arXiv subjects

Aldo Treves

Publications and source records attributed to Aldo Treves.

At least 73 records · Page 4Linked to original sources

arXiv2003

Host galaxies and black hole masses of low and high luminosity radio loud active nuclei

We investigate the host galaxy luminosities of BL Lac Objects (BLLs) and Radio Loud Quasars (RLQs) at z<0.5 imaged with the Hubble Space Telescope (HST). From a homogeneous treatment of the data we construct the host galaxy luminosity functions (HGLFs) and find that RLQ hosts are ~0.5 mag brighter than those of BLL: _BLL= -23.5. For both classes the HGLFs exhibit a remarkably different distribution with respect to that of normal (inactive) ellipticals, with clear preference for more luminous galaxies to show nuclear activity. We make use of the black hole mass -- bulge luminosity (M[BH] -L[bulge]) relation, derived for nearby inactive ellipticals, to estimate the central black hole mass in our sample of radio loud active galaxies. In spite of a ~2 order of magnitude difference of intrinsic nuclear luminosity BLL and RLQ have BH of similar mass _BLL= 5.6x10^{8}, <M[BH]/M[sun]_RLQ = 1.0 10^{9}). This implies that the two types of objects are radiating at very different rates with respect to their Eddington luminosity.

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arXiv2001

The black hole mass of BL Lacs from stellar velocity dispersion of the host galaxy

We present the first results from our on-going program to estimate black hole masses [M(BH)] of nearby BL Lac objects. The estimates are based on stellar velocity dispersion (sigma) of the BL Lac host galaxies from optical spectroscopy, and the recently found tight correlation between M{BH} and sigma in nearby early-type galaxies. For the first three BL Lacs, we find log M(BH) = 7.5 - 8.7 and M(BH)/M(host) = 0.03 - 0.1.

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arXiv2000

Near-infrared imaging of the host galaxies of three radio-loud quasars at z = 1.5

We present high spatial resolution near-infrared H-band (1.65 microns) images, taken with ISAAC on UT1 of ESO VLT, of three radio-loud quasars at z = 1.5, as a pilot study for imaging of a larger sample of radio-loud and radio-quiet quasars in the redshift range 1 < z < 2. We are able to clearly detect the host galaxy in two quasars (PKS 0000-177 and PKS 0348-120) and marginally in the third (PKS 0402-362). The host galaxies appear compact (average bulge scale-length R(e) = 4 kpc) and luminous (average M(H) = -27.6+-0.1). They are 2.5 mag more luminous than the typical galaxy luminosity (M*(H) = -25.0+-0.2), and are comparable to the hosts of low redshift radio-loud quasars (M(H) = -26), taking into account passive stellar evolution. Their luminosities are also similar to those of high redshift radio galaxies. All three quasars have at least one close companion galaxy at a projected distance < 50 kpc from the quasar, assuming they are at the same redshift.

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arXiv2000

Multiband optical polarimetry of the BL Lac object PKS 2155--304. Intranight and long term variability

The polarized and total flux of the BL Lac object PKS 2155-304 were monitored intensively and simultaneously in the optical UBVRI bands with the Turin photopolarimeter at the CASLEO 2.15 m telescope during 4 campaigns in June, August, November 1998 and August 1999. The effective observation time amounted to ~47 hours. PKS 2155-304 showed a linear polarization percentage (P) usually ranging between 3% and 7% and a polarization position angle (PA) mainly between 70 and 120 deg. The highest temporal resolution of our observations, 15 minutes, is unprecedented for polarimetric monitoring of this source, and has allowed us to detect amplitude variations of the linear polarization percentage from 6 to 7.5% in time scales of hours. In some nights the polarization percentage seems to increase toward shorter wavelengths, however the polarized spectrum does not vary significantly with time. The most remarkable variability event occurred on 1998 June 18, when the degree of linear polarization decreased by more than a factor 2 in one day in all bands, while the PA rotated by 90 deg. This is consistent with the presence of two emission components, of different polarization degree and position angle. Intranight variability of P and PA can be interpreted with small amplitude physical or geometrical changes within the jet. Measurements of the circular polarization over time intervals of days set upper limits of 0.2%. Simultaneous photometry taken with the Turin Photopolarimeter and with a CCD camera at Cordoba Astronomical Observatory did not show light variations correlated with those of the linearly polarized flux.

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arXiv2000

Ultra-strong Elelctric Fields and Vacuum Breakdown: Search for an Astrophysical Scenario

In some models of $γ$-ray bursts super-strong electric fields ($E\sim 10^{14} {\rm statvolt cm}^{-1}$) have been surmized. Such large fields may provide copious pair production through vacuum polarization. Here we examine various astrophysical scenarios where huge electric field can be present. It is shown that when the conditions for quantistic vacuum breakdown are met, pair creation through conventional processes is likely to be always more important. Although of great physical interest, quantum pair production seems therefore of little astrophysical relevance.

