Relativistic ionized accretion disc models of MCG–6-30-15

We present BeppoSAX observations of Nova Velorum 1999 (V382 Vel), carried out in a broad X-ray band covering 0.1–300 keV only 15 d after the discovery and again after 6 months. The nova was detected at day 15 with the BeppoSAX instruments which measured a flux F _x≃1.8×10⁻¹¹ erg cm⁻² s⁻¹ in the 0.1–10 keV range and a 2 σ upper limit F _x<6.7×10⁻¹² erg cm⁻² s⁻¹ in the 15–60 keV range. We attribute the emission to shocked nebular ejecta at a plasma temperature kT ≃6 keV . At six months no bright component emerged in the 15–60 keV range, but a bright central supersoft X-ray source appeared. The hot nebular component previously detected had cooled to a plasma temperature kT <1 keV . There was strong intrinsic absorption of the ejecta in the first observation and not in the second, because the column density of neutral hydrogen decreased from N (H)≃1.7×10²³ to N (H)≃10²¹ cm⁻² (close to the interstellar value). The unabsorbed X-ray flux also decreased from F _x=4.3×10⁻¹¹ to F _x≃10⁻¹² erg cm⁻² s⁻¹ .     

The imprint of massive black hole formation models on the LISA data stream

The formation, merging and accretion history of massive black holes (MBHs) along the hierarchical build-up of cosmic structures leaves a unique imprint on the background of gravitational waves (GWs) at mHz frequencies. We study here, by means of dedicated simulations of black hole build-up, the possibility of constraining different models of black hole cosmic evolution using future GW space-borne missions, such as LISA . We consider two main scenarios for black hole formation, namely, one where seeds are light (  ≃10² M_⊙  , remnant of Population III stars) and one where seeds are heavy (  ≳10⁴ M_⊙  , direct collapse). In all the models we have investigated, MBH binary coalescences do not produce a stochastic GW background, but rather, a set of individual resolved events. Detection of several hundreds merging events in a 3-yr LISA mission will be the sign of a heavy seed scenario with efficient formation of black hole seeds in a large fraction of high-redshift haloes. At the other extreme, a low event rate, about a few tens in 3 yr, is peculiar of scenarios where either the seeds are light, and many coalescences do not fall into the LISA band, or seeds are massive, but rare. In this case a decisive diagnostic is provided by the shape of the mass distribution of detected events. Light binaries  ( m < 10⁴ M_⊙)  are predicted in a fairly large number in Population III remnant models, but are totally absent in direct collapse models. Finally, a further, helpful diagnostic of black hole formation models lies in the distribution of the mass ratios in binary coalescences. While heavy seed models predict that most of the detected events involve equal-mass binaries, in the case of light seeds, mass ratios are equally distributed in the range 0.1–1.     

Large-scale magnetic fields: galaxy two-point correlation function

We study the effect of large-scale tangled magnetic fields on the galaxy two-point correlation function in the redshift space. We show that (i) the magnetic field effects can be comparable to the gravity-induced clustering for present magnetic field strength   B ₀≃ 5 × 10⁻⁸ G  , (ii) the absence of this signal from the present data gives an upper bound   B ₀≲ 3 × 10⁻⁸ G  and (iii) the future data can probe the magnetic fields of  ≃10⁻⁸ G  . A comparison with other constraints on the present magnetic field shows that they are marginally compatible. However, if the magnetic fields corresponding to   B ₀≃ 10⁻⁸ G  existed at the last scattering surface, they will cause unacceptably large cosmic microwave background radiation anisotropies.     

