ASCA observations of deep ROSAT fields — III. The discovery of an obscured Type 2 AGN at z = 0.67

We report on the discovery of a narrow-emission-line object at z  = 0.672 detected in a deep ASCA survey. The object, AXJ 0341.4–4453, has a flux in the 2–10 keV band of 1.1 ± 0.27 × 10⁻¹³ erg s⁻¹ cm⁻², corresponding to a luminosity of 1.8 × 10⁴⁴ erg s⁻¹ ( q ₀ = 0.5, H ₀ = 50 km s⁻¹ Mpc⁻¹). It is also marginally detected in the ROSAT 0.5–2 keV band with a flux 5.8 × 10⁻¹⁵ erg s⁻¹ cm⁻². Both the ASCA data alone and the combined ROSAT/ASCA data show a very hard X-ray spectrum, consistent with either a flat power law (α < 0.1) or photoelectric absorption with a column of n _H > 4 × 10²² cm⁻² (α = 1). The optical spectrum shows the high-ionization, narrow emission lines typical of a Seyfert 2 galaxy. We suggest that this object may be typical of the hard sources required to explain the remainder of the X-ray background at hard energies.     

Quiescent times in gamma-ray bursts – II. Dormant periods in the central engine?

Within the framework of the internal–external shocks model for γ -ray bursts, we study the various mechanisms that can give rise to quiescent times in the observed γ -ray light curves. In particular, we look for the signatures that can provide us with evidence as to whether or not the central engine goes dormant for a period of time comparable to the duration of the gaps. We show that the properties of the prompt γ -ray and X-ray emission can, in principle, determine whether the quiescent episodes are caused by a modulated relativistic wind or a switching off of the central engine. We suggest that detailed observations of the prompt afterglow emission from the reverse shock will strongly constrain the possible mechanisms for the production of quiescent times in γ -ray bursts.     

Reflections on reflexions – II. Effects of light echoes on the luminosity and spectra of Type Ia supernovae

In this paper, we present and discuss the effects of scattered light echoes (LEs) on the luminosity and spectral appearance of Type Ia supernovae (SNe). After introducing the basic concept of LE spectral synthesis by means of LE models and real observations, we investigate the deviations from pure SN spectra, light and colour curves, the signatures that witness the presence of an LE and the possible inferences on the extinction law. The effects on the photometric parameters and spectral features are also discussed. In particular, for the case of circumstellar dust, LEs are found to introduce an apparent relation between the post-maximum decline rate and the absolute luminosity, which is most likely going to affect the well-known Pskowski–Phillips relation.     

ASCA and ROSAT observations of nearby cluster cooling flows

We present a detailed analysis of the X-ray properties of the cooling flows in a sample of nearby, X-ray-bright clusters of galaxies using high-quality ASCA spectra and ROSAT X-ray images. We demonstrate the need for multiphase models to consistently explain the spectral and imaging X-ray data for the clusters. The mass deposition rates of the cooling flows, independently determined from the ASCA spectra and ROSAT images, exhibit reasonable agreement. We confirm the presence of intrinsic X-ray absorption in the clusters using a variety of spectral models. We also report detections of 100-μm infrared emission, spatially coincident with the cooling flows, in several of the systems studied. The observed infrared fluxes and flux limits are in good agreement with the predicted values owing to reprocessed X-ray emission from the cooling flows. We present precise measurements of the abundances of iron, magnesium, silicon and sulphur in the central regions of the Virgo and Centaurus clusters. Our results firmly favour models in which a high mass fraction (70–80 per cent) of the iron in the X-ray gas in these regions originates from Type Ia supernovae. Finally, we present a series of methods which may be used to estimate the ages of cooling flows from X-ray data. The results for the present sample of clusters indicate ages of between 2.5 and 7 Gyr. If the ages of cooling flows are primarily set by subcluster merger events, then our results suggest that in the largest clusters, mergers with subclusters with masses of ∼30 per cent of the final cluster mass are likely to disrupt cooling flows.     

Massive stars exploding in a He-rich circumstellar medium – II. The transitional case of SN 2005la

We present photometric and spectroscopic data of the peculiar SN 2005la, an object which shows an optical light curve with some luminosity fluctuations and spectra with comparably strong narrow hydrogen and helium lines, probably of circumstellar nature. The increasing full width at half-maximum velocity of these lines is indicative of an acceleration of the circumstellar material. SN 2005la exhibits hybrid properties, sharing some similarities with both Type IIn supernovae and 2006jc-like (Type Ibn) events. We propose that the progenitor of SN 2005la was a very young Wolf–Rayet (WN-type) star which experienced mass ejection episodes shortly before core collapse.     

