arXiv subjects

G. Henri

Publications and source records attributed to G. Henri.

144 records · Page 8Linked to original sources

arXiv2004

Applying Magnetized Accretion-Ejection Models to Microquasars: a preliminary step

We present in this proceeding some aspects of a model that should explain the spectral state changes observed in microquasars. In this model, ejection is assumed to take place only in the innermost disc region where a large scale magnetic field is anchored. Then, in opposite to conventional ADAF models, the accretion energy can be efficiently converted in ejection and not advected inside the horizon. We propose that changes of the disc physical state (e.g. transition from optically thick to optically thin states) can strongly modify the magnetic accretion-ejection structure resulting in the spectral variability. After a short description of our scenario, we give some details concerning the dynamically self-consistent magnetized accretion-ejection model used in our computation. We also present some preliminary results of spectral energy distribution.

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arXiv2004

Discovery of a new INTEGRAL source: IGR J19140+0951

IGR J19140+0951 (formerly known as IGR J19140+098) was discovered with the INTEGRAL satellite in March 2003. We report the details of the discovery, using an improved position for the analysis. We have performed a simultaneous study of the 5-100 keV JEM-X and ISGRI spectra from which we can distinguish two different states. From the results of our analysis we propose that IGR J19140+0951 is a persistent Galactic X-ray binary, probably hosting a neutron star although a black hole cannot be completely ruled out.

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arXiv2004

INTEGRAL observations of the transient source IGR J19140+098

IGR J19140+098 was discovered during the early INTEGRAL observations of GRS1915+105 in March 2003. The following observations by INTEGRAL and RXTE show significant variability on various time scales from 100s to 1ks, but no pulsations. The ISGRI spectra show strong spectral variability in the 20-80keV range. Combined JEMX_2-ISGRI spectra are well fitted by a power-law and a thermal emission model with a rather high temperature. RXTE data also reveal an apparently broad ionised iron line not detected in earlier INTEGRAL observations. These results are compatible with a galactic X-ray binary source.

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arXiv2003

The XMM/BeppoSAX observation of Mkn 841

Mkn 841 has been observed simultaneously by XMM and BeppoSAX in January 2001. Due to operational contingency, the 30ks XMM observation was split into two parts, separated by about 15 hours. We first report the presence of a narrow iron line which appears to be rapidly variable between the two pointings, requiring a non-standard interpretation. We then focus on the analysis of the broad band (0.3-200 keV) continuum using the XMM/EPIC, RGS and SAX/PDS data. The Mkn 841 spectrum is well fitted by a comptonization model in a geometry more photon-fed than a simple slab geometry above a passive disk. It presents a relatively large reflection (R>2) which does not agree with an apparently weak iron line. It also show the presence of a strong soft excess wellfitted by a comptonized spectrum in a cool plasma, suggesting the presence of a multi-temperature corona.

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arXiv2002

A rapidly variable narrow X-ray iron line in Mkn 841

We report on the detection of a rapidly variable narrow Fe K$α$ line in Mkn 841. The source has been observed two times by XMM-Newton and simultaneously with BeppoSAX. The two observations, of about 10ks long each, were separated by $\sim$ 15 hours. The line flux reaches a maximum during the first observation and is significantly reduced in the second one. The continuum shape and flux, instead, keep roughly constant between the two pointings. Such rapid variability of a narrow (unresolved by the XMM-pn instrument) line has never been reported in the past. These results are not easily explained in the standard cold reflection model where the narrow line component is supposed to be produced far from the primary X-ray source (e.g. from the torus) and is thus not expected to vary rapidly. Different interpretations are discussed.

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arXiv2001

Pair creation at shocks. Application to the high energy emission of compact objects

We investigate the effect of pair creation on a shock structure. Particles, accelerated in the shock via the first order Fermi process, are supposed to cool by inverse Compton process on external soft photons, resulting in a cut-off power law shape of the particle distribution function. The high energy photons produced are thus able to create pairs, through photon-photon annihilation. The increase of the pair pressure may then be sufficient to modify the shock profile. We show that there is even a limit of the pair pressure (of the order of 20% of the ram pressure of the upstream flow) above which the shock cannot exist any longer. Conversely, significant changes of the flow velocity profile will also modify the spectral index and the high energy cut--off of the particle distribution function. Hence the number of particles able to trigger the pair creation process will change, modifying the pair creation rate accordingly. Taking into account these different processes, we self-consistently derive the flow velocity profile and the particle distribution function. We show that, in some region of the parameter space, the system can converge towards stationary states where pair creation and hydrodynamical effects balance. We discuss the application of this model to explain the high energy emission observed in compact objects. We show that hard X-ray spectra ($α_{X}< 1.$) are only obtained for small pair pressure and we don't expect any strong annihilation line in this case. We suggest also a possible variability mechanism if the soft photon compactness depends itself on the pair density of the hot plasma, such as expected in reillumination models.

