arXiv subjects

Gabriele Ghisellini

Publications and source records attributed to Gabriele Ghisellini.

At least 109 records · Page 6Linked to original sources

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

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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arXiv1998

Rapid variability in the synchrotron self Compton model for blazars

Blazars are characterized by large amplitude and fast variability, indicating that the electron distribution is rapidly changing, often on time scales shorter than the light crossing time. The emitting region is sufficiently compact to let radiative losses dominate the cooling of high energy electrons. We study the time dependent behaviour of the electron distribution after episodic electron injection phases, and calculate the observed synchrotron and self Compton radiation spectra. Since photons produced in different part of the source have different travel times, the observed spectrum is produced by the electron distribution at different stages of evolution. Even a homogeneous source then resembles an inhomogeneous one. Time delays between the light curves of fluxes at different frequencies are possible, as illustrated for the specific case of the BL Lac object Mkn 421.

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arXiv1998

Blazars: recent developments

Recent observational and theoretical results on blazars are presented and discussed. We are beginning to understand the rich phenomenology of blazars, and we are finding trends which will hopefully lead us to unveil the physics of these extreme sources. Limits and constraints on how jets originate can be derived by their intense and variable $γ$--ray emission. We are now starting to study their variability behavior simultaneously at different frequencies, to find delays and lags, to put constraints on emission models and source geometry. We observe trends in the overall spectral energy distribution. Also the problems of the overall energetics of jets and of their matter content are now being tackled.

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arXiv1998

The power of jets: blazars vs galactic superluminals

Estimates on different scales of the power of relativistic bulk motion in extragalactic and galactic jets are presented. The power in the jets and the power produced by the accretion disk are found to be roughly equal. This may suggest an important role of the magnetic field in extracting both the gravitational energy of the accreting matter and the rotational energy of the black hole. A method to derive the minimum power of jets is discussed, and used to find lower limits to the jet power of blazars and the galactic superluminal source GRS 1915+105. The matter content of jets is discussed, finding that electron positron pairs cannot be dynamically important. The jet power can initially be in the form of Poynting flux, gradually accelerating matter until equipartition between bulk motion of the particles and Poynting flux is reached.

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arXiv1998

Rapid variability of blazars

Blazars are characterized by large amplitude and fast variability, indicating that the electron distribution is rapidly changing, often on time scales shorter than the light crossing time. We study the time dependent behavior of the electron distribution after episodic electron injection phases, and calculate the observed synchrotron and self Compton radiation spectra. Since photons produced in different part of the source have different travel times, the observed spectrum is produced by the electron distribution at different stages of evolution. Time delays between the light curves of fluxes at different frequencies are possible, as illustrated for the specific case of the BL Lac object Mkn 421.

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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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arXiv1997

Unification of all Blazars

The overall spectra (SED) of blazars, from radio to gamma-ray energies, seem to obey well defined trends, with a continuity of properties between blazars of different classes. To quantify this statement we can either investigate their observed properties (see Fossati et al., this volume), or try to determine their intrinsic physical parameters by applying specific models, and trying to fit their SED. Results of the latter approach are reported here. We applied simple, one-zone, homogeneous models to all blazars strongly detected in the gamma-ray band, assuming or not the presence of seed (for the inverse Compton process) photons produced outside the active region. Our results suggest that the SEDs of blazars are ruled by the amount of radiative cooling suffered by the electrons producing most of the emission. In turn, the amount of cooling is ruled by the amount of the external photon emission, which can be identified with radiation coming from the broad line clouds. Blazar SEDs are therefore organized in a sequence: objects with no or very weak emission lines (X-ray selected BL Lacs, or HBL) are characterized by very high electron energies and a Compton luminosity of the same order of the synchrotron one. These are TeV sources. BL Lacertae objects selected in the radio band (or LBL) are characterized by smaller electron energies, more total power and more line luminosity, and by a larger ratio of the Compton to synchrotron luminosity. They are GeV sources. Increasing the total intrinsic power and the line emission luminosity, we have smaller still electron energies (more cooling) and greater still Compton to synchrotron power ratio. These are flat spectrum radio quasars, with a high energy peak located at MeV-GeV energies.

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arXiv1996

The Synchrotron Boiler: a Thermalizer in Seyfert Galaxies

There are difficulties in understanding what keeps the plasma thermalized in compact sources, especially during rapid variations of the emitted flux. Particle-particle collisions are too inefficient in hot rarefied plasmas, and a faster process is called for. Synchrotron absorption is such a process. We show that relativistic electrons can thermalize in a few synchrotron cooling times by emitting and absorbing cyclo-synchrotron photons. The resulting equilibrium distribution is a Maxwellian at low energies, with a high energy power law tail when Compton cooling is important. Assuming that the particles emit completely self absorbed synchrotron radiation while they at the same time Compton scatter ambient UV photons, we calculate the time dependent behavior of the distribution function, and the final high energy spectra.

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arXiv1996

Time dependent spectra of Blazars

We calculate the time dependent electron distribution under the assumption of continuous injection of new relativistic particles, and assuming radiative cooling (by sinchrotron and inverse Compton) and particle escape. Resulting photon spectra are calculated taking into account the time delays introduced by the different light travel times across the source. We apply these calculations to the varying X-ray spectrum of Mkn 421.