Simulating the formation of a protocluster at z∼ 2

We present results from two high-resolution hydrodynamical simulations of protocluster regions at   z ≃ 2.1  . The simulations have been compared to observational results for the so-called Spiderweb galaxy system, the core of a putative protocluster region at   z = 2.16  , found around a radio galaxy. The simulated regions have been chosen so as to form a poor cluster with   M ₂₀₀≃ 10¹⁴  h ⁻¹ M_⊙  (C1) and a rich cluster with   M ₂₀₀≃ 2 × 10¹⁵  h ⁻¹ M_⊙  (C2) at   z = 0  . The simulated protoclusters show evidence of ongoing assembly of a dominating central galaxy. The stellar mass of the brightest cluster galaxy of the C2 system is in excess with respect to observational estimates for the Spiderweb galaxy, with a total star formation rate which is also larger than indicated by observations. We find that the projected velocities of galaxies in the C2 cluster are consistent with observations, while those measured for the poorer cluster C1 are too low compared with the observed velocities. We argue that the Spiderweb complex resembles the high-redshift progenitor of a rich galaxy cluster. Our results indicate that the included supernovae feedback is not enough to suppress star formation in these systems, supporting the need of introducing active galactic nuclei feedback. According to our simulations, a diffuse atmosphere of hot gas in hydrostatic equilibrium should already be present at this redshift, and enriched at a level comparable to that of nearby galaxy clusters. The presence of this gas should be detectable with future deep X-ray observations.     

High-energy emission from the starburst galaxy NGC 253

Measurement sensitivity in the energetic γ-ray region has improved considerably and is about to increase further in the near future, motivating a detailed calculation of high-energy (HE; ≥100 MeV) and very high-energy (VHE; ≥100 GeV) γ-ray emission from the nearby starburst galaxy NGC 253. Adopting the convection–diffusion model for energetic electron and proton propagation, and accounting for all the relevant hadronic and leptonic processes, we determine the steady-state energy distributions of these particles by a detailed numerical treatment. The electron distribution is directly normalized by the measured synchrotron radio emission from the central starburst region; a commonly expected theoretical relation is then used to normalize the proton spectrum in this region. Doing so fully specifies the electron spectrum throughout the galactic disc and, with an assumed spatial profile of the magnetic field, the predicted radio emission from the full disc matches well the observed spectrum, confirming the validity of our treatment. The resulting radiative yields of both particles are calculated; the integrated HE and VHE fluxes from the entire disc are predicted to be   f (≥100 MeV) ≃ (1.8^+1.5_−0.8) × 10⁻⁸ cm⁻² s⁻¹  and   f (≥100 GeV) ≃ (3.6^+3.4_−1.7) × 10⁻¹² cm⁻² s⁻¹  , with a central magnetic field value   B ₀≃ 190 ± 10 μ  G. We discuss the feasibility of measuring emission at these levels with the space-borne Fermi and ground-based Cherenkov telescopes.     

Low-frequency gravitational waves from cosmological compact binaries

We consider gravitational waves emitted by various populations of compact binaries at cosmological distances. We use population synthesis models to characterize the properties of double neutron stars, double black holes and double white dwarf binaries, and white dwarf–neutron star, white dwarf–black hole and black hole–neutron star systems.
We use the observationally determined cosmic star formation history to reconstruct the redshift distribution of these sources and their merging rate evolution.
The gravitational signals emitted by each source during its early spiralling in phase add randomly to produce a stochastic background in the low-frequency band with spectral strain amplitude between ~10⁻¹⁸ and ~5×10⁻¹⁷ Hz^−1/2 at frequencies in the interval ~5×10⁻⁶–5×10⁻⁵ Hz.
The overall signal, which at frequencies above 10⁻⁴ Hz is largely dominated by double white dwarf systems, might be detectable with LISA in the frequency range 1–10 mHz and acts like a confusion-limited noise component, which might limit the LISA sensitivity at frequencies above 1 mHz.     

