Aims. The detection of old neutron stars and stellar mass black holes in isolation is one of the most sought after goals of compact object astrophysics. Microlensing surveys may help in achieving ...this aim because the lensing mechanism is independent of the emission properties of the lens. Several black hole candidates have indeed been detected by means of microlensing observations have been reported in the literature. The identification of counterparts, especially in the X-rays, would be a strong argument in favor of the compact nature of these lenses. Methods. We perform a cross-correlation between the catalogs of microlensing events produced by the OGLE, MACHO, and MOA teams, and those of X-rays sources from the data acquired by the XMM-Newton and Chandra satellites. On the basis of our previous work, we select only microlensing events with durations longer than one hundred days, which should contain a large fraction of lenses as compact objects. Our matching criterion takes into account the positional coincidence on the sky. Results. We find a single match between a microlensing event, OGLE-2004-BLG-081 (tE ~ 103 days), and the X-ray source 2XMM J180540.5-273427. The angular separation is ~0.5 arcsec, i.e. well within the 90% error box of the X-ray source. The hardness ratios reported in the 2XMM catalog imply that it has a hard spectrum with a peak between 2 keV and 4.5 keV or it has a softer but highly absorbed spectrum. Moreover, the microlensing event is not fully constrained, and other authors propose a possible association of the source star with either a flaring cataclysmic variable or a RS Canum Venaticorum-like star. Conclusions. The very small angular separation (within uncertainties) is a strong indicator that 2XMM J180540.5-273427 is the X-ray counterpart of the OGLE event. However, the uncertainties in the nature of both the lensed system and the lens itself challenge the interpretation of 2XMM J180540.5-273427 as the first confirmed isolated black hole identified so far. In any case, it verifies the diagnostic capacity of microlensing surveys and open the pathway for further identifications of black hole or neutron star candidates.
Aims. Multiwavelength variability of blazars offers indirect, but very effective, insight into their powerful engines and on the mechanisms through which energy is propagated from the centre down the ...jet. The BL Lac object Mkn 421 is a TeV emitter, a bright blazar at all wavelengths, and therefore an excellent target for variability studies. Methods. We activated INTEGRAL observations of Mkn 421 in an active state on 16–21 April 2013, and complemented them with Fermi-LAT data. Results. We obtained well sampled optical, soft, and hard X-ray light curves that show the presence of two flares and time-resolved spectra in the 3.5–60 keV (JEM-X and IBIS/ISGRI) and 0.1–100 GeV (Fermi-LAT) ranges. The average flux in the 20–100 keV range is 9.1 × 10-11 erg s-1 cm-2 (~4.5 mCrab) and the nuclear average apparent magnitude, corrected for Galactic extinction, is V ≃ 12.2. In the time-resolved X-ray spectra, which are described by broken power laws and, marginally better, by log-parabolic laws, we see a hardening that correlates with flux increase, as expected in refreshed energy injections in a population of electrons that later cool via synchrotron radiation. The hardness ratios between the JEM-X fluxes in two different bands and between the JEM-X and IBIS/ISGRI fluxes confirm this trend. During the observation, the variability level increases monotonically from the optical to the hard X-rays, while the large LAT errors do not allow a significant assessment of the MeV-GeV variability. The cross-correlation analysis during the onset of the most prominent flare suggests a monotonically increasing delay of the lower frequency emission with respect to that at higher frequency, with a maximum time-lag of about 70 min, that is however not well constrained. The spectral energy distributions from the optical to the TeV domain were compared to homogeneous models of blazar emission based on synchrotron radiation and synchrotron self-Compton scattering. They represent a satisfactory description, except in the state corresponding to the LAT softest spectrum and highest flux. Conclusions. Multiwavelength variability of Mkn 421 can be very complex, with patterns changing from epoch to epoch down to intra-day timescales, depending on the emission state. This makes accurate monitoring of this source during bright hard X-ray states necessary and calls for the elaboration of multicomponent, multizone, time-dependent models.
A study of six low-redshift quasar pairs Farina, E. P.; Falomo, R.; Treves, A.
Monthly notices of the Royal Astronomical Society,
08/2011, Letnik:
415, Številka:
4
Journal Article
Recenzirano
Odprti dostop
The dynamical properties of six Sloan Digital Sky Survey quasar pairs at z≤ 0.8 are investigated. The pairs have proper transverse separation R
⊥≤ 500 kpc, and velocity difference along the line of ...sight ΔV
r≤ 500 km s−1. If they are bound systems their dynamical mass can be evaluated and compared with that of host galaxies. Evidence is found of an excess of the former mass with respect to the latter. This suggests that these quasar pairs are hosted by galaxies with massive dark haloes or that they reside in a group/cluster of galaxies.
We report on the analysis of the photometric and spectroscopic properties of a sample of 29 low-redshift (z < 0.6) QSOs for which both Hubble Space Telescope (HST) WFPC2 images and ultraviolet HST ...FOS spectra are available. For each object we measure the R-band absolute magnitude of the host galaxy, the C iv(1550 Å) linewidth and the 1350 Å continuum luminosity. From these quantities we can estimate the black hole (BH) mass through the MBH–Lbulge relation for inactive galaxies, and from the virial method based on the kinematics of the regions emitting the broad-lines. The comparison of the masses derived from the two methods yields information on the geometry of the gas emitting regions bound to the massive BH. The cumulative distribution of the linewidths is consistent with that produced by matter laying in planes with inclinations uniformly distributed between ∼10° and ∼50°, which corresponds to a geometrical factor f∼ 1.3. Our results are compared with those of the literature and discussed within the unified model of active galactic nuclei.