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arXiv1996

X-Ray Emission in Gamma-Ray Blazars

Although the gamma-ray emission in blazars dominates the power output, there are crucial informations carried by the X-rays. Indeed, their paucity, together with the short variability timescales observed both at X and gamma-ray energies constrains the location, along the jet, of the active region responsible for most of the emission. Furthermore, the knowledge of the X-ray flux and spectrum helps in defining the overall spectral energy distribution (SED) of blazars, allowing to discriminate between models.

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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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arXiv1996

X-ray variability and correlations in the two phase disk-corona model for Seyfert galaxies

We discuss in detail the broad band X-ray variability expected in Seyfert galaxies in the framework of a Compton cooled corona model. Variations in the optical depth tau of the corona cause spectral changes in the Comptonized emission in the sense that the spectrum steepens and the temperature decreases with increasing tau. If the corona is pair dominated, tau is determined by the compactness (proportional to luminosity for fixed size). This yields a definite relation between spectral shape and intensity implying a variation in intensity of a factor 10 for a change of 0.2 in the 2-10 keV spectral index. For low pair density coronae, no definite prediction is possible. In the absence of substantial changes in the structure of the corona, the soft thermal component correlates with the medium-hard Comptonized component. Examples are computed using the ROSAT response matrix, showing that current observations of NGC 5548 and Mrk 766 showing different behaviours can be reconciled with the model. The spectral behaviour of Mrk 766 strongly suggests that the coronal plasma in this source is not pair dominated.

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arXiv1995

ROSAT observations of radio-selected BL Lac objects

ROSAT observations of a homogeneous sample of radio--selected BL Lac objects are presented. Results of a detailed spectral analysis in the soft 0.1-2.0 keV energy range are discussed and compared with similar previously published results. The X--ray spectral shape is discussed in relation to the overall energy distribution with particular emphasys on the high energy gamma--ray emission recently detected by CGRO--EGRET for about half of the objects in the sample. Dividing the objects in our sample on the basis of the radio to X--ray flux ratio (alpha_rx) we show that objects with alpha_rx > 0.75 have flatter X--ray spectra and are likely to be stronger gamma--ray emitters than objects with alpha_rx < 0.75. Moreover we note that the value of the peak energy of the synchrotron component, in a E * F(E) plot, correlates with alpha_rx and alpha_x.

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arXiv1995

THE VELOCITY OF THE EMITTING PLASMA OF THE SUPERLUMINAL GALACTIC SOURCE GRS 1915+105

We calculate the flux and size ratio of radio emitting features moving in antiparallel directions at relativistic speeds, taking into account that the pattern we observe may move at a velocity different from the one of the emitting plasma. Both velocities can be determined in sources in which both the approaching and the receding features are observed. This is the case of the galactic superluminal source GRS 1915+105, for which the pattern speed (responsible for the apparent superluminal velocity) was found to be 0.92$c$. For this source we find that the velocity of the plasma is 0.73$c$. The found velocity helps decreasing the estimate of the associated kinetic power. Since intrinsically identical knots will be observed to have different sizes, high resolution radio observations of GRS 1915+105 will be a test for the proposed difference of the pattern and plasma velocities.

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arXiv1994

A Model for the X-Ray and UV Emission from Seyfert Galaxies and Galactic Black Holes

We propose that the X--ray emission from radio quiet AGN and galactic black holes is due to Comptonization of soft thermal photons emitted by the underlying accretion disk in localized structures (blobs). The power per unit area produced by the blobs, impinging on the disk, can easily dominate the radiation internally produced by the disk. In this case the electron temperature and the high energy spectrum can be determined in a similar way as in the previously studied homogeneous model (Haardt \& Maraschi 1991). However in the present model: a) the emitted spectrum is largely independent of the {\it fraction} of gravitational power dissipated in the blobs; b) the X--ray spectrum can be harder depending on a form factor of the blobs; c) the UV (or soft X--ray for galactic objects) luminosity that is not intercepted by the blobs can be larger than the X--ray luminosity. In the framework of a simplified accretion disk $α-Ω$ dynamo model, we make order of magnitude estimates of the number of active blobs, their size, luminosity and hence their compactness, finding values in agreement with what is observed. The expected UV to X--ray spectra and correlations of X--ray and UV light curves are discussed.

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arXiv1994

On Thermal Comptonization in e+- Pair Plasmas

We study $e^{\pm}$ pair plasmas in pair equilibrium, which emit high energy radiation by thermal Comptonization of soft photons. We find that the maximum luminosity to size ratio of the source (i.e. the compactness) depends not only on the hot plasma temperature, but also on the spectral index of the resulting Comptonized spectrum. In the observationally interesting range, sources of same compactness can be hotter if their spectrum is steeper. Instruments observing in the 50--500 keV energy range, such as OSSE on board CGRO, and especially the future SAX satellite, can be more successful in detecting sources moderately steep, the flattest sources being characterized by an high energy cut--off at too low frequencies. For any given pair of values of spectral index and temperature, Comptonization theory alone fixes the $ratio$ of the compactnesses in hard and soft photons. However, if the source is pair dominated, the absolute values of the two compactnesses are fixed. Therefore there is a a one--to--one correspondence between the physical parameters of the source (the compactnesses in soft and hard photons) and the observable quantities (spectral index and temperature). This correspondence can be extremely useful in interpreting the physical behaviour of the sources, especially during variations, and can help discriminating between different models for the high energy emission of compact sources.

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