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arXiv1999

Isolated Neutron Stars: Accretors and Coolers

As many as $10^9$ neutron stars populate the Galaxy, but only $\approx 10^3$ are directly observed as pulsars or as accreting sources in X-ray binaries. In principle also the accretion of the interstellar medium may make isolated neutron stars shine, and their weak luminosity could be detected in soft X-rays. Recent ROSAT observations have convincingly shown that neutron stars accreting from the interstellar medium are extremely rare, if observed at all, in contrast with earlier theoretical predictions. Until now two possible explanations for their elusiveness have been proposed: their velocity distribution may peak at $\sim 200-400 {\rm km s}^{-1}$, as inferred from pulsar statistics, and this would severely choke accretion; the magnetic field may decay on timescales $\sim 10^8-10^9$ yr, preventing a large fraction of neutron stars from entering the accretor stage. The search for accreting neutron stars has produced up to now a handful of promising candidates. While little doubt is left that these objects are indeed isolated neutron stars, the nature of their emission is still controversial. In particular accreting objects can be confused with much younger, cooling neutron stars. However, a combination of observations and theoretical modeling may help in discriminating between the two classes.

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arXiv1999

The HST Survey of BL Lacertae Objects. I. Surface Brightness Profiles, Magnitudes, and Radii of Host Galaxies

We report on a large HST imaging survey of BL Lac objects, at spatial resolution ~10 times better than previous ground-based surveys. We focus on data reduction and analysis, describing the procedures used to model the host galaxy surface brightness radial profiles. A total of 69 host galaxies were resolved out of 110 objects observed, including almost all sources at z < 0.5. We classify them morphologically by fitting with either an exponential disk or a de~Vaucouleurs profile; when one fit is preferred over the other, in 58 of 69 cases, it is invariably the elliptical morphology. This is a very strong result given the large number of BL Lac objects, the unprecedented spatial resolution, and the homogeneity of the data set. With the present reclassification of the host galaxy of 1418+546 as an elliptical, there remain no undisputed examples of a disk galaxy hosting a BL Lac nucleus. This implies that, at 99% confidence, fewer than 7% of BL Lacs can be in disk galaxies. The apparent magnitude of the host galaxies varies with distance as expected if the absolute magnitudes are approximately the same, with a spread of +-1 mag, out to redshift z < 0.5. At larger redshifts, only 6 of 23 BL Lacs are resolved so the present data do not constrain possible luminosity evolution of the host galaxies. The collective Hubble diagram for BL Lac host galaxies and radio galaxies strongly supports their unification.

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arXiv1999

The HST Survey of BL Lacertae Objects. II. Host Galaxies

We have used the HST WFPC2 camera to survey 132 BL Lac objects comprising seven complete radio-, X-ray-, and optically-selected samples. We obtained useful images for 110 targets spanning the redshift range 0 < z < 1.3. In two thirds of the BL Lac images, host galaxies are detected, including nearly all for z < 0.5 (58 of 63). The highest redshift host galaxy detected is in a BL Lac object at z=0.664. In 58 of the 72 resolved host galaxies, a de Vaucouleurs profile is significantly preferred, at >99% confidence, over a pure exponential disk; the two fits are comparable in the remaining 14 cases. These results limit the number of disk systems to at most 8% of BL Lacs (at 99% confidence), and are consistent with all BL~Lac host galaxies being ellipticals. The detected host galaxies are luminous ellipticals with a median absolute K-corrected magnitude of M_R= -23.7 +- 0.6 mag, at least one magnitude brighter than M* and comparable to brightest cluster galaxies. The galaxy morphologies are generally smooth and undisturbed, with small or negligible ellipticities (<0.2). There is no correlation between host galaxy and observed nuclear magnitude or estimated jet power corrected for beaming. If black hole mass is correlated linearly with bulge mass in general, this implies a large range in Eddington ratio. Present data strongly support the unification picture with FR I galaxies constituting the bulk of the parent population of BL Lac objects.

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arXiv1998

HST snapshot imaging of BL Lac objects

Snapshot images of about 100 BL Lac objects were obtained with WFPC2 on HST. Sources from various samples, in the redshift range 0.05 to 1.2, were observed and 61 resolved (51 with known z). The high resolution and homogeneity of the images allow us to address the properties of the immediate environments of BL Lacs with unprecedented capability. Host galaxies of BL Lacs are luminous ellipticals (on average 1 mag brighter than L*) with no or little disturbed morphology. The nucleus, that is always well centered onto the galaxy, contributes in the optical (R band) to about half of the total luminosity of the object (range of the contribution from 0.1 to 10). The undisturbed morphology suggests that the nuclear activity has marginal effect on the overall properties of the hosts. Nonetheless several examples of close companions have been detected. The luminosity distribution of host galaxies is compared with that of a large sample of FR-I radio galaxies.

