ALBERT

Description: Quasar accretion disc winds observed via broad absorption lines (BALs) in the UV produce strong continuous absorption in X-rays. The X-ray absorber is believed to serve critically as a radiative shield to keep the outflow ionizations low enough for radiative driving. However, previous studies have shown that ‘mini-BAL’ and narrow absorption line (NAL) outflows have dramatically less X-ray absorption than BALs. Here, we examine X-ray and rest-frame UV spectra of eight mini-BAL quasars with outflow speeds in the range 0.1–0.2c to test the hypothesis that these extreme speeds require a strong shield. We find that the X-ray absorption is weak or moderate, with neutral-equivalent column densities N H  〈 few 10 22 cm –2 , consistent with mini-BALs at lower speeds. We use photoionization models to show that the amount of shielding consistent with our data is too weak to control the outflow ionizations and, therefore, it is not important for the acceleration. Shielding in complex geometries also seems unlikely because the alleged shield would need to extinguish the ionizing far-UV flux while avoiding detection in X-rays and the near-UV. We argue that the outflow ionizations are kept moderate, instead, by high gas densities in small clouds. If the mini-BALs form at radial distances of the order of R ~ 2 pc from the central quasar (broadly consistent with theoretical models and with the mini-BAL variabilities observed here and in previous work), and the total column densities in the mini-BAL gas are N H 10 21 cm –2 , then the total radial extent of outflow clouds is only R clouds 3 x 10 13 cm in cases of no/weak shielding or R clouds 3 x 10 14 cm behind the maximum shield allowed by our data. This implies radial filling factors R clouds / R 5 x 10 – 6 or 5 x 10 – 5 for the unshielded or maximally shielded cases, respectively. Compared to the transverse sizes 8 x 10 15 cm (based on measured line depths), the outflows have shapes like thin ‘pancakes’ viewed face-on, or they occupy larger volumes like a spray of many dense clouds with a small volume filling factor. These results favour models with magnetic confinement in magnetic disc winds. To the extent that BALs, mini-BALs and NALs probe the same general outflow phenomenon, our result for dense substructures should apply to all three outflow types.

Description: We present a census of z abs 2 intrinsic (those showing partial coverage) and associated ( z abs  ~ z em ) quasar absorption-line systems detected in the Hubble Space Telescope archive of Space Telescope Imaging Spectrograph echelle spectra. This work complements the Misawa et al. survey of 2 〈 z em  〈 4 quasars that selects systems using similar techniques. We confirm the existence of so-called strong N v intrinsic systems (where the equivalent width of H i Lyα is small compared to N v 1238) presented in that work, but find no convincing cases of ‘strong C iv ’ intrinsic systems at low redshift/luminosity. Moreover, we also report on the existence of ‘strong O vi ’ systems. From a comparison of partial coverage results as a function of ion, we conclude that systems selected by the N v ion have the highest probability of being intrinsic. By contrast, the C iv and O vi ions are poor selectors. Of the 30 O vi systems tested, only two of the systems in the spectrum on 3C 351 show convincing evidence for partial coverage. However, there is an ~3 excess in the number of absorbers near the quasar redshift (| v | ≤ 5000 km s –1 ) over absorbers at large redshift differences. In at least two cases, the associated O vi systems are known not to arise close to the accretion disc of the quasar.

Description: We present the results of a joint Swift -NuSTAR spectroscopy campaign on M31. We focus on the five brightest globular cluster X-ray sources in our fields. Two of these had previously been argued to be black hole candidates on the basis of apparent hard-state spectra at luminosities above those for which neutron stars are in hard states. We show that these two sources are likely to be Z-sources (i.e. low magnetic field neutron stars accreting near their Eddington limits), or perhaps bright atoll sources (low magnetic field neutron stars which are just a bit fainter than this level) on the basis of simultaneous Swift and NuSTAR spectra which cover a broader range of energies. These new observations reveal spectral curvature above 6–8 keV that would be hard to detect without the broader energy coverage the NuSTAR data provide relative to Chandra and XMM–Newton . We show that the other three sources are also likely to be bright neutron star X-ray binaries, rather than black hole X-ray binaries. We discuss why it should already have been realized that it was unlikely that these objects were black holes on the basis of their being persistent sources, and we re-examine past work which suggested that tidal capture products would be persistently bright X-ray emitters. We discuss how this problem is likely due to neglecting disc winds in older work that predict which systems will be persistent and which will be transient.

Description: We report the discovery of a new ‘changing-look' quasar, SDSS J101152.98+544206.4, through repeat spectroscopy from the Time Domain Spectroscopic Survey. This is an addition to a small but growing set of quasars whose blue continua and broad optical emission lines have been observed to decline by a large factor on a time-scale of approximately a decade. The 5100 Å monochromatic continuum luminosity of this quasar drops by a factor of 〉9.8 in a rest-frame time interval of 〈9.7 yr, while the broad Hα luminosity drops by a factor of 55 in the same amount of time. The width of the broad Hα line increases in the dim state such that the black hole mass derived from the appropriate single-epoch scaling relation agrees between the two epochs within a factor of 3. The fluxes of the narrow emission lines do not appear to change between epochs. The light curve obtained by the Catalina Sky Survey suggests that the transition occurs within a rest-frame time interval of approximately 500 d. We examine three possible mechanisms for this transition suggested in the recent literature. An abrupt change in the reddening towards the central engine is disfavoured by the substantial difference between the time-scale to obscure the central engine and the observed time-scale of the transition. A decaying tidal disruption flare is consistent with the decay rate of the light curve but not with the prolonged bright state preceding the decay; nor can this scenario provide the power required by the luminosities of the emission lines. An abrupt drop in the accretion rate on to the supermassive black hole appears to be the most plausible explanation for the rapid dimming.