The SCUBA HAlf Degree Extragalactic Survey – VI. 350-μm mapping of submillimetre galaxies

A follow-up survey using the Submillimetre High-Angular Resolution Camera (SHARC-II) at 350 μm has been carried out to map the regions around several 850-μm-selected sources from the Submillimetre HAlf Degree Extragalactic Survey (SHADES). These observations probe the infrared (IR) luminosities and hence star formation rates in the largest existing, most robust sample of submillimetre galaxies (SMGs). We measure 350-μm flux densities for 24 850-μm sources, seven of which are detected at ≥2.5σ within a 10 arcsec search radius of the 850-μm positions. When results from the literature are included the total number of 350-μm flux density constraints of SHADES SMGs is 31, with 15 detections. We fit a modified blackbody to the far-IR (FIR) photometry of each SMG, and confirm that typical SMGs are dust-rich  ( M _dust≃ 9 × 10⁸ M_⊙)  , luminous  ( L _FIR≃ 2 × 10¹² L_⊙)  star-forming galaxies with intrinsic dust temperatures of ≃35 K and star formation rates of  ≃400 M_⊙ yr⁻¹  . We have measured the temperature distribution of SMGs and find that the underlying distribution is slightly broader than implied by the error bars, and that most SMGs are at 28 K with a few hotter. We also place new constraints on the 350-μm source counts, N ₃₅₀(>25 mJy) ∼ 200–500 deg⁻².     

Wide triplets of galaxies: collapse on spatial scales of ∼1 Mpc

We study triple systems of galaxies with mean projected harmonic separation ≃0.6  h ⁻¹ Mpc     We call the systems 'wide triplets', in contrast to compact triplets with mean projected harmonic separation ≃0.04  h ⁻¹ Mpc, studied by Karachentsev et al. Data are taken for 108 wide triplets from a list compiled by Trofimov & Chernin; at least one-third of them are considered to be probably isolated physical systems. With typical crossing times of about the Hubble time, the wide triplets seem to be in a state of ongoing collapse. This is confirmed by a set of computer models which simulate well the observational characteristics of the ensemble of wide triplets. The simulations also give a statistical estimate of the total mass of a typical wide triplet: it proves to be ≃10¹³ M_⊙. This figure indicates that the dark matter mass is 15–30 times the mass of baryonic matter in the systems. The dynamics of wide triplets, as well as their dark matter content, provide new direct cosmological constraints by establishing that hierarchical evolution is occurring on a mass scale of ∼10¹³ M_⊙ and a spatial scale of ∼1 Mpc.     

A survey of hard spectrum ROSAT sources – I. X-ray source catalogue

We present a catalogue of 147 serendipitous X-ray sources selected to have hard spectra ( α <0.5) from a survey of 188 ROSAT fields. Such sources must be the dominant contributors to the X-ray background at faint fluxes. We have used Monte Carlo simulations to verify that our technique is very efficient at selecting hard sources: the survey has 10 times as much effective area for hard sources as it has for soft sources above a 0.5–2 keV flux level of 10⁻¹⁴ erg cm⁻² s⁻¹. The distribution of best-fitting spectral slopes of the hard sources suggests that a typical ROSAT hard source in our survey has a spectral slope α ∼0. The hard sources have a steep number flux relation (d N /d S ∝ S ^{− γ} with a best-fitting value of γ =2.72±0.12) and make up about 15 per cent of all 0.5–2 keV sources with S >10⁻¹⁴ erg cm⁻² s⁻¹. If their N ( S ) continues to fainter fluxes, the hard sources will comprise ∼40 per cent of sources with 5×10⁻¹⁵< S <10⁻¹⁴ erg cm⁻² s⁻¹. The population of hard sources can therefore account for the harder average spectra of ROSAT sources with S <10⁻¹⁴ erg cm⁻² s⁻¹. They probably make a strong contribution to the X-ray background at faint fluxes and could be the solution to the X-ray background spectral paradox.     

Impact of cosmic rays on Population III star formation

We explore the implications of a possible cosmic-ray (CR) background generated during the first supernova explosions that end the brief lives of massive Population III stars. We show that such a CR background could have significantly influenced the cooling and collapse of primordial gas clouds in minihaloes around redshifts of   z ∼ 15–20  , provided the CR flux was sufficient to yield an ionization rate greater than about 10⁻¹⁹ s⁻¹ near the centre of the minihalo. The presence of CRs with energies  ≲10⁷  eV would indirectly enhance the molecular cooling in these regions, and we estimate that the resulting lower temperatures in these minihaloes would yield a characteristic stellar mass as low as  ∼10 M_⊙  . CRs have a less-pronounced effect on the cooling and collapse of primordial gas clouds inside more massive dark matter haloes with virial masses  ≳10⁸ M_⊙  at the later stages of cosmological structure formation around   z ∼ 10–15  . In these clouds, even without CR flux the molecular abundance is already sufficient to allow cooling to the floor set by the temperature of the cosmic microwave background.     

