1,721,279 research outputs found
A simple disc wind model for broad absorption line quasars
No full textApproximately 20 per cent of quasi-stellar objects (QSOs) exhibit broad, blue-shifted absorption lines in their ultraviolet spectra. Such features provide clear evidence for significant outflows from these systems, most likely in the form of accretion disc winds. These winds may represent the ‘quasar’ mode of feedback that is often invoked in galaxy formation/evolution models, and they are also key to unification scenarios for active galactic nuclei (AGN) and QSOs. To test these ideas, we construct a simple benchmark model of an equatorial, biconical accretion disc wind in a QSO and use a Monte Carlo ionization/radiative transfer code to calculate the ultraviolet spectra as a function of viewing angle. We find that for plausible outflow parameters, sightlines looking directly into the wind cone do produce broad, blue-shifted absorption features in the transitions typically seen in broad absorption line (BAL) QSOs. However, our benchmark model is intrinsically X-ray weak in order to prevent overionization of the outflow, and the wind does not yet produce collisionally excited line emission at the level observed in non-BAL QSOs. As a first step towards addressing these shortcomings, we discuss the sensitivity of our results to changes in the assumed X-ray luminosity and mass-loss rate, Ṁwind. In the context of our adopted geometry, Ṁwind ∼ Ṁacc is required in order to produce significant BAL features. The kinetic luminosity and momentum carried by such outflows would be sufficient to provide significant feedback
Quasar emission lines as probes of orientation: implications for disc wind geometries and unification
Get PDFThe incidence of broad absorption lines (BALs) in quasar samples is often interpreted in the context of a geometric unification model consisting of an accretion disc and an associated outflow. We use the Sloan Digital Sky Survey quasar sample to test this model by examining the equivalent widths (EWs) of C iv 1550 Å, Mg ii 2800 Å, [O iii] 5007 Å and C iii] 1909 Å. We find that the emission line EW distributions in BAL and non-BAL quasars are remarkably similar – a property that is inconsistent with scenarios in which a BAL outflow rises equatorially from a geometrically thin, optically thick accretion disc. We construct simple models to predict the distributions from various geometries; these models confirm the above finding and disfavour equatorial geometries. We show that obscuration, line anisotropy and general relativistic effects on the disc continuum are unlikely to hide an EW inclination dependence. We carefully examine the radio and polarization properties of BAL quasars. Both suggest that they are most likely viewed (on average) from intermediate inclinations, between type 1 and type 2 active galactic nuclei (AGN). We also find that the low-ionization BAL quasars in our sample are not confined to one region of the ‘Eigenvector 1’ parameter space. Overall, our work leads to one of the following conclusions, or some combination thereof: (i) the continuum does not emit like a geometrically thin, optically thick disc; (ii) BAL quasars are viewed from similar angles to non-BAL quasars, that is, low inclinations and (iii) geometric unification does not explain the fraction of BALs in quasar samples
Episodic absorption in the outflow of V603 Aquilae
No full textWe report on the time-dependent behaviour of ultraviolet spectral lines in Hubble Space Telescope Goddard High-Resolution Spectrograph data of the classical nova V603 Aql. In particular, episodic blueshifted absorption (extending to ∼−2500 km s−1) is present, with a variability time-scale down to ∼1 min. The data provide a rare opportunity to study the rapid evolution of absorption structures that may be associated with accretion-disc winds in cataclysmic variables. At least three absorption events are recorded (at blueward velocities only) over ∼5 h, each lasting ∼10–15 min. The derived velocity, acceleration and optical depth properties provide an empirical picture of stochastically variable structures in the outflow, with no evidence for short-term (less than ∼1 h) cyclic or modulated behaviour in the overall absorption properties. In contrast, the emission components of the ultraviolet resonance lines are very stable in velocity and strength in this low-inclination system. On at least two occasions there is an intriguing short-term ‘flare’ in the ultraviolet continuum flux (of up to ∼40 per cent). Though there is no clear one-to-one relation in these data between the continuum fluctuations and the occurrence of the absorption events, the time-scales for the two variable phenomena are essentially the same. The irregular absorption episodes in the ultraviolet data of V603 Aql presently defy a clear physical interpretation. Their overall characteristics are discussed in the context of instabilities in radiation-pressure-driven disc winds