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arXiv1998

Science with IUE and the Growth of an Astrophysical Group in Milan: from X-Ray Binaries to AGN

The contribution of an astrophysics group based in Milan to the science with the IUE satellite during its almost 20 years lifetime has focussed on high energy sources, of both galactic (LMXRB, HMXRB, and black hole candidates) and extragalactic (AGN) nature. The results of this long term research and in particular of the latest multiwavelength campaigns conducted simultaneously with IUE are reviewed here.

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arXiv1998

BeppoSAX Observations of the BL Lac Object Mkn 501 in an Intermediate State

The BL Lac object Mkn 501 was observed with the BeppoSAX satellite at three epochs in April-May 1998, simultaneously with the Whipple and HEGRA Cherenkov telescopes. The X-ray spectrum is well detected up to 70 keV and it exhibits, at all epochs, a continuous curvature, which is here modeled with three power-laws of increasingly steeper index at larger energies. In the nu*f_nu representation the spectrum exhibits a peak at ~20 keV, which is interpreted as the maximum of the synchrotron emission. This implies that the synchrotron peak energy has lowered by an order of magnitude with respect to the powerful X-ray outburst observed in April 1997. The simultaneous TeV flux was comparable to the lowest levels observed for Mkn 501, possibly suggesting that the peak of the inverse Compton radiation had also shifted toward lower energies.

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arXiv1998

HST Observations of the Host Galaxies of BL Lacertae Objects

Six BL Lac objects from the complete 1 Jy radio-selected sample of 34 objects were observed in Cycle 5 with the HST WFPC2 camera to an equivalent limiting flux of mu_I~26 mag/arcsec^2. Here we report results for the second half of this sample, as well as new results for the first three objects, discussed previously by Falomo et al. (1997). In addition, we have analyzed in the same way HST images of three X-ray-selected BL Lacs observed by Jannuzi et al. (1997). The ensemble of 9 BL Lac objects spans the redshift range from z=0.19 to ~1. Host galaxies are clearly detected in seven cases, while the other two, at z~0.258 (redshift highly uncertain) and z=0.997, are not resolved. The HST images constitute a homogeneous data set with unprecedented morphological information between a few tenths of an arcsecond and several arcseconds from the nucleus, allowing us in 6 of the 7 detected host galaxies to rule out definitively a pure disk light profile. The host galaxies are luminous ellipticals with an average absolute magnitude of M_I~-24.6 mag (with dispersion 0.7 mag), more than a magnitude brighter than L* and comparable to brightest cluster galaxies. The morphologies are generally smooth and have small ellipticities (epsilon<0.2). Given such roundness, there is no obvious alignment with the more linear radio structures. In the six cases for which we have HST WFPC2 images in two filters, the derived color profiles show no strong spatial gradients and are as expected for K-corrected passively evolving elliptical galaxies. The host galaxies of the radio-selected and X-ray-selected BL Lacs for this very limited sample are comparable in both morphology and luminosity.

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arXiv1997

BeppoSAX Observations of Unprecedented Synchrotron Activity in the BL Lac Object Mkn 501

The BL Lac object Mkn 501, one of the only three extragalactic sources (with Mkn 421 and 1ES 2344+514) so far detected at TeV energies, was observed with the BeppoSAX satellite on 7, 11, and 16 April 1997 during a phase of high activity at TeV energies, as monitored with the Whipple, HEGRA and CAT Cherenkov telescopes. Over the whole 0.1-200 keV range the spectrum was exceptionally hard (alpha =< 1, with F_nu ~ nu^{-alpha}) indicating that the X-ray power output peaked at (or above) ~100 keV. This represents a shift of at least two orders of magnitude with respect to previous observations of Mkn 501, a behavior never seen before in this or any other blazar. The overall X-ray spectrum hardens with increasing intensity and, at each epoch, it is softer at larger energies. The correlated variability from soft X-rays to the TeV band points to models in which the same population of relativistic electrons produces the X-ray continuum via synchrotron radiation and the TeV emission by inverse Compton scattering of the synchrotron photons or other seed photons. For the first time in any blazar the synchrotron power is observed to peak at hard X-ray energies. The large shift of the synchrotron peak frequency with respect to previous observations of Mkn 501 implies that intrinsic changes in the relativistic electron spectrum caused the increase in emitted power. Due to the very high electron energies, the inverse Compton process is limited by the Klein-Nishina regime. This implies a quasi-linear (as opposed to quadratic) relation of the variability amplitude in the TeV and hard X-ray ranges (for the SSC model) and an increase of the inverse Compton peak frequency smaller than that of the synchrotron peak frequency.