Gravitational waves from scattering of stellar-mass black holes in galactic nuclei

Stellar-mass black holes (BHs) are expected to segregate and form a steep density cusp around supermassive black holes (SMBHs) in galactic nuclei. We follow the evolution of a multimass system of BHs and stars by numerically integrating the Fokker–Planck energy diffusion equations for a variety of BH mass distributions. We find that the BHs 'self-segregate', and that the rarest, most massive BHs dominate the scattering rate closest to the SMBH  (≲10⁻¹ pc)  . BH–BH binaries form out of gravitational wave emission during BH encounters. We find that the expected rate of BH coalescence events detectable by Advanced LIGO is  ∼1–10² yr⁻¹  , depending on the initial mass function of stars in galactic nuclei and the mass of the most massive BHs. We find that the actual merger rate is likely ∼10 times larger than this due to the intrinsic scatter of stellar densities in many different galaxies. The BH binaries that form this way in galactic nuclei have significant eccentricities as they enter the LIGO band (90 per cent with   e > 0.9  ), and are therefore distinguishable from other binaries, which circularize before becoming detectable. We also show that eccentric mergers can be detected to larger distances and greater BH masses than circular mergers, up to  ∼700 M_⊙  . Future ground-based gravitational wave observatories will be able to constrain both the mass function of BHs and stars in galactic nuclei.     

Universal gas density and temperature profile

We explore possibilities of collapse and star formation in Population III objects exposed to the external ultraviolet background (UVB) radiation. Assuming spherical symmetry, we solve self-consistently radiative transfer of photons, non-equilibrium H₂ chemistry and gas hydrodynamics. Although the UVB does suppress the formation of low-mass objects, the negative feedback turns out to be weaker than previously suggested. In particular, the cut-off scale of collapse drops significantly below the virial temperature T _vir∼10⁴ K at weak UV intensities ( J ₂₁≲10⁻²) , owing to both self-shielding of the gas and H₂ cooling. Clouds above this cut-off tend to contract highly dynamically, further promoting self-shielding and H₂ formation. For plausible radiation intensities and spectra, the collapsing gas can cool efficiently to temperatures well below 10⁴ K before rotationally supported and the final H₂ fraction reaches ∼ 10⁻³.
Our results imply that star formation can take place in low-mass objects collapsing in the UVB. The threshold baryon mass for star formation is ∼ 10⁹ M_⊙ for clouds collapsing at redshifts z ≲3 , but drops significantly at higher redshifts. In a conventional cold dark matter universe, the latter coincides roughly with that of the 1 σ density fluctuations. Objects near and above this threshold can thus constitute 'building blocks' of luminous structures, and we discuss their links to dwarf spheroidal/elliptical galaxies and faint blue objects. These results suggest that the UVB can play a key role in regulating the star formation history of the Universe.     

The size and shape of local voids

We study the size and shape of low-density regions in the local Universe, which we identify in the smoothed density field of the PSCz flux-limited IRAS galaxy catalogue. After quantifying the systematic biases that enter the detection of voids using our data set and method, we identify, using a smoothing length of 5  h ⁻¹ Mpc, 14 voids within 80  h ⁻¹ Mpc (having volumes 10³  h ⁻³ Mpc³) and, using a smoothing length of 10  h ⁻¹ Mpc, eight voids within 130  h ⁻¹ Mpc (having volumes  8×10³ h⁻³ Mpc³)  . We study the void size distribution and morphologies and find that there is roughly an equal number of prolate and oblate-like spheroidal voids. We compare the measured PSCz void shape and size distributions with those expected in six different cold dark matter (CDM) models and find that only the size distribution can discriminate between models. The models preferred by the PSCz data are those with intermediate values of   σ ₈(≃0.83)  , independent of cosmology.     