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arXiv2001

Pair Creation at Shocks: Application to the High Energy Emission of Compact objects

We investigate the effect of pair creation on a shock structure. Actually, particles accelerated by a shock can be sufficiently energetic to boost, via Inverse Compton (IC) process for example, surrounding soft photons above the rest mass electron energy and thus to trigger the pair creation process. The increase of the associated pair pressure is thus able to disrupt the plasma flow and possibly, for too high pressure, to smooth it completely. Reversely, significant changes of the flow velocity profile may modify the distribution function of the accelerated particles, modifying consequently the pair creation rate. Stationary states are then obtained by solving self-consistently for the particle distribution function and the flow velocity profile. We discuss our results and the application of these processes to the high energy emission and variability of compact objects.

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arXiv2000

Pair Creation at Shocks: Application to Agns

We investigate the effect of pair creation on a shock structure. Actually, particles accelerated by a shock can be sufficiently energetic to boost, via Inverse Compton process for example, surrounding soft photons above the rest mass electron energy and thus to trigger the pair creation process. The increase of the associated pair pressure is thus able to disrupt the plasma flow and possibly, for too high pressure, to smooth it completely. Reversely, significant changes of the flow velocity profile may modify the distribution function of the accelerated particles in such a way that the number of particles enable to trigger the pair creation process may change, modifying consequently the pair creation rate. We propose here, using simplifying assumptions, to study the global behavior of a shock structure in an environment dominated by pairs.

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arXiv1999

Search for optical microvariability in a large sample of Seyfert I galaxies

We present results of an optical (I band) monitoring of a sample of 22 Seyfert I galaxies. We aimed to detect microvariability with time resolution from ~ 6 minutes down to 30 seconds for the most luminous one. It is the largest survey ever done in the search of rapid optical variations in Seyfert galaxies. We used differential photometry and a new method of analysis between galaxy and comparison stars light curves in order to minimize the influence of the intrinsic variabilities of the latter. We thus obtain precision on standard deviation measurements less than 1% and generally of the order of 0.5%. We obtain no clear detection of microvariability in any of these objects. In the hypothesis where optical microvariability could be due to synchrotron emission of a non thermal electrons population, we discuss the physical constraints imposed by these results.

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arXiv1999

Active Galactic Nuclei as High Energy Engines

Active Galactic Nuclei are considered as possible sites of cosmic ray acceleration and some of them have been observed as high energy gamma ray emitters (Blazars). There naturally comes an appealing idea that the acceleration of the highest energy cosmic rays in the AGNs has a signature in the form of gamma ray emission and high energy neutrino emission through the collisions of very high energy protons with soft photons. Moreover it is often said that electrons cannot reach enough energy through Fermi acceleration to account for the highest energy photons observed with ground Cerenkov telescopes. In this paper, we discussed these points and show that the fast variability of the flares recently observed rules out the assumption of a Fermi acceleration of protons. We show that Fermi acceleration of electrons is enough to account for the gamma spectra, their shape, cut-off and their variability. Moreover the spectral break is nicely explained by invoking an gamma-ray photosphere. Nevertheless we give estimates of the high energy cosmic ray generation in AGNs and of the resultant neutrino flux, that turns out to be very sensitive to the spectral index of the proton distribution.

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arXiv1998

Anisotropic Illumination in AGNs. The reflected component. Comparison to hard X-ray spectra from Seyfert Galaxies

We calculate the reflection component predicted by the anisotropic illumination model of Henri & Petrucci 1997. This component appears to be more important than for isotropic models. The overall X/gamma spectrum is found to be strongly angle-dependent. When the accretion disc is seen with a nearly edge-on orientation the reflection hump is weak, while a face-on viewing angle leads to a proeminent reflection hump with an equivalent reflection coefficient R close to 50. Such reflection dominated Seyfert 1s galaxies are not observed. By fitting observed X/gamma spectra, we derive inclination angles of 70 degrees for MCG 8-11 -11 and 80 degrees for IC 4329a and NGC 4151. Although the model succeed in reproducing individual observed spectra, it requires all the Seyfert 1s observed in the X-ray band to be seen with large inclination angles. Such a situation is highly improbable. On the other hand, we show that the ionisation of a fair part of the reflecting disk could represent an interesting improvement of the model, consistent with the data and relaxing the constraints on the high energy cut-off in Seyfert galaxies.