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arXiv1997

Multiwavelength Monitoring of the BL Lacertae Object PKS 2155-304 in May 1994. II. The IUE Campaign

PKS 2155-304, the brightest BL Lac object in the ultraviolet sky, was monitored with the IUE satellite at ~1 hour time-resolution for ten nearly uninterrupted days in May 1994. The campaign, which was coordinated with EUVE, ROSAT, and ASCA monitoring, along with optical and radio observations from the ground, yielded the largest set of spectra and the richest short time scale variability information ever gathered for a blazar at UV wavelengths. The source flared dramatically during the first day, with an increase by a factor ~2.2 in an hour and a half. In subsequent days, the flux maintained a nearly constant level for ~5 days, then flared with ~35% amplitude for two days. The same variability was seen in both short- and long-wavelength IUE light curves, with zero formal lag (~<2 hr), except during the rapid initial flare, when the variations were not resolved. Spectral index variations were small and not clearly correlated with flux. The flux variability observed in the present monitoring is so rapid that for the first time, based on the UV emission alone, the traditional Delta L/Delta t limit indicating relativistic beaming is exceeded. The most rapid variations, under the likely assumption of synchrotron radiation, lead to a lower limit of 1 G on the magnetic field strength in the UV emitting region. These results are compared with earlier intensive monitoring of PKS 2155-304 with IUE in November 1991, when the UV flux variations had completely different characteristics.

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arXiv1996

Old Isolated Accreting Neutron Stars: The Diffuse X--ray Emission From The Galactic Center

The contribution of weakly--magnetized ($B\sim 10^9$ G) neutron stars accreting the interstellar medium to the diffuse X--ray emission observed in the Galactic Center is investigated. It is shown that, under rather conservative assumptions about the neutron stars and gas distributions, the accretion luminosity can account for a sizable fraction, possibly most, of the detected X--ray flux in the 2.5--7 keV band. In particular, model results are compared with {\it Granat\/} data and show a general agreement in both the flux energy and radial distributions.

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arXiv1995

The observability of old neutron stars accreting the interstellar medium: III. The solar proximity

Old isolated accreting neutron stars may show up among unidentified soft X--ray sources detected by the ROSAT All Sky Survey. We argue that the chances of identification are greater for ONSs located in the closest overdense regions of the solar neighbourhood. In particular, we consider the neutral hydrogen wall in the second Galactic quadrant ($15^\circ < l < 150^\circ$) which shrinks the estimated contour of the Local Bubble to $\approx 16-30$ pc. Due to their vicinity, we expect $\sim 10$ ONSs to be detectable at a relatively high flux limit ($\sim 0.1$ counts/s) in the 0.2--2.4 keV band. This implies that about 5 \% of unidentified sources above this threshold could be ONSs. No optical counterpart is present, but EUV emission from these objects could be detected using EUVE Lex filter at the highest sensitivity limits.

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arXiv1995

OLD ISOLATED ACCRETING NEUTRON STARS: CONTRIBUTION TO THE SOFT X--RAY BACKGROUND IN THE 0.5--2 KEV BAND

The issue of the observability of Old Isolated Neutron Stars (ONSs) accreting from the interstellar gas, is reconsidered using the spectra presented in Zampieri {\it et al.\/} (1995). In particular, we focus our attention on the overall soft X--ray emission of ONSs, which may provide a substantial contribution to the X--ray background in the 0.5--2 keV band. It is found that if accretion is channeled into polar caps by a magnetic field of $\sim 10^9$ G, up to $\sim$ $12-25 \%$ of the unresolved soft excess observed at high latitudes in the X--ray background can be explained in terms of emission from accreting isolated ONSs. However, this would imply the detection with ROSAT of $\sim 10$ sources deg$^{-2}$ above a threshold of $\sim 10^{-14} \, {\rm erg\, cm^{-2} s^{-1}}$. A brief reconsideration of the observability of isolated ONSs as individual X--ray sources is also presented.

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arXiv1995

On the electrodynamics of a magnetized rotating accretor

We consider the current flow in an accreting rotating neutron star (X-ray pulsator) and propose that the active magnetosphere is limited by the Alfvèn surface. Formulae for the luminosity and torque due to currents circulating in the neutron star are given, and astrophysical consequences for systems like Her X-1 and LMXRB are shortly discussed.

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