Description: As part of our ongoing archival X-ray survey of galaxy clusters for tidal flares, we present evidence of an X-ray transient source within 1 arcmin of the core of Abell 1795. The extreme variability (a factor of nearly 50), luminosity (〉2 10 42 erg s –1 ), long duration (〉5 yr) and supersoft X-ray spectrum (〈0.1 keV) are characteristic signatures of a stellar tidal disruption event according to theoretical predictions and to existing X-ray observations, implying a massive 10 5 M black hole at the centre of that galaxy. The large number of X-ray source counts (~700) and long temporal baseline (~12 yr with Chandra and XMM–Newton ) make this one of the best sampled examples of any tidal flare candidate to date. The transient may be the same EUV source originally found contaminating the diffuse intracluster medium observations of Bowyer, Berghöfer and Korpela, which would make it the only tidal flare candidate with reported EUV observations and implies an early source luminosity 1–2 orders of magnitude greater. If the host galaxy is a cluster member then it must be a dwarf galaxy, an order of magnitude less massive than the quiescent galaxy Henize 2–10 which hosts a massive black hole that is difficult to reconcile with its low mass. The unusual faintness of the host galaxy may be explained by tidal stripping in the cluster core.

Description: We perform an exploratory study of the physical properties of accretion flows and jets in low-luminosity active galactic nuclei (LLAGNs) by modelling the spectral energy distributions (SEDs) of 12 LLAGNs in low-ionization nuclear emission-line regions (LINERs). These SEDs we constructed from high-resolution radio, X-ray and optical/ultraviolet (UV) observations of the immediate vicinity of the black hole. We adopt a coupled accretion-jet model comprising an inner advection-dominated accretion flow (ADAF) and an outer standard thin disc. We present best-fitting models in which either the ADAF or the jet dominates the X-ray emission. Six sources in our sample display an optical–UV excess with respect to ADAF and jet models; this excess can be explained as emission from the truncated disc with transition radii 30–225 R S in four of them. In almost all sources the optical emission can also be attributed to unresolved, old stellar clusters with masses ~10 7 –10 8 M . We find evidence for a correlation between the accretion rate and jet power and an anticorrelation between the radio loudness and the accretion rate. We confirm previous findings that the radio emission is severely underpredicted by ADAF models and explained by the relativistic jet. We find evidence for a non-linear relation between the X-ray and bolometric luminosities and a slight IR excess in the average model SED compared to that of quasars. We suggest that the hardness of the X-ray spectrum can be used to identify the X-ray emission mechanism and discuss directions for progress in understanding the origin of the X-rays.

Description: We report on the first detection of extreme-ultraviolet (EUV) absorption variability in the Ne  viii 770, 780 mini-broad absorption line (mini-BAL) in the spectrum of the quasar (QSO) PG 1206+459. The observed equivalent width (EW) of the Ne  viii doublet shows a ~4 variation over a time-scale of 2.8 months in the QSO's rest frame. Both members of the Ne  viii doublet exhibit non-black saturation, indicating partial coverage of the continuum source. An increase in the Ne  viii covering fraction from f c = 0.59 ± 0.05 to 0.72 ± 0.03 is observed over the same period. The Ne  viii profiles are too highly saturated to be susceptible to changes in the ionization state of the absorbing gas. In fact, we do not observe any significant variation in the EW and/or column density after correcting the spectra for partial coverage. We, thus, propose transverse motions of the absorbing gas as the cause of the observed variability. Using a simple model of a transiting cloud we estimate a transverse speed of ~1800 km s –1 . For Keplerian motion, this corresponds to a distance between the absorber and the central engine of ~1.3 pc, which places the absorber just outside the broad-line region. We further estimate a density of ~5 x 10 6  cm –3 and a kinetic luminosity of ~10 43 –10 44  erg s –1 . Such large kinetic powers suggest that outflows detected via EUV lines are potentially major contributors to active galactic nuclei feedback.

Description: We present a study of the morphology and intensity of star formation in the host galaxies of eight Palomar-Green quasars using observations with the Hubble Space Telescope . Our observations are motivated by recent evidence for a close relationship between black hole growth and the stellar mass evolution in its host galaxy. We use narrow-band [O ii ]3727, Hβ, [O iii ]5007 and Paα images, taken with the Wide Field Planetary Camera 2 and NICMOS instruments, to map the morphology of line-emitting regions, and, after extinction corrections, diagnose the excitation mechanism and infer star-formation rates. Significant challenges in this type of work are the separation of the quasar light from the stellar continuum and the quasar-excited gas from the star-forming regions. To this end, we present a novel technique for image decomposition and subtraction of quasar light. Our primary result is the detection of extended line-emitting regions with sizes ranging from 0.5 to 5 kpc and distributed symmetrically around the nucleus, powered primarily by star formation. We determine star-formation rates of the order of a few tens of M yr –1 . The host galaxies of our target quasars have stellar masses of the order of 10 11 M and specific star-formation rates on a par with those of M82 and luminous infrared galaxies. As such they fall at the upper envelope or just above the star-formation mass sequence in the specific star formation versus stellar mass diagram. We see a clear trend of increasing star-formation rate with quasar luminosity, reinforcing the link between the growth of the stellar mass of the host and the black hole mass found by other authors.