Modeling the spectral signatures of accretion disk winds: a new Monte Carlo approach
No full textBipolar outflows are present in many disk-accreting astrophysical systems. In disk-accreting cataclysmic variables (CVs), these outflows are responsible for most of the strong features in the ultraviolet spectra of these systems. However, there have been few attempts to model these features quantitatively. Here we describe a new, hybrid Monte Carlo/Sobolev code, which allows us to synthesize the complete spectrum of a disk-dominated, mass-losing CV. The line profiles that we calculate for C IV resemble those calculated by previous workers when an identical geometry is assumed. However, our synthetic spectra exhibit not only the well-known resonance lines of O VI, N V, Si IV, and C IV but, with an appropriate choice of mass-loss rate and wind geometry, also many lines originating from excited lower states. Many of these lines have already been seen in the far-ultraviolet spectra of CVs obtained with the Hopkins Ultraviolet Telescope, the Far Ultraviolet Spectroscopic Explorer, and the Hubble Space Telescope. In order to illustrate the degree to which we are currently able to reproduce observed spectra, we finally present a preliminary fit to the Hopkins Ultraviolet Telescope spectrum of the dwarf nova Z Cam in outburst
The IPHAS catalogue of H alpha emission-line sources in the northern Galactic plane
Get PDFWe present a catalogue of point-source H alpha emission-line objects selected from the INT/WFC Photometric Ha Survey (IPHAS) of the northern Galactic plane. The catalogue covers the magnitude range 13 <= r' <= 19.5 and includes Northern hemisphere sources in the Galactic latitude range -5 degrees < b < 5 degrees. It is derived from similar to 1500 deg(2) worth of imaging data, which represents 80 per cent of the final IPHAS survey area. The electronic version of the catalogue will be updated once the full survey data become available. In total, the present catalogue contains 4853 point sources that exhibit strong photometric evidence for Ha emission. We have so far analysed spectra for similar to 300 of these sources, confirming more than 95 per cent of them as genuine emission-line stars. A wide range of stellar populations are represented in the catalogue, including early-type emission-line stars, active late-type stars, interacting binaries, young stellar objects and compact nebulae.
The spatial distribution of catalogue objects shows overdensities near sites of recent or current star formation, as well as possible evidence for the warp of the Galactic plane. Photometrically, the incidence of Ha emission is bimodally distributed in (r' - i'). The blue peak is made up mostly of early-type emission-line stars, whereas the red peak may signal an increasing contribution from other objects, such as young/active low-mass stars. We have cross-matched our H alpha-excess catalogue against the emission-line star catalogue of Kohoutek & Wehmeyer, as well as against sources in SIMBAD. We find that fewer than 10 per cent of our sources can be matched to known objects of any type. Thus IPHAS is uncovering an order of magnitude more faint (r' > 13) emission-line objects than were previously known in the Milky Way
An ultracompact X-ray binary in the globular cluster NGC 1851
No full textWe present far-ultraviolet photometry obtained with the Hubble Space Telescope of the low-mass X-ray binary 4U 0513-40 in the globular cluster NGC 1851. Our observations reveal a clear, roughly sinusoidal periodic signal with P sime 17 minutes and amplitude 3%-10%. The signal appears fully coherent and can be modeled as a simple reprocessing effect associated with the changing projected area presented by the irradiated face of a white dwarf donor star in the system. All of these properties suggest that the signal we have detected is orbital in nature, thus confirming 4U 0513-40 as an ultracompact X-ray binary (UCXB). All four confirmed UCXBs in globular clusters have orbital periods below 30 minutes, whereas almost all UCXBs in the Galactic field have orbital periods longer than this. This suggests that dynamical formation processes dominate UCXB production in clusters, producing a different orbital period distribution than observed among field UCXBs. Based on the likely system parameters, we show that 4U 0513-40 should be a strong gravitational wave source and may be detectable by Laser Interferometer Space Antenna over the course of a multiyear mission
A triple star origin for T Pyx and other short-period recurrent novae