X-ray observations unveiled various types of radio-silent isolated neutron stars (INSs), phenomenologically very diverse, e.g. the ∼Myr old X-ray-dim INSs (XDINSs) and the ∼kyr old magnetars. ...Although their phenomenology is much diverse, the similar periods (P = 2-10 s) and magnetic fields ( 1014 G) suggest that XDINSs are evolved magnetars, possibly born from similar populations of supermassive stars. One way to test this hypothesis is to identify their parental star clusters by extrapolating backwards the NS velocity vector in the Galactic potential. By using the information on the age and space velocity of the XDINS RX J1856.5−3754, we computed backwards its orbit in the Galactic potential and searched for its parental stellar cluster by means of a closest approach criterion. We found a very likely association with the Upper Scorpius OB association, for a NS age of 0.42 ± 0.08 Myr, a radial velocity V
NS
r
= 67 ± 13 km s−1, and a present-time parallactic distance d
NS
π = 123+ 11
− 15 pc. Our result confirms that the 'true' NS age is much lower than the spin-down age (τsd = 3.8 Myr), and is in good agreement with the cooling age, as computed within standard cooling scenarios. The mismatch between the spin-down and the dynamical/cooling age would require either an anomalously large breaking index (n ∼ 20) or a decaying magnetic field with initial value B
0 1014 G. Unfortunately, owing to the uncertainty on the age of the Upper Scorpius OB association and the masses of its members, we cannot yet draw firm conclusions on the estimated mass of the RX J1856.5−3754 progenitor.
Starting from the quasar sample of the Sloan Digital Sky Survey (SDSS) for which the C iv line is observed, we use an analysis scheme to derive the z-dependence of the maximum mass of active black ...holes, which overcomes the problems related to the Malmquist bias. The same procedure is applied to the low-redshift sample of SDSS quasars for which Hβ measurements are available. Combining with the results from the previously studied Mg ii sample, we find that the maximum mass of the quasar population increases as (1 +z)1.64±0.04 in the redshift range 0.1 ⪅z⪅ 4, which includes the epoch of maximum quasar activity.
Aims. We present optical spectroscopy for a sample of BL Lac objects, to determine their redshift, to study their broad emission line properties, and to characterize their close environment. Methods. ...Twelve objects were observed using the ESO 3.6 m and NOT 2.5 m telescopes, obtaining spectra for the BL Lacs and for nearby sources. Results. For seven of these objects, nuclear emission lines and/or absorption lines from the host galaxy were detected. In all the four cases where absorption lines were revealed, the host galaxy has been resolved with HST or ground-based imaging. The broad H_{\alpha} luminosities (or their upper limits) of the BL Lacs are similar to those of radio-loud quasars. For two BL Lacs, spectroscopy of close companions indicates that they are at the redshift of the BL Lacs, and therefore physically associated and probably interacting. Five BL Lacs have a featureless spectrum. In these cases, we apply a new technique to derive lower limits for their redshift, which are consistent with the lower limits deduced from imaging.
At least three general categories of environmental pressure - predation, resource distribution, and demographics - shape the costs and benefits of group-living for animals. Among the demographic ...factors that influence individual survival and reproduction, the composition of social groups can play an important role. Census data drawn from 26 populations of howler monkeys (Alouatta spp.) were used to determine if the composition of groups explained variation in their reproductive performance. Each group's reproductive performance was estimated by calculating the difference between the observed number of immatures and the number expected from its population average. Of four group structure variables tested, only one - the residual of the adult and subadult sex ratio - was a consistent correlate of reproductive performance across the howler monkey populations. Groups with a greater proportion of adult and subadult males contained more juveniles than expected from the population average. I propose that the survival or retention of immatures within howler monkey groups depends in part on the behavior of resident males. Of particular importance, the relative proportions of resident males and females were more informative than the absolute number of males or females. On this basis, I evaluate the possible role of males in protection from predation, conspecific aggression, and resource competition. The techniques used here can also be used to forecast major changes in demographic structure within populations.
Starting from the ∼50 000 quasars of the Sloan Digital Sky Survey for which Mg ii line width and 3000 Å monochromatic flux are available, we aim to study the dependence of the mass of active black ...holes on redshift. We focus on the observed distribution in the full width at half–maximum–nuclear luminosity plane, which can be reproduced at all redshifts assuming a limiting MBH, a maximum Eddington ratio and a minimum luminosity (due to the survey flux limit). We study the z–dependence of the best–fitting parameters of assumed distributions at increasing redshift and find that the maximum mass of the quasar population evolves as log (MBH(max)/M⊙) ∼ 0.3z+ 9, while the maximum Eddington ratio (∼0.45) is practically independent of cosmic time. These results are unaffected by the Malmquist bias.
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