Decay of the vacuum energy into cosmic microwave background photons

We examine the possibility of the decay of the vacuum energy into a homogeneous distribution of a thermalized cosmic microwave background (CMB), which is characteristic of an adiabatic vacuum energy decay into photons. It is shown that observations of the primordial density fluctuation spectrum, obtained from CMB and galaxy distribution data, restrict the possible decay rate. When photon creation due to an adiabatic vacuum energy decay takes place, the standard linear temperature dependence   T ( z ) = T ₀(1 + z )  is modified, where T ₀ is the present CMB temperature, and can be parametrized by a modified CMB temperature dependence     . From the observed CMB and galaxy distribution data, a strong limit on the maximum value of the decay rate is obtained by placing a maximum value  β_max≃ 3.4 × 10⁻³  on the β parameter.     

The space density of cataclysmic variables: constraints from the ROSAT North Ecliptic Pole survey

We use the ROSAT North Ecliptic Pole (NEP) survey to construct a small, but purely X-ray flux-limited sample of cataclysmic variable stars (CVs). The sample includes only four systems, two of which (RX J1715.6+6856 and RX J1831.7+6511) are new discoveries. We present time-resolved spectroscopy of the new CVs and measure orbital periods of 1.64 ± 0.02 and 4.01 ± 0.03 h for RX J1715.6+6856 and RX J1831.7+6511, respectively. We also estimate distances for all the CVs in our sample, based mainly on their apparent brightness in the infrared. The space density of the CV population represented by our small sample is  1.1^+2.3_−0.7× 10⁻⁵ pc⁻³  . We can also place upper limits on the space density of any subpopulation of CVs too faint to be included in the NEP survey. In particular, we show that if the overall space density of CVs is as high as  2 × 10⁻⁴ pc⁻³  (as has been predicted theoretically), the vast majority of CVs must be fainter than   L _X≃ 2 × 10²⁹ erg s⁻¹  .     

The local star formation rate and radio luminosity density

We present a new determination of the local volume-averaged star formation rate from the 1.4-GHz luminosity function of star forming galaxies. Our sample, taken from the   B ≤12  Revised Shapley–Ames catalogue (231 normal spiral galaxies over an effective area of 7.1 sr) has ≃100 per cent complete radio detections and is insensitive to dust obscuration and cirrus contamination. After removal of known active galaxies, the best-fitting Schechter function has a faint-end slope of  −1.27±0.07  in agreement with the local H α luminosity function, characteristic luminosity   L ∗=(2.6±0.7)×10²² W Hz⁻¹  and density   φ ∗=(4.8±1.1)×10⁻⁴ Mpc⁻³.  The inferred local radio luminosity density of  (1.73±0.37±0.03)×10¹⁹ W Hz⁻¹ Mpc⁻³  (Poisson noise, large-scale structure fluctuations) implies a volume-averaged star formation rate ∼2 times larger than the Gallego et al. H α estimate, i.e.   ρ _1.4 GHz=(2.10±0.45±0.04)×10⁻² M_⊙ yr⁻¹ Mpc⁻³  for a Salpeter initial mass function from  0.1–125 M_⊙  and Hubble constant of 50 km s⁻¹ Mpc⁻¹. We demonstrate that the Balmer decrement is a highly unreliable extinction estimator, and argue that optical–ultraviolet (UV) star formation rates (SFRs) are easily underestimated, particularly at high redshift.     

Investigating ultra-long gravitational waves with measurements of pulsar rotational parameters

A method is suggested with which to explore the gravitational wave background (GWB) in the frequency range 10⁻¹²–10⁻⁸ Hz. This method is based on the precise measurements of pulsar rotational parameters: the influence of gravitational waves (GWs) in this frequency range will affect these parameters and therefore some conclusions about the energy density of the GWB can be made using analysis of the derivatives of pulsar rotational frequency. The calculated values of the second derivative from a number of pulsars limit the density of the GWB, Ω_gw, as follows:  Ω_gw < 2 × 10⁻⁶  . Also, the time series of the frequency ν of different pulsars in a pulsar array can be cross-correlated pairwise in the same manner as in anomalous residuals analysis, thus providing the possibility of GWB detection in the ultra-low-frequency range.     

