arXiv subjects

Annalisa Celotti

Publications and source records attributed to Annalisa Celotti.

69 records · Page 4Linked to original sources

arXiv2000

Blank Field X-Ray Sources

The X-ray sky is not as well known as is sometimes thought. We report on our search of minority populations (Kim & Elvis 1999). One of the most intriguing is that of `blank field sources', i.e. bright ROSAT sources (F(X)>1e-13 erg/cm2/s) with no optical counterpart on the Palomar Sky Survey (to O=21.5) within their 39"(99%) radius error circle. The nature of Blank Field sources is unknown: no known extragalactic population has such extreme X-ray to optical ratios (f(X)/f(V) >60). Moreover blank field source tend to have much flatter PSPC spectra compared to radio-quiet AGN. Both properties suggest obscuration. `Blank field sources' could be: Quasar-2s, low-mass AGNs, isolated neutron stars, high redshift clusters of galaxies, failed clusters, AGNs with no big blue bump. Identification with any of these populations would be an interesting discovery.

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arXiv2000

SISSA Searching for blazars: the BLEIS project

We present the aim and the status of the BLEIS project currently under development at SISSA. This project consists in selecting a complete Blazar sample from deep optical data, down to B=24.6, V=24.4 and I=23.7 (80% completness). The optical images are taken from the ESO Imaging Survey (EIS - Wide), a public survey covering ~16 deg2 of the Southern Emisphere. This new Blazar sample, thanks to the different energy band of the selection and to the faintness of EIS images, will be useful not only in understanding the physics of such objects, but also in evolutive and cosmological studies. It will also be a new test for the unified models of AGNs and will be helpful in making predictions for future deep surveys.

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arXiv2000

Does the unification of BL Lac and FR I radio galaxies require jet velocity structures?

We explore the viability of the unification of BL Lacs and FR I radio galaxies by comparing the core emission of radio galaxies with those of BL Lacs of similar extended radio power, taking advantage of the newly measured optical nuclear luminosity of FR I sources. The spectral properties of complete samples are also studied in the radio-optical luminosity plane: starting from the Spectral Energy Distribution (SED) of BL Lacs, we calculate the predicted luminosity of FR I nuclei in the frame of a simple one--zone model, by properly taking into account the relativistic transformations. We find that the bulk Lorentz factors required by the spread in the observed luminosities in all bands are significantly smaller than those implied by other, both observational and theoretical, considerations. This discrepancy is also reflected in the fact that FR I nuclei are over-luminous by a factor of 10-10^4, with respect to the predictions, both in the radio and in the optical band. In order to reconcile these results with the unification scheme, velocity structures in the jet are suggested, where a fast spine is surrounded by a slow (but still relativistic) layer so that the emission at different angles is dominated by different velocity components: the fast one dominates the emission in BL Lacs while the slow layer dominates the emission in misaligned objects. Furthermore for the lowest luminosity BL Lacs it has to be also postulated that their beaming factor in the radio band is lower than in the optical (and X-ray), as would result from deceleration of the jet. The self-consistency of the unification model therefore requires that both intrinsic differences in the SED and different beaming properties play a substantial role in characterizing the phenomenology of these sources.

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arXiv2000

The HST view of the FR I / FR II dichotomy

In order to explore how the FR I / FR II dichotomy is related to the nuclear properties of radio galaxies, we studied a complete sample of 26 nearby FR II radio galaxies using Hubble Space Telescope (HST) images and compared them with a sample of FR I previously analyzed. FR I nuclei lie in the radio-optical luminosity plane along a tight linear correlation, which argues for a common synchrotron origin. FR II show a more complex behavior, which is however clearly related to their optical spectral classification. Broad line FR II radio galaxies (BLRG) are located overall well above the FR I correlation, suggesting that a contribution from thermal (disc) emission is present. Three narrow line (NLRG) and one weak line radio galaxy (WLRG), in which no nuclear source is seen, can be interpreted as the obscured counter-parts of BLRG, in agreement with the current unification schemes. Conversely, in 5 sources of the sample, all of them NLRG or WLRG, optical cores are located on the same correlation defined by FR I and with similar radio and optical luminosities. This suggests that, in analogy to FR I, the emission is dominated by synchrotron radiation and represents the optical counter-part of the non-thermal radio cores. Interestingly, all these galaxies are located in clusters, an environment typical of FR I. These results imply that, at least at low redshifts, the FR II population is not homogeneous. Furthermore, the traditional dichotomy between edge darkened and brightened radio morphology is not univocally connected with the innermost nuclear structure, as we find FR~II with FR I-like nuclei and this has interesting bearings from the point of view of the AGN unified models.

