Helen M. Johnston

An ultra-relativistic outflow from a neutron star accreting gas from a companion

Collimated relativistic outflows—also known as jets—are amongst the most energetic phenomena in the Universe. They are associated with supermassive black holes in distant active galactic nuclei, accreting stellar-mass black holes and neutron stars in binary systems and are believed to be responsible for γ-ray bursts. The physics of these jets, however, remains something of a mystery in that their bulk velocities, compositions and energetics remain poorly determined. Here we report the discovery of an ultra-relativistic outflow from a neutron star accreting gas within a binary stellar system. The velocity of the outflow is comparable to the fastest-moving flows observed from active galactic nuclei, and its strength is modulated by the rate of accretion of material onto the neutron star. Shocks are energized further downstream in the flow, which are themselves moving at mildly relativistic bulk velocities and are the sites of the observed synchrotron emission from the jet. We conclude that the generation of highly relativistic outflows does not require properties that are unique to black holes, such as an event horizon.

Radio galaxies in the 2SLAQ Luminous Red Galaxy Survey - I. The evolution of low-power radio galaxies to z ∼ 0.7

We have combined optical data from the 2dF-SDSS (Sloan Digital Sky Survey) LRG (Luminous Red Galaxy) and QSO (quasi-stellar object) (2SLAQ) redshift survey with radio measurements from the 1.4 GHz VLA (Very Large Array) FIRST (Faint Images of the Radio Sky at Twentycm) and NVSS (NRAO VLA Sky Survey) surveys to identify a volume-limited sample of 391 radio galaxies at redshift 0.4 < z < 0.7. By determining an accurate radio luminosity function for luminous early-type galaxies in this redshift range, we can investigate the cosmic evolution of the radio-galaxy population over a wide range in radio luminosity. The low-power radio galaxies in our LRG sample (those with 1.4 GHz radio luminosities in the range 10 24 to 10 25 WH z −1 , corresponding to Fanaroff‐Riley I (FR I) radio galaxies in the local Universe) undergo significant cosmic evolution over the redshift range 0 < z < 0.7, consistent with pure luminosity evolution of the form (1 + z) k , where k = 2.0 ± 0.3. Our results appear to rule out (at the 6‐7σ level) models in which low-power radio galaxies undergo no cosmic evolution. The most powerful radio galaxies in our sample (with radio luminosities above 10 26 WH z −1 ) may undergo more rapid evolution over the same redshift range. The evolution seen in the low-power radio-galaxy population implies that the total energy input into massive early-type galaxies from active galactic nucleus (AGN) heating increases with redshift, and was at least 50 per cent higher at z ∼ 0.55 (the median redshift of the 2SLAQ LRG sample) than in the local universe.

The clustering of radio galaxies at z≃ 0.55 from the 2SLAQ LRG survey

We examine the clustering properties of low-power radio galaxies at redshift 0.4 < z < 0.8, using data from the 2SLAQ Luminous Red Galaxy (LRG) survey, and find that radio-detected LRGs (with typical optical luminosities of 3–5L∗ and 1.4 GHz radio powers in the range 1024–1026WHz−1) are significantly more clustered than a matched population of radio-quiet (≲1024WHz−1) LRGs with the same distribution in optical luminosity and colour. The measured scalelength of the two-point cross-correlation function between the full LRG sample and the radio-detected LRGs is 9.57 ± 0.50 h−1 Mpc, compared to 8.47 ± 0.27 h<sup.−1 Mpc for the matched sample of radio-quiet LRGs; while the implied scalelength of the auto-correlation function, r0, is 12.3 ± 1.2 h−1 Mpc and 9.02 ± 0.52 h−1 Mpc for the radio-detected and radio-quiet samples, respectively. We further interpret our clustering measurements in the halo model framework and demonstrate that the radio-detected LRGs have typical halo masses of 10.1 ± 1.4 × 1013 h−1M⊙ and bias of 2.96 ± 0.17, compared to 6.44 ± 0.32 × 1013 h−1M⊙ and 2.49 ± 0.02 for the radio-quiet sample. A model in which the radio-detected LRGs are almost all central galaxies within haloes provides the best fit to the measured clustering, and we estimate that at least 30 per cent of all 2SLAQ LRGs with the same clustering amplitude as the radio-detected LRGs are currently radio loud. Our results imply that radio-detected galaxies in the 2SLAQ LRG sample typically occupy more massive haloes than other LRGS of the same optical luminosity, so the probability of finding a radio-loud active galactic nucleus (AGN) in a massive galaxy at z∼0.55 is influenced by the halo mass and/or cluster environment in addition to the well-known dependence on optical luminosity. If we model the radio-detected fraction of LRGs, Frad, as a function of halo mass M, then the data are well-fitted by a power law of the form Frad ∝ M0.65±0.23. The observed relationship between radio emission and clustering strength could plausibly arise either through a higher fuelling rate of gas on to the central black holes of galaxies in the most massive haloes (producing more powerful radio jets) or through the presence of a denser IGM (which would provide a more efficient working surface for the jets, thus boosting their observed radio luminosity). Further work is needed to determine which of these effects is dominant.