The redshift evolution of clustering in the Hubble Deep Field

We present a correlation function analysis for the catalogue of photometric redshifts obtained from the Hubble Deep Field image by Fernandez-Soto, Lanzetta & Yahil. By dividing the catalogue into redshift bins of width Δ z =0.4 we measured the angular correlation function w ( θ ) as a function of redshift up to z ∼4.8. From these measurements we derive the trend of the correlation length r ₀. We find that r ₀( z ) is roughly constant with look-back time up to z ≃2, and then increases to higher values at z ≳2.4. We estimate the values of r ₀, assuming ξ ( r , z )=[ r r ₀( z )]^{− γ} , γ =1.8 and various geometries. For Ω₀=1 we find r ₀( z =3)≃7.00±4.87  h ⁻¹ Mpc, in good agreement with the values obtained from analysis of the Lyman break galaxies.     

Peculiar nature of hard X-ray eclipse in SS433 from INTEGRAL observations

The analysis of hard X-ray INTEGRAL observations (2003–2008) of superaccreting Galactic microquasar SS433 at precessional phases of the source with the maximum disc opening angle is carried out. It is found that the shape and width of the primary X-ray eclipse are strongly variable, suggesting additional absorption in dense stellar wind and gas outflows from the optical A7I component and the wind–wind collision region. The independence of the observed hard X-ray spectrum on the accretion disc precessional phase suggests that hard X-ray emission (20–100 keV) is formed in an extended, hot, quasi-isothermal corona, probably heated by interaction of relativistic jet with inhomogeneous wind outflow from the precessing supercritical accretion disc. A joint modelling of X-ray eclipsing and precessional hard X-ray variability of SS433 revealed by INTEGRAL by a geometrical model suggests the binary mass ratio   q = m_x / m _v ≃  0.25–0.5. The absolute minimum of joint orbital and precessional  χ²  residuals is reached at   q ≃ 0.3  . The found binary mass ratio range allows us to explain the substantial precessional variability of the minimum brightness at the middle of the primary optical eclipse. For the mass function of the optical star   f _v = 0.268 M_⊙  as derived from Hillwig & Gies data, the obtained value of   q ≃ 0.3  yields the masses of the components   m_x ≃ 5.3 M_⊙, m _v ≃ 17.7 M_⊙  , confirming the black hole nature of the compact object in SS433.     

Clustering of X-ray selected active galactic nuclei

A total of 235 active galactic nuclei (AGN) from two different soft X-ray surveys [the ROSAT Deep Survey (DRS) and the ROSAT International X-ray Optical Survey (RIXOS)] with redshifts between 0 and 3.5 are used to study the clustering of X-ray selected AGN and its evolution. A 2σ significant detection of clustering of such objects is found on scales < 40–80 h ⁻¹ Mpc in the RIXOS sample, while no clustering is detected on any scales in the DRS sample. Assuming a single power-law model for the spatial correlation function (SCF), quantitative limits on the AGN clustering have been obtained: a comoving correlation length 1.5 ≲  r ₀ ≲ 3.3  h ⁻¹ Mpc is implied for comoving evolution, while 1.9 ≲  r ₀ ≲ 4.8 for stable clustering and 2.2 ≲  r ₀ ≲ 5.5 for linear evolution. These values are consistent with the correlation lengths and evolutions obtained for galaxy samples, but imply smaller amplitude or faster evolution than recent ultraviolet and optically selected AGN samples. We also constrain the ratio of bias parameters between X-ray selected AGN and IRAS galaxies to be ≲ 1.7 on scales ≲ 10  h ⁻¹ Mpc, a somewhat smaller value than is inferred from local large-scale dynamical studies.