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arXiv2000

On the Formation of Jets

The phenomenology of jets associated with a variety of black hole systems is summarized, emphasizing the constraints imposed on their origin. Models of jet formation are reviewed, focusing in particular on recent ideas concerning MHD models. Finally, the potential for advancing our understanding of jets both through future observations - especially forthcoming X-ray missions - and for elucidating some crucial theoretical questions is highlighted.

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arXiv1999

Constraints on the emission mechanisms of gamma-ray bursts

If the emission of gamma-ray bursts were due to the synchrotron process in the standard internal shock scenario, then the typical observed spectrum should have a slope F(nu) \propto nu^{-1/2}, which strongly conflicts with the much harder spectra observed. This directly follows from the cooling time being much shorter than the dynamical time. Particle re-acceleration, deviations from equipartition, fastly changing magnetic fields and adiabatic losses are found to be inadequate to account for this discrepancy. We also find that in the internal shock scenario the relativistic inverse Compton scattering is always as important as the synchrotron process, and faces the same problems. This indicates that the burst emission is not produced by relativistic electrons emitting synchrotron and inverse Compton radiation.

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arXiv1999

Astrophysical evidence for the existence of black holes

Following a short account of the history of the idea of black holes, we present a review of the current status of the search for observational evidence of their existence aimed at an audience of relativists rather than astronomers or astrophysicists. We focus on two different regimes: that of stellar-mass black holes and that of black holes with the masses of galactic nuclei.

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arXiv1999

Compton Dragged Gamma--Ray Bursts associated with Supernovae

It is proposed that the gamma-ray photons that characterize the prompt emission of Gamma-Ray Bursts are produced through the Compton drag process, caused by the interaction of a relativistic fireball with a very dense soft photon bath. If gamma-ray bursts are indeed associated with Supernovae, then the exploding star can provide enough soft photons for radiative drag to be effective. This model accounts for the basic properties of gamma-ray bursts, i.e. the overall energetics, the peak frequency of the spectrum and the fast variability, with an efficiency which can exceed 50%. In this scenario there is no need for particle acceleration in relativistic collisionless shocks. Furthermore, though Poynting flux may be important in accelerating the outflow, no magnetic field is required in the gamma-ray production. The drag also naturally limits the relativistic expansion of the fireball to Gamma < 10^4.

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arXiv1999

The HST view of FR I radio galaxies: evidence for non-thermal nuclear sources

Unresolved nuclear sources are detected by the Hubble Space Telescope in the great majority of a complete sample of 33 FR I radio galaxies belonging to the 3CR catalogue. The optical flux of these Central Compact Cores (CCC) shows a striking linear correlation with the radio core one over four decades, arguing for a non--thermal synchrotron origin of the CCC radiation. We also find evidence that this emission is anisotropic, which leads us to identify CCCs with the misoriented relativistic jet component which dominates in BL Lac objects. This interpretation is also supported by the similarity in the radio-to-optical and optical spectral indices. The high rate of CCC detection (85 %) suggests that a `standard' pc scale, geometrically thick torus is not present in low luminosity radio-galaxies. Thus the lack of broad lines in FR I cannot be attributed to obscuration. CCC fluxes also represent upper limits to any thermal/disc emission. For a $10^9 M_{\sun}$ black hole, typical of FR I sources, these limits translate into a fraction as small as less than $10^{-7}-10^{-5}$ of the Eddington luminosity.