The γ-ray binary LS 5039: mass and orbit constraints from MOST observations

The results of a coordinated space-based photometric and ground-based spectroscopic observing campaign on the enigmatic γ -ray binary LS 5039 are reported. 16 d of observations from the MOST satellite have been combined with high-resolution optical echelle spectroscopy from the 2.3-m ANU Telescope in Siding Spring, Australia. These observat ions were used to measure the orbital parameters of the binary and to study the properties of stellar wind from the O primary. We found that any broad-band optical photometric variability at the orbital period is below the 2 mmag level, supporting the scenario that the orbital eccentricity of the system is near the 0.24 ± 0.08 value implied by our spectroscopy, which is lower than values previously obtained by other workers. The low amplitude optical variability also implies the component masses are at the higher end of estimates based on the primary’s O6.5V((f)) spectral type with

Optical spectroscopy of GX 339-4 during the high—soft and low—hard states — II. Line ionization and emission region

We have carried out observations of the X-ray transient GX 339-4 during its high-soft and low-hard X-ray spectral states. Our high-resolution spectroscopic observation in 1999 April suggests that the H alpha line has a single-peaked profile in the low-hard state as speculated in our previous paper. The He II lambda 4686 line, however, has a double-peaked profile in both the high-soft and low-hard states. This suggests that the line-emission mechanism is different in the two states. Our interpretation is that double-peaked lines are emitted from a temperature-inversion layer on the accretion disc surface when it is irradiatively heated by soft X-rays. Single-peaked lines may be emitted from outflow/wind matter driven by hard X-ray heating. We have constructed a simple plane-parallel model and we use it to illustrate that a temperature-inversion layer can be formed at the disc surface under X-ray illumination. We also discuss the conditions required for the formation of temperature inversion and line emission. Based on the velocity separations measured for the double-peaked lines in the high-soft state, we propose that GX 339-4 is a low-inclination binary system. The orbital inclination is about 15 degrees if the orbital period is 14.8 h.

Optical properties of high-frequency radio sources from the Australia Telescope 20 GHz (AT20G) Survey

Our current understanding of radio-loud active galactic nuclei (AGN) comes predominantly from studies at frequencies of 5 GHz and below. With the recent completion of the Australia Telescope 20 GHz (AT20G) survey, we can now gain insight into the high-frequency radio properties of AGN. This paper presents supplementary information on the AT20G sources in the form of optical counterparts and redshifts. Optical counterparts were identified using the SuperCOSMOS data base and redshifts were found from either the 6dF Galaxy Survey or the literature. We also report 144 new redshifts. For AT20G sources outside the Galactic plane, 78.5 per cent have optical identifications and 30.9 per cent have redshift information. The optical identification rate also increases with increasing flux density. Targets which had optical spectra available were examined to obtain a spectral classification. There appear to be two distinct AT20G populations; the high luminosity quasars that are generally associated with point-source optical counterparts and exhibit strong emission lines in the optical spectrum, and the lower luminosity radio galaxies that are generally associated with passive galaxies in both the optical images and spectroscopic properties. It is suggested that these different populations can be associated with different accretion modes (cold-mode or hot-mode). We find that the cold-mode sources have a steeper spectral index and produce more luminous radio lobes, but generally reside in smaller host galaxies than their hot-mode counterparts. This can be attributed to the fact that they are accreting material more efficiently. Lastly, we compare the AT20G survey with the S-cubed semi-empirical (S3-SEX) models and conclude that the S3-SEX models need refining to correctly model the compact cores of AGN. The AT20G survey provides the ideal sample to do this.