Get PDFRecurrent novae are star systems in which a massive white dwarf accretes material at such a high rate that it undergoes thermonuclear runaways every 1-100 yr. They are the only class of novae in which the white dwarf can grow in mass, making some of these systems strong Type Ia supernova progenitor candidates. Almost all known recurrent novae are long-period (Porb ? 12h) binary systems in which the requisite mass supply rate can be provided by an evolved (sub-)giant donor star. However, at least two recurrent novae are short-period (Porb ? 3h) binaries in which mass transfer would normally be driven by gravitational radiation at rates three to four orders of magnitude smaller than required. Here, we show that the prototype of this class - T Pyxidis - has a distant proper motion companion and therefore likely evolved from a hierarchical triple star system. Triple evolution can naturally produce exotic compact binaries as a result of three-body dynamics, either by Kozai-Lidov eccentricity cycles in dynamically stable systems or via mass-loss-induced dynamical instabilities. By numerically evolving triple progenitors with physically reasonable parameters forward in time, we show explicitly that the inner binary can become so eccentric that mass transfer is triggered at periastron, driving the secondary out of thermal equilibrium. We suggest that short-period recurrent novae likely evolved via this extreme state, explaining their departure from standard binary evolution tracks.</p
A far-ultraviolet survey of 47 Tucanae. I. Imaging
No full textWe present results from the imaging portion of a far-ultraviolet (FUV) survey of the core of 47 Tucanae. We have detected 767 FUV sources, 527 of which have optical counterparts in archival HST/WFPC2 images of the same field. Most of our FUV sources are main-sequence (MS) turnoff stars near the detection limit of our survey. However, the FUV/optical color-magnitude diagram (CMD) also reveals 19 blue stragglers (BSs), 17 white dwarfs (WDs), and 16 cataclysmic variable (CV) candidates. The BSs lie on the extended cluster MS, and four of them are variable in the FUV data. The WDs occupy the top of the cluster cooling sequence, down to an effective temperature of Teff~=20,000 K. Our FUV source catalog probably contains many additional, cooler WDs without optical counterparts. Finally, the CV candidates are objects between the WD cooling track and the extended cluster MS. Four of the CV candidates are previously known or suspected cataclysmics. All of these are bright and variable in the FUV. Another CV candidate is associated with the semidetached binary system V36 that was recently found by M. D. Albrow and coworkers. V36 has an orbital period of 0.4 or 0.8 days, blue optical colors, and is located within 1" of a Chandra X-ray source. A few of the remaining CV candidates may represent chance superpositions or SMC interlopers, but at least half are expected to be real cluster members with peculiar colors. However, only a few of these CV candidates are possible counterparts to Chandra X-ray sources. Thus, it is not yet clear which, if any, of them are true CVs, rather than noninteracting MS/WD binaries or helium WDs
Plateaus, dips, and rebrightenings during the outbursts of WZ Sge: no magnetic propeller, but a veiling curtain
No full textWZ Sge is the prototype of highly evolved, low-accretion rate dwarf novae (DNe). During the decline from eruptions, its light curve displays a ‘dip’ followed by ≃10 ‘echo outbursts’. The standard disc instability model does not account for this behaviour, which is also seen in other low-accretion rate DNe. One recent interpretation for these rapid brightness changes is that they represent transitions into and out of a magnetic propeller regime. Here, we test this scenario with time-resolved, ultraviolet spectroscopy taken with the Hubble Space Telescope just before, during and after the dip in WZ Sge’s 2001 eruption. We find no distinctive or unique signatures that could be attributed to a propeller in either the time-averaged UV spectrum or the variability spectrum. Thus the data do not support the magnetic propeller scenario. Instead of resolving the mystery of WZ Sge’s outburst light curve, our study has actually added another: the origin of the narrow absorption features seen in all outburst phases. We show explicitly that these features are likely formed in a high-density ‘veiling curtain’ with a characteristic temperature T≈17,000 KT≃17,000 K. However, the nature and origin of this veil are unclear. Given that WZ Sge-type DNe are the most intrinsically common class of accreting white dwarfs, resolving these questions should be a high priority
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