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arXiv1999

Bursts from internal shocks: is it really synchrotron emission?

Within the standard internal shock scenario, synchrotron emission would produce a spectrum with slope F(v) proportional to v^(-1/2), as immediate consequence of the cooling timescale being shorter than the integration time. This is in disagreement with the harder observed spectra, indicating that a different mechanism is responsible for the burst emission. Furthermore in this scenario pair production is expected when photons produced by inverse Compton emission are taken into account.

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arXiv1999

Quasi-Thermal Comptonization and GRBs

Quasi-thermal Comptonization is an attractive alternative to the synchrotron process to explain the spectra of GRBs, even if we maintain other important properties of the internal shock scenario, implying a compact emitting region and an equipartition magnetic field. Photon-photon absorption and electron-positron pairs can play a crucial role: this process may lock the effective temperature in a narrow range and may be the reason why burst spectra have high energy cut-offs close to the rest mass-energy of the electron. If the progenitors of GRB are hypernovae, the circum-burst matter is dominated by the wind of the pre-hypernova star. The presence of this dense material has strong effects on the generation of the radiation of the burst and its afterglow.

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arXiv1998

Testing the FR I/BL Lac unifying model with HST observations

Hubble Space Telescope (HST) observations provide a novel way of testing unified models for FR I radio sources and BL Lac objects. The detection of extended dust discs in some radio galaxies provides information on their jet orientation. Given this, the strength of the compact nuclear sources of FR I and BL Lacs can be compared with model predictions. As a pilot project, we selected five radio galaxies which show extended nuclear discs in the HST images. The relative orientation of the projected radio-jets and of the extended nuclear discs indicates that they are not perpendicular, as the simplest geometrical model would suggest, but that they form an angle of ~ 20 - 40 degrees with the symmetry axis of the disc: a significant change of orientation occurs between the innermost AGN structure and the kpc-scale. Nevertheless, the discs appear to be useful indicators of the radio sources orientation since the angles formed by the disc axis and the jet with the line of sight differ by only ~ 10 - 20 degrees. At the center of each disc an unresolved nuclear source is present. We compared its luminosity with the optical core luminosity of BL Lacs selected for having similar host galaxy magnitude and extended radio luminosity. The BL Lac cores are between 2 E2 and 3 E5 times brighter than the corresponding radio galaxies ones. The FR I/BL Lac core luminosity ratio shows a suggestive correlation with the orientation of the radio galaxies with respect to the line of sight. The behavior of this ratio is quantitatively consistent with a scenario in which the emission in the FR I and BL Lac is dominated by the beamed radiation from a relativistic jet with Doppler factor ~ 5 - 10, thus supporting the basic features of the proposed unification schemes.

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arXiv1998

Rapid variability of BL Lacs and time lags: what can we learn?

The quasi-symmetric shape of the lightcurves, as seen in the X-rays and in the optical, together with the fast variations of the flux observed, suggest that the cooling time for the highest energy electrons may be shorter than the light crossing time $R/c$. Moreover the existence of time lags between lightcurves at different frequencies can be interpreted, in the frame of homogeneous models, as due to the different cooling times of electrons of different energy. To reproduce in detail the variability pattern at different wavelengths we need to study the time dependent behavior of the emitting particle distribution and to take into account of the different light travel times of photons produced in different regions of the source. We apply the model to PKS 2155-304 (1991 and 1994 campaigns).

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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

Jets and accretion processes in Active Galactic Nuclei: further clues

We present evidence in favour of a link between the luminosity radiatively dissipated in the central engine of radio-loud Active Galactic Nuclei and the kinetic power in their jets. This piece of evidence is based on the relation we find between the luminosity in broad emission lines and the kinetic power in pc-scale radio jets, for a sample of radio-loud quasars for which suitable data are available in the literature. We find that the ionizing luminosity and the kinetic one are of the same order of magnitude, suggesting that the processes responsible for them are somehow related. A strong magnetic field in equipartition with the radiation field could be responsible for regulating both processes. BL Lac objects seem to follow a similar behaviour, but with comparatively fainter broad line emission.

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