of the system in August found it in a deep low state, the light curves appearing to retain a bright and faint phase morphology although the bright interval showed significant phase drifts compared with the earlier ROSAT data. At this time, the system appeared optically bright, while polarimetry pointed to one active pole. A modest recovery of intensity, albeit accompanied by an apparently chaotic extreme ultraviolet (EUV) light curve, was subsequently recorded during the coordinated run in late September. This brightening was confirmed by a second ROSAT pointing in early October, although the EUV brightening is matched by a marked decay in its optical output. These observations, all of which found the system in a fainter EUV state than seen in 1993, add to the catalogue of accretion mode behaviour in QS Tel. However, as yet, the data base of observations does not allow us to establish whether the accretion mode is dictated by accretion-rate dependent penetration of the magnetosphere or asynchronous rotation. The ASCA spectrum obtained during the coordinated observation is adequately described by a thermal spectrum with a temperature of about 4.4 keV and an absorbing column density of less than 8×1020 cm−2. At the same time, the 1150–2600 A UV HST spectrum shows a strong, broad Lyα absorption profile which we attribute to the white dwarf. We infer an effective temperature of 16 000–19 000 K and a mass between 0.5 and 0.93 M⊙ for the white dwarf, at least some of the uncertainty arising because the profile is very probably contaminated and broadened by Zeeman features. The orbital variation seen by HST can be matched by a blackbody-like spectrum with a temperature above 24 000 K. The near UV part of the spectrum contains evidence of phase-variable cyclotron harmonics.

‘The definitive version is available at www.blackwell-synergy.com.’ Copyright Blackwell Publishing. DOI: 10.1046/j.1365-8711.1998.01844.x [Full text of this article is not available in the UHRA]

Results of the first low-resolution (6 A), time-resolved Hubble Space Telescope (HST) Faint Object Spectrograph (FOS) observation of the prototypical strong-field intermediate polar system, PQ Gem, are presented. The AAl150-2600 continuum light curve is dominated by the 13.9-min rotational signature of the white dwarf at all UV wavelengths covered, with a broadly constant fractional modulation depth. The rotational profile contains a dip which is deepest in the far-UVand which we believe, like its X-ray counterpart, is caused by stream occultation of the white dwarf. The continuum and emission-line fluxes are also modulated on the 14.5-min beat period but, remarkably, vary in antiphase. This complex behaviour facilitates the identification and partial isolation of two spin­ modulated spectral components and a beat component. One spin component has a blue spectral distribution whose temperature is :$ 50000 K if no allowance is made for the absorbing effects of the stream, but may be much hotter (consistent with earlier X-ray estimates) if, as seems likely, the absorber is not completely optically thick. The other spin-modulated component has a red spectral distribution whose temperature (:$ 10000 K) and luminosity probably associate it with the magnetospheric accretion flow itself. The beat continuum component has a temperature in the region of 17 000 K and appears to be radiated by a region whose size is comparable to that of the white dwarf. The beat pulsation in the emission lines may also originate from a region of similar dimensions. We consider one- and two-site hypotheses to explain the antiphased line and continuum beat modulations, but are unable to arrive at a convincing solution.

The optical emission-line properties of the nova-like cataclysmic variable V795 Her are studied using high-resolution Isaac Newton Telescope [Intermediate Dispersion Spectrograph (IDS) and Faint-Object Spectrograph (FOS)] data secured during 1994 June. The observed Balmer and He emission lines, and He I lambda 4471 absorption, modulate on the 2.6-h orbital period of this system. The detailed nature of these changes is complex, however, with the incidence of separate low- and high-velocity fluctuations. Low-intensity 'S'-wave features are apparent in the blue and red outer wings of the Balmer lines, extending to similar to +/- 1600 km s(-1). These phased changes are seen as two synchronized components with net blueshift and redshift. The Balmer profiles are also affected by absorption which dominates during the first part of the cycle. We propose an empirical interpretation of this behaviour in terms of a single broad emission feature which follows a high-velocity 'S' wave, plus a superimposed, sympathetically phased, absorption component. The associated physical interpretation is that of a gas stream (seen in absorption) that overflows the outer accretion disc and re-impacts on the inner disc regions.The 'double-peaked' inner cores of the Balmer lines have relatively fixed central velocities, and do not reveal an obvious 'S'-wave variation. This inner-core emission likely arises from material located in the outer regions of the accretion disc. An similar to 10 per cent variation is present in the continuum flux of V795 Her, restricted to less than 25 per cent of the system orbit. We find some correspondence between the overall optical emission changes and those found by us in the UV resonance lines of V795 Her in a separate study based on Hubble Space Telescope data.We discuss our observational results in terms of magnetic white dwarf and disc-overflow gas stream scenarios. Constraints are presented that argue against recently proposed models for V795 Her which invoke channelled accretion columns flowing on to a synchronously rotating white dwarf. Disc-overflow model simulations are presented which quantitatively account for several of the observed dynamical properties of V795 Her. There remain some discrepancies, however, which call for a more detailed physical model.

