K. F. Gunn

Radio observations of the 13hXMM–Newton/ROSAT Deep X‐ray Survey Area

In order to determine the relationship between the faint X-ray and faint radio source populations, and hence to help understand the X-ray and radio emission mechanisms in those faint source populations, we have made a deep 1.4-GHz Very Large Array radio survey of the 13 h + 38° XMM-Newton/ROSAT X-ray Survey Area. From a combined data set of 10-h, B-configuration data and 14-h, A-configuration data, maps with 3.35-arcsec resolution and a noise limit of 7.5 μJy were constructed. A complete sample of 449 sources was detected within a 30-arcmin diameter region above a 4a detection limit of 30 μJy, at the map centre, making this one of the deepest radio surveys at this frequency. The differential source count shows a significant upturn at submilliJansky flux densities, similar to that seen in other deep surveys at 1.4 GHz (e.g. the Phoenix survey), but larger than that seen in the Hubble Deep Field (HDF) which may have been selected to be underdense. This upturn is well modelled by the emergence of a population of medium-redshift star-forming galaxies which dominate at faint flux densities. The brighter source counts are well modelled by active galactic nuclei.

GRB 011121: A Massive Star Progenitor

Of the cosmological gamma-ray bursts, GRB 011121 has the lowest redshift, z = 0.36. More importantly, the multicolor excess in the afterglow detected in the Hubble Space Telescope (HST) light curves is compelling observational evidence of an underlying supernova. Here we present near-infrared and radio observations of the afterglow, and from our comprehensive afterglow modeling, we find evidence favoring a wind-fed circumburst medium. Lacking X-ray data, we are unable to conclusively measure the mass-loss rate, , but obtain an estimate, ~ 2 × 10-7/vw3 M☉ yr-1, where vw3 is the speed of the wind from the progenitor in units of 103 km s-1. This is similar to that inferred for the progenitor of the Type Ibc supernova SN 1998bw that has been associated with the peculiar burst GRB 980425. Our data, taken in conjunction with the HST results of Bloom et al., provide a consistent picture: the long-duration GRB 011121 had a massive star progenitor that exploded as a supernova at about the same time as the gamma-ray burst event. Finally, we note that the gamma-ray profile of GRB 011121 is similar to that of GRB 980425.

Revision of the Properties of the GRS 1915+105 Jets: Clues from the Large-Scale Structure

The jets of GRS 1915+105 carry a considerable energy away from the central source into the interstellar medium (ISM). The similarity of the jets of this source and jets in radio galaxies or radio-loud quasars suggests that we should detect large-scale, synchrotron-emitting radio structures surrounding GRS 1915+105. However, these large structures have not been found. We show that by adapting a model for the radio lobes of extragalactic jet sources we predict a radio surface brightness of the equivalent structures of GRS 1915+105 below the current detection limits. The model uses an energy transport rate of the jets averaged over the jet lifetime. This transport rate is found to be considerably lower than the power of the jets during the rare major ejection events. Thus, the lobes contain less energy than would be inferred from these events and produce a lower radio luminosity. The model also predicts a lifetime of the jets of order 106 yr and a gas density of the ISM in the vicinity of GRS 1915+105 of ~150 cm-3. The impact sites of the jets are identified with two IRAS regions with a flat radio spectrum located on either side of GRS 1915+105. Observations of molecular lines and dust emission from these objects are consistent with our interpretation. Distance estimates for the IRAS regions give 6.5 kpc, and our model implies that this is also the distance to GRS 1915+105. This low distance estimate in combination with the observed motions of jet ejections on small scales yields a jet velocity of about 0.7c and an angle of 53° of the jets to our line of sight.

X-ray spectra of sources in the 13HXMM–Newton/Chandra deep field

We present the X-ray spectra of 86 optically identified sources in the 13^H XMM–Newton/Chandra deep field which have >70 X-ray counts. The majority of these sources have 2–10 keV fluxes between 10^(−1)5 and 5 × 10^(−1)4 erg cm^(−2) s^(−1). The sample consists of 50 broad-line active galactic nuclei (BLAGN), 25 narrow emission-line galaxies (NELGs), six absorption-line galaxies and five Galactic stars. The majority (42/50) of the BLAGN have X-ray spectra which are consistent with a power-law shape. They have a mean photon index 〈Γ〉= 2.0 ± 0.1 and an intrinsic dispersion σ_Γ= 0.4 ± 0.1. Three of the BLAGN show curved spectra, with more emission near the high- and low-energy ends of the spectrum relative to the emission in the 1–2 keV range than can be reproduced by the power-law model. Five BLAGN show a deficit of soft X-rays, indicating absorption. We consider a source to be significantly absorbed if a power-law model fit is rejected with >99 per cent confidence and an absorbed power-law model produces an acceptable fit, or if the best-fitting power law is abnormally hard (Γ < 1). Significant absorption is more common in the NELGs (13/25) and absorption-line galaxies (2/6) than in the BLAGN (5/50), but is not universal in any of these classes of object. The majority of the 20 absorbed sources have X-ray spectra consistent with a simple cold photoelectric absorber, but a significant minority (6/20) require more complex models with either an additional component of soft X-ray emitting plasma, or an ionized absorber. Of the 16 narrow emission- and absorption-line galaxies which do not show evidence for X-ray absorption, only two objects are likely to be powered by star formation, and both have 2–10 keV X-ray luminosities of ≤ 10^(40) erg s^(−1). The X-ray emission in the other 14 unabsorbed NELGs and galaxies is most likely powered by AGN, which are not detected in the optical because they are outshone by their luminous host galaxies. The Galactic stars show multitemperature thermal spectra which peak between 0.5 and 1 keV. Star/AGN discrimination is possible for four of the five stars solely from their X-ray spectra.