Abundance stratification in Type Ia supernovae – II. The rapidly declining, spectroscopically normal SN 2004eo

The variation in properties of Type Ia supernovae, the thermonuclear explosions of Chandrasekhar-mass carbon–oxygen white dwarfs, is caused by different nucleosynthetic outcomes of these explosions, which can be traced from the distribution of abundances in the ejecta. The composition stratification of the spectroscopically normal but rapidly declining SN 2004eo is studied by performing spectrum synthesis of a time-series of spectra obtained before and after maximum, and of one nebular spectrum obtained about eight months later. Early-time spectra indicate that the outer ejecta are dominated by oxygen and silicon, and contain other intermediate-mass elements, implying that the outer part of the star was subject only to partial burning. In the inner part, nuclear statistical equilibrium (NSE) material dominates, but the production of ⁵⁶Ni was limited to  ∼0.43 ± 0.05   M_⊙  . An innermost zone containing  ∼0.25   M_⊙  of stable Fe-group material is also present. The relatively small amount of NSE material synthesized by SN 2004eo explains both the dimness and the rapidly evolving light curve of this supernova.     

ROSAT PSPC observations of nearby spiral galaxies – II. Statistical properties

We present a statistical analysis of the largest X-ray survey of nearby spiral galaxies in which diffuse emission has been separated from discrete source contributions. Regression and rank-order correlation analyses are used to compare X-ray properties, such as total, source and diffuse luminosities and diffuse emission temperature, with a variety of physical and multiwavelength properties, such as galaxy mass, type and activity, and optical and infrared luminosity.
The results are discussed in terms of the way in which hot gas and discrete X-ray sources scale with the mass and activity of galaxies, and with the star formation rate. We find that the X-ray properties of starburst galaxies are dependent primarily on their star-forming activity, whilst for more quiescent galaxies, galaxy mass is the more important parameter. One of the most intriguing results is the tight linear scaling between far-infrared and diffuse X-ray luminosity across the sample, even though the hot gas changes from a hydrostatic corona to a free wind across the activity range sampled here.     

The Deutsch field gamma-ray pulsar – II. Application of the model

A new model for gamma-ray pulsars presented by Higgins & Henriksen is applied to the cases of the seven known gamma-ray pulsars. Those pulsars that are not presently observed in gamma-rays, but are candidates for observation by the next generation of gamma-ray telescopes, are discussed. The case of millisecond pulsars is discussed, and it is shown that these objects should radiate at detectable levels, in opposition to the predictions of other gamma-ray pulsar models.     

The X-ray luminosity function of AGN at z∼ 3

We combine Lyman-break colour selection with ultradeep (≳200 ks) Chandra X-ray imaging over a survey area of ∼0.35 deg² to select high-redshift active galactic nuclei (AGN). Applying careful corrections for both the optical and X-ray selection functions, the data allow us to make the most accurate determination to date of the faint end of the X-ray luminosity function (XLF) at   z ∼ 3  . Our methodology recovers a number density of X-ray sources at this redshift which is at least as high as previous surveys, demonstrating that it is an effective way of selecting high z AGN. Comparing to results at   z = 1  , we find no evidence that the faint slope of the XLF flattens at high z , but we do find significant (factor ∼3.6) negative evolution of the space density of low luminosity AGN. Combining with bright end data from very wide surveys we also see marginal evidence for continued positive evolution of the characteristic break luminosity   L _*  . Our data therefore support models of luminosity-dependent density evolution between   z = 1  and   z = 3  . A sharp upturn in the the XLF is seen at the very lowest luminosities  ( L _X≲ 10^42.5 erg s⁻¹)  , most likely due to the contribution of pure X-ray starburst galaxies at very faint fluxes.     