We present infrared (0.97-2.45 micron) spectra of the intermediate polars PQ Gem (RE0751+14), BG CMi and EX Hya. The spectra show strong Paschen, Brackett and HeI emission lines from the accretion disc/stream. The infrared continua of PQ Gem and BG CMi can be represented by blackbodies of temperatures 4500 K and 5100 K, respectively, or by power-laws of the form f_nu \propto nu^(0.6,0.9), respectively, and show no evidence of secondary star features. The continuum of EX Hya is doiminated by water bands from the red-dwarf secondary star, which has a spectral type of ~M3. Despite showing circular polarization, PQ Gem and BG CMi show no evidence for cyclotron humps and hence we are unable to measure their magnetic field strengths; any cyclotron emission present must contribute less than ~3 per cent of the infrared continuum flux., 7 pages, 2 figures

We report spin-resolved optical photometry and spectroscopy of the intermediate polar BG Canis Minoris. New 913-s pulsation ephemerides are derived. We find that a cubic fit provides a better description of the O-C values than the hitherto assumed quadratic ephemeris at a confidence level of 97.3 per cent. From the spectroscopy, we find that the violet-to-red (V/R) ratio of the emission lines varies with the 913-s period but not at the alternative period of 847 s detected in the X-ray band. This suggests that 913 s is the most probable spin period of the white dwarf. However, we do find evidence that the emission line intensities and equivalent widths are modulated at 847 s

We present optical spectroscopy, multicolour photometry and polarimetry of the new intermediate polar, RE0751+14, discovered during the ROSAT WFC survey. The wings of its emission lines exhibit radial velocity motion with a period ∼13.9 min, confirming that this is the rotation period of the white dwarf. Intriguingly, the R- and I-band photometric light curves reveal a double-peaked profile at the rotation period. In contrast, the B-band light curve, at least in our longest (∼5 h) time-series in that colour, is apparently dominated by a 14.5-min, broadly sinusoidal pulsation. There is supporting evidence from a short NOT observation at an earlier epoch that a modulation with a slightly longer period than the rotation period makes an increasing contribution at blue wavelengths

We have made a detailed study of a sample of 28 hot DA white dwarfs detected in the ROSAT EUV and soft X-ray all-sky-survey.

We present the results of ROSAT X-ray and EUV observations of the eclipsing AM Her star UZ For. Data have been obtained during faint and bright phases and, most interestingly, during the rise interval when the emission region appears over the limb of the white dwarf. The source is seen weakly in the X-ray telescope during the faint phase. We find that the rise to the bright phase is resolved, which is direct evidence of a spatially extended soft X-ray emission region and which places constraints on the size of the emission region. Using the bright-phase spectrum and constraints on the flux per unit area on the white dwarf, we find 19 eV < kT(bb) < 25 eV, 7 x 10(19) cm-2 < N(H) < 1.7 x 10(20) cm-2 and a lower limit to the bolometric blackbody luminosity of 1.5 x 10(31) erg s-1 cm-2 d100(2), where d100 is the distance in units of 100 pc. We also show evidence that the blackbody temperature in the rise interval is approximately 10 per cent lower than in the bright phase. The soft X-ray excess is at least a factor of 7.

The first all-sky survey for cosmic sources of extreme-ultraviolet radiation has been carried out with the UK Wide Field Camera on ROSAT. A first reduction of the survey data has yielded a catalogue of 383 relatively bright EUV sources, forming the WFC Bright Source Catalogue. This represents a 30-fold increase in the number of astrophysical objects detected in the ∼ 60-200 eV energy band and covers a flux range, in each of the two survey bands, of more than 2000. A search of the (typically ∼ 1-arcmin) error circles of the WFC sources, using a variety of catalogues and the SIMBAD data base, has identified probable optical counterparts of ∼73 per cent, including many active stars, white dwarf stars and a variety of other galactic and extragalactic objects

The Ultraviolet/Optical Telescope (UVOT) is one of three instruments onboard theSwift observatory. The photometric calibration has been published, and this paperfollows up with details on other aspects of the calibration including a measurement ofthe point spread function with an assessment of the orbital variation and the effect onphotometry. A correction for large scale variations in sensitivity over the field of viewis described, as well as a model of the coincidence loss which is used to assess the coin-cidence correction in extended regions. We have provided a correction for the detectordistortion and measured the resulting internal astrometric accuracy of the UVOT,also giving the absolute accuracy with respect to the International Celestial ReferenceSystem. We have compiled statistics on the background count rates, and discuss thesources of the background, including instrumental scattered light. In each case we de-scribe any impact on UVOT measurements, whether any correction is applied in thestandard pipeline data processing or whether further steps are recommended.Keywords: instrumentation:photometers – instrumentation:detectors – astrometry