X‐ray and optical properties of X‐ray sources in the 13hr XMM‐Newton/Chandra deep survey

The 13hr XMM-Newton/Chandra deep survey is the first of two extremely deep XMM-Newton fields observed by the XMM-OM consortium, A 120 ks Chandra mosaic, covering 0.2 deg(2), provides sensitive, confusion-free point source detection with sub-arcsecond positions. while the 200 ks XMM-Newton observation provides high quality X-ray spectroscopy over the same sky area. We have optical spectroscopic identifications for 70 X-ray sources. Of these, 42 are broad emission-line AGN with a wide range of redshifts. The optical counterparts of a further 23 sources are narrow emission line galaxies and absorption line galaxies. These 23 sources all lie at z < I and typically have lower X-ray luminosities than the broad-line AGN. About half of them show significant X-ray absorption and are almost certainly intrinsically absorbed AGN. However some of them have unabsorbed, AGN-like, power-law components in their X-ray spectra, but do not show broad emission lines in their optical spectra, These sources may be weak, unobscured AGN in bright galaxies and their existence at low redshifts could be a consequence of the strong cosmological evolution of AGN characteristic luminosities.

XMM–Newton 13H deep field – I. X-ray sources

We present the results of a deep X-ray survey conducted with XMM‐Newton, centred on the UK ROSAT 13 H deep field area. This region covers 0.18 deg 2 , and is the first of the two areas covered with XMM‐Newton as part of an extensive multiwavelength survey designed to study the nature and evolution of the faint X-ray source population. We have produced detailed Monte Carlo simulations to obtain a quantitative characterization of the source detection procedure and to assess the reliability of the resultant sourcelist. We use the simulations to establish a likelihood threshold, above which we expect less than seven (3 per cent) of our sources to be spurious. We present the final catalogue of 225 sources. Within the central 9 arcmin, 68 per cent of source positions are accurate to 2 arcsec, making optical follow-up relatively straightforward. We construct the N (>S) relation in four energy bands: 0.2‐0.5, 0.5‐2, 2‐5 and 5‐10 keV. In all but our highest energy band we find that the source counts can be represented by a double power law with a bright-end slope consistent with the Euclidean case and a break around 10 −14y erg cm −2 s −1 . Below this flux, the counts exhibit a flattening. Our source counts reach densities of 700, 1300, 900 and 300 deg −2 at fluxes of 4.1 × 10 −16 , 4.5 × 10 −16 , 1.1 × 10 −15 and 5.3 × 10 −15 erg cm −2 s −1 in the 0.2‐0.5, 0.5‐2, 2‐5 and 5‐10 keV energy bands, respectively. We have compared our source counts with those in the two Chandra deep fields and Lockman hole, and found our source counts to be amongst the highest of these fields in all energy bands. We resolve >51 per cent (>50 per cent) of the X-ray background emission in the 1‐2 keV (2‐5 keV) energy bands. Ke yw ords: surveys ‐ galaxies: active ‐ quasars: general ‐ X-rays: galaxies.

A medium-deep Chandra and Subaru survey of the 13-h XMM/ROSAT deep survey area

We present the results of a Chandra ACIS-I survey of a high-latitude region at 13 h +38degrees which was earlier observed with ROSAT and which has recently been observed by XMM-Newton for 200 ks. XMM-Newton will provide good-quality X-ray spectra for over 200 sources with fluxes around the knee of the log N / log S, which are responsible for the bulk of the X-ray background. The main aim of the Chandra observations is to provide arcsecond, or better, positions, and hence reliable identifications, for the XMM-Newton sources. The ACIS-I observations were arranged in a mosaic of four 30-ks pointings, covering almost all of the 15-arcmin radius XMM-Newton /ROSAT field. We detect 214 Chandra sources above a Cash likelihood statistic of 25, which approximates to 5sigma significance, to a limiting flux of similar to1.3 x 10(-15) erg cm(-2) s(-1) (0.5-7 keV). Optical counterparts are derived from a Subaru SuprimeCam image reaching to R similar to 27. The very large majority of the Chandra sources have an optical counterpart, with the distribution peaking at 23 < R < 24, although 14 have no counterpart to R = 27. The fraction of X-ray sources with no identification brighter than R = 27 is similar to that found in deeper Chandra surveys.The majority of the identifications are with galaxies. As found in other Chandra surveys, there is a very wide range of optical magnitudes for a given X-ray flux, implying a range of emission mechanisms, and many sources have high L-X/L-opt ratios, implying absorption at moderate redshift. Comparison with the earlier ROSAT survey shows that the accuracy of the ROSAT positions agrees very well with the predictions from simulations by M-c Hardy et al. and that the large majority of the identifications were correct.