Polarization observations of the radio cores of AGN — I. A sample of quasars

We present total-intensity and linear-polarization observations at a single epoch for a sample of 11 quasars and one BL Lac object. The data were taken with the VLA A array at λλ 20, 18, 6 and 2 cm. We examine the variation of the degree of polarization, p , and polarization position angle, PA, with wavelength, and attempt to determine the rotation measure, RM, of the cores in these sources. The degree of polarization does not exhibit any systematic variation with wavelength, the median values ranging from 2.3 to 3.5 per cent at the different wavelengths. The variation of PA with λ² is not linear over the entire wavelength range. However, for most sources the λλ 20-, 18- and 6-cm PAs do follow such a linear relationship, yielding a median |RM| of about 15 rad m⁻². In contrast, the λλ 6- and 2-cm observations give a median |RM| of about 129 rad m⁻². The long-wavelength emission is likely to originate from a spatially different part of the milliarcsec-scale jet from the λ 2-cm emission, which could turn over at a higher frequency and is likely to be more compact and located closer to the quasar nucleus. We have attempted to obtain linear fits over the entire wavelength range allowing for n  π ambiguities in the PAs, but the fits are not statistically significant. The low values of RM for these core-dominated sources suggest that either the radio emission from the jet intercepts few of the emission-line clouds and their confining medium, or the clouds have a small filling factor and are possibly magnetically confined.     

The ROSAT Brightest Cluster Sample – IV. The extended sample

We present a low-flux extension of the X-ray-selected ROSAT Brightest Cluster Sample (BCS) published in Paper I of this series. Like the original BCS and employing an identical selection procedure, the BCS extension is compiled from ROSAT All-Sky Survey (RASS) data in the northern hemisphere ( δ ≥0°) and at high Galactic latitudes (| b |≥20°). It comprises 99 X-ray-selected clusters of galaxies with measured redshifts z ≤0.3 (as well as eight more at z >0.3) and total fluxes between 2.8×10⁻¹² and 4.4×10⁻¹² erg cm⁻² s⁻¹ in the 0.1–2.4 keV band (the latter value being the flux limit of the original BCS). The extension can be combined with the main sample published in 1998 to form the homogeneously selected extended BCS (eBCS), the largest and statistically best understood cluster sample to emerge from the RASS to date.
The nominal completeness of the combined sample (defined with respect to a power-law fit to the bright end of the BCS log  N –log  S distribution) is relatively low at 75 per cent (compared with 90 per cent for the high-flux sample of Paper I). However, just as for the original BCS, this incompleteness can be accurately quantified, and thus statistically corrected for, as a function of X-ray luminosity and redshift.
In addition to its importance for improved statistical studies of the properties of clusters in the local Universe, the low-flux extension of the BCS is also intended to serve as a finding list for X-ray-bright clusters in the northern hemisphere which we hope will prove useful in the preparation of cluster observations with the next generation of X-ray telescopes such as Chandra and XMM-Newton .
An electronic version of the eBCS can be obtained from the following URL: http://www.ifa.hawaii.edu/~ebeling/clusters/BCS.html.     

Compact radio sources variable at 151 MHz – II. High-resolution observations of an unbiased sample

High‐resolution observations, made with the Very Large Array (VLA) at 330 MHz, 1.4 GHz and 8.4 GHz and with the Ryle Telescope at 15 GHz, are presented of a sample of 23 sources which are variable at 151 MHz, concluding the observations of an unbiased sample of 40 such sources. The 8.4 GHz emission of almost all of the sources is dominated by structure on a scale ≲0.1 arcsec – the spectra of these compact components are such that they will also dominate the emission at 151 MHz; the number of sources for which this is not the case is consistent with the number of spurious variables expected to be found in the sample. About two-thirds of the sources have the self-absorbed, or flat, spectra expected from their size. The majority (∼75 per cent) of the sources belong to the compact steep-spectrum (CSS) class, but are generally more compact than other CSS sources; the CSS variables also appear to exhibit more spectral ageing than typical CSS sources, which may indicate a difference in the nature of the sources.     

SN 2005cs in M51 – I. The first month of evolution of a subluminous SN II plateau

Early-time optical observations of supernova (SN) 2005cs in the Whirlpool Galaxy (M51) are reported. Photometric data suggest that SN 2005cs is a moderately underluminous Type II plateau SN (SN IIP). The SN was unusually blue at early epochs (   U − B ≈−0.9  about three days after explosion) which indicates very high continuum temperatures. The spectra show relatively narrow P Cygni features, suggesting ejecta velocities lower than observed in more typical SNe IIP. The earliest spectra show weak absorption features in the blue wing of the He  i 5876-Å absorption component and, less clearly, of Hβ and Hα. Based on spectral modelling, two different interpretations can be proposed: these features may either be due to high-velocity H and He  i components, or (more likely) be produced by different ions (N  ii , Si  ii ). Analogies with the low-luminosity, ⁵⁶Ni-poor, low-velocity SNe IIP are also discussed. While a more extended spectral coverage is necessary in order to determine accurately the properties of the progenitor star, published estimates of the progenitor mass seem not to be consistent with stellar evolution models.