A new search for distant radio galaxies in the Southern hemisphere – III. Optical spectroscopy and analysis of the MRCR–SUMSS sample

We have compiled a sample of 234 ultra-steep-spectrum(USS)-selected radio sources in order to nd high-redshift radio galaxies (HzRGs). The sample is in the southern sky at 40 2. We analyse the K{z distribution and compare 4arcsec-aperture magnitudes with 64-kpc aperture magnitudes in several surveys from the literature; the MRCR{SUMSS sample is found to be consistent with models for 10 11 {10 12 M galaxies. Dispersions about the ts in the K{z plot support passive evolution of radio galaxy hosts since z > 3. By comparing USS-selected samples in the literature, we nd that the resultant median redshift of the samples shown is not dependent on the ux density distribution or selection frequency of each sample. In addition, our nding that the majority of the radio spectral energy distributions remain straight over a wide frequency range suggests that a k-correction is not responsible for the success of USS-selection in identifying high redshift radio galaxies and therefore the steep radio spectra may be intrinsic to the source or a product of the environment. Two galaxies have been found to have both compact radio structures and strong self-absorption in the Ly line, suggesting they are surrounded by a dense medium. For the bulk of the sources, spectral line ratios show that photoionisation is the primary excitation process.

A search for distant radio galaxies from SUMSS and NVSS – II. Optical spectroscopy

This is the second in a series of papers presenting observations and results for a sample of 76 ultra-steep-spectrum (USS) radio sources in the southern hemisphere designed to nd galaxies at high redshift. Here we focus on the optical spectroscopy program for 53 galaxies in the sample. We report 35 spectroscopic redshifts, based on observations with the Very Large Telescope (VLT), the New Technology Telescope (NTT) and the Australian National University’s 2.3m telescope; they include v e radio galaxies with z > 3. Spectroscopic redshifts for the remaining 18 galaxies could not be conrmed: three are occulted by Galactic stars, eight show continuum emission but no discernible spectral lines, whilst the remaining seven galaxies are undetected in medium-deep VLT integrations. The latter are either at very high redshift (z > 7) or heavily obscured by dust. A discussion of the eciency of the USS technique is presented. Based on the similar space density of z > 3 radio galaxies in our sample compared with other USS-selected samples, we argue that USS selection at 843{1400MHz is an ecien t and reliable technique for nding distant radio galaxies.

Optical spectroscopy of GX 339−4 during the high–soft and low–hard states – I

We carried out spectroscopic observations of the candidate black hole binary GX 339−4 during its low–hard and high–soft X-ray states. We have found that the spectrum is dominated by emission lines of neutral elements with asymmetric, round-topped profiles in the low–hard state. In the high–soft state, however, the emission lines from both neutral and ionized elements have unambiguously resolved double-peaked profiles. The detection of double-peaked emission lines in the high–soft state, with a larger peak separation for higher ionization lines, indicates the presence of an irradiatively heated accretion disc. The round-topped lines in the low–hard state are probably caused by a dense matter outflow from an inflated non-Keplerian accretion disc. Our data do not show velocity modulations of the line centres caused by the orbital motion of the compact object, neither do the line basewidths show substantial variations in each observational epoch. There are no detectable absorption lines from the companion star. All these features are consistent with those of a system with a low-mass companion star and low orbital inclination.

MRC B1221−423: a compact steep‐spectrum radio source in a merging galaxy

We present BVRIK images and spectroscopic observations of the z = 0.17 host galaxy of the compact steep-spectrum radio source MRC B1221 423. This is a young (� 10 5 yr) radio source with double lobes lying well within the visible galaxy. The host galaxy is undergoing tidal interaction with a nearby companion, with shells, tidal tails, and knotty star-forming regions all visible. We analyse the images of the galaxy and its companion pixel-by-pixel, first using colour-magnitude diagrams, and then fitting stellar population models to the spectral energy distributions of each pixel. We also present medium-resolution spectroscopy of the system. The pixels separate cleanly in colour-magnitude diagrams, with pixels of different colours occupying distinct regions of the host galaxy and its companion. Fitting stellar population models to these colours, we have estimated the age of each population. We find three distinct groups of ages: an old population (� � 15 Gyr) in the outskirts of the host galaxy; an intermediate-age population (� � 300 Myr) around the nucleus and tidal tail, and a young population (� < 10 Myr) in the nucleus and blue “knots”. The spectrum of the nucleus shows numerous strong emission lines, including [O i] �6300, [O ii] �3727, [S ii] ��6716, 6731, H�, and [N ii] ��6548, 6583, characteristic of a LINER spectrum. The companion galaxy shows much narrower emission lines with very different line ratios, characteristic of a starburst galaxy. We have evidence for three distinct episodes of star formation in B1221 423. The correlation of age with position suggests the two most recent episodes were triggered by tidal interactions with the companion galaxy. The evidence points to the AGN in the centre of B1221 423 having been “caught in the act” of ignition. However, none of the components we have identified is as young as the radio source, implying that the delay between the interaction and the triggering of the AGN is at least 3×10 8 years.