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arXiv1998

The terminal bulk Lorentz factor of relativistic electron-positron jets

We present numerical simulation of bulk Lorentz factor of relativistic electron-positron jet driven by Compton rocket effect from accretion disc radiation. The plasma is assumed to have a power-law distribution $n_{e}(γ) \propto γ^{-s}$ whith $1 < γ< γ_{max}$ and is continuously reheated to compensate for radiation losses. We include full Klein-Nishina (hereafter KN) cross section, and study the role of energy upper cut-off $γ_{max}$, spectral index $s$, and source compactness. We determine terminal bulk Lorentz factor in the case of supermassive black holes relevant to AGN and stellar black holes relevant to galactic microquasars. In the latter case, Klein-Nishina cross section effect are more important, and induce terminal bulk Lorentz factor smaller than in the former case. Our result are in good agreement with bulk Lorentz factors observed in galactic sources (GRS1915+105, GROJ1655-40) and extragalactic ones. Differences in scattered radiation and acceleration mechanism efficiency in AGN environment can be responsible for the variety of relativistic motion in those objects. We also take into account the influence of the size of the accretion disc; if the external radius is small enough, the bulk Lorentz factor can be as high as 60.

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arXiv1997

An Anisotropic Illumination Model of Seyfert I Galaxies

We develop a self-consistent model of Seyfert galaxies continuum emission. High energy emission is produced by Inverse Compton process on soft photons emitted as thermal radiation by the accretion disk. Thermal emission is, at turn, entirely due to reprocessing of the impinging high energy photons. The high energy source is assumed to be an optically thin plasma of highly relativistic leptons ($e^+-e^-$), at rest at a given height on the disk axis. This warm source could be the result of a strong schock between an abortive jet coming from the disk and the interstellar medium. Such a geometry is highly anisotropic, which has a strong influence on Compton process. Monte-Carlo simulations allow the superposition of a reflected component (the so-called high energy bump) to the UV to X-ray spectrum obtained with our model, leading us to a first comparison with observations by fitting the high energy spectra of NGC4151 and IC4329a.

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arXiv1997

The terminal bulk Lorentz factor of relativistic electron-positron jet

We present numerical simulation of bulk Lorentz factor of relativistic electron positron jet driven by Compton rocket effect from accretion disk radiation. The plasma is assumed to have a power-law energy distribution and is continuously reheated to compensate for radiation losses. We include Klein-Nishina (hereafter KN) corrections, and study the role of energy upper cut-off, spectral index, and source compactness. We determine terminal bulk Lorentz factor in the case of supermassive black holes relevant to AGN and stellar black holes relevant to galactic microquasars. In the latter case, effects of KN corrections are more important and induce terminal bulk Lorentz factor smaller than in the former case. The result can explain the low bulk Lorentz factors for galactic sources (GRS1915+105, GROJ1655-40) compared to extragalactic ones. We also take into account the influence of the size of the accretion disk; if the external radius is small enough, the bulk Lorentz factor can be as high as 60, which is comparable to the values needed to explain extragalactic gamma-ray bursts.

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arXiv1997

Anisotropic illumination of AGN's accretion disk by a non thermal source II: General relativistic effects

In a previous paper (Henri & Petrucci, hereafter paper I), we have derived a new model in order to explain the UV and X-ray emission of radio quiet AGNs. This model assumes that a point source of relativistic leptons (e+,e-) illuminates the accretion disk of the AGN by Inverse Compton process. This disk is supposed to be simply represented by a finite slab which radiates only the energy reprocessed from the hot source. The radiation field within the hot source region is therefore highly anisotropic, which strongly influences the Inverse Compton process. The different Eddington parameters characterizing the radiative balance of this system have been calculated self-consistently in the Newtonian case (paper I) giving a universal spectrum for a given inclination angle. In this paper, we take into account relativistic effects by including the gravitational redshift, the Doppler boosting and the gravitational focusing due to the central supermassive black hole. This has the effect of modifying the radial temperature profile in the innermost region of the disk (at some gravitational radii). However, the spectrum is hardly different