Constraints on the distribution of absorption in the X-ray-selected AGN population found in the 13HXMM-Newton/Chandra deep field

We present an analysis of the X-ray properties of sources detected in the 13 H XMM‐Newton deep (200-ks) field. In order to constrain the absorbed active galactic nuclei (AGN) population, we use extensive Monte Carlo simulations to compare directly the X-ray colours of observed sources with those predicted by several model distributions. In particular, we have carried out our comparisons over the entire 0.2‐10 keV energy range of the XMM‐Newton cameras, making our analysis sensitive to a large range of absorbing column densities. We have tested the simplest form of the unified scheme, whereby the intrinsic luminosity function of absorbed AG Ni sset to be the same as that of their unabsorbed brethren, coupled with various model distributions of absorption. Of the tested distributions, the best-fitting model has the fraction of AGN with absorbing column N H, proportional to (log N H) 8 .W e h ave also tested two extensions to the unified scheme: an evolving absorption scenario, in which the fraction of absorbed sources is larger at higher redshifts, and a luminosity-dependent model in which high-luminosity AGN are less likely to be absorbed. Both of these models provide poorer matches to the observed X-ray colour distributions than the best-fitting simple unified model. We find that a luminosity-dependent density evolution luminosity function reproduces poorly the 0.5‐2 keV source counts seen in the 13 H field. Field-to-field variations could be the cause of this disparity. Computing the X-ray colours with a simple absorbed power-law spectral model is found to overpredict, by a factor of 2, the fraction of hard sources that are completely absorbed below 0.5 keV, implying that an additional source of soft-band flux must be present in a number of the absorbed sources. The tested synthesis models predict that around 16 per cent of the detections in the 13 H field are due to AGN at z > 3. However, so far, only a single AGN with z > 3 has been identified in our approximately 50 per cent complete optical spectroscopy follow-up programme. Finally, we use our simulations to demonstrate the efficacy of a hardness ratio selection scheme at selecting absorbed sources for further study. Using this selection scheme, we show that around 40 per cent of the 13 H sample are expected to be AGN with N H > 10 22 cm −2 . Ke yw ords: surveys ‐ galaxies: active ‐ quasars: general ‐ X-rays: galaxies.

Starburst activity in a ROSAT narrow emission-line galaxy

We present multiwaveband photometric and optical spectropolarimetric observations of the R = 15.9 narrow emission line galaxy R117 A which lies on the edge of the error circle of the ROSAT X-ray source R117 (from M c Hardy et al. 1998). The overall spectral energy distribution of the galaxy is well modelled by a combination of a normal spiral galaxy and a moderate-strength burst of star formation. The far infrared and radio emission is extended along the major axis of the galaxy, indicating an extended starburst. On positional grounds, the galaxy is a good candidate for the identification of R117 and the observed X-ray flux is very close to what would be expected from a starburst of the observed far infra-red and radio fluxes. Although an obscured high redshift QSO cannot be entirely ruled out as contributing some fraction of the X-ray flux, we find no candidates to K = 20.8 within the X-ray errorbox and so conclude that R117 A is responsible for a large fraction, if not all, of the X-ray emission from R117. Searches for indicators of an obscured AGN in R117 A have so far proven negative; deep spectropolarimetric observations show no signs of broad lines to a limit of one per cent and, for the observed far infra-red and radio emission, we would expect a ten times greater X-ray flux if the overall emission were powered by an AGN. We therefore conclude that the X-ray emission from R117 is dominated by starburst emission from the galaxy R117 A.

Is GRS 1915+105 A Microquasar?

The large mechanical luminosity of the jets of GRS 1915+105 should give rise to luminous emission regions, similar to those observed in radio galaxies, where the jets interact with the gas surrounding the source. However, no radio synchrotron emission of the expected morphology has been found. Here we present the results of a study suggesting that radio bremsstrahlung from the compressed and heated ISM in front of the jets should be detectable, while the synchrotron lobes may be too faint. We identify these jet impact sites with two well-known IRAS regions. This identification suggests a distance of GRS 1915+105 of 6.5 ± 1.6 kpc, significantly closer than the usually assumed distance of 11–12 kpc.We discuss the implications of this reduced distance estimate. The non-detection of the synchrotron radio lobes implies a significant fraction of non-radiating particles, possibly protons, in the jets. The apparent motion of small-scale jet components is not superluminal, so if superluminal motion is required for an object to be termed a microquasar, GRS 1915+105 actually does not qualify. The mass of the black hole in the system is increased to 21±9 M⊙, while the mechanical luminosity of the jets is reduced to 14% of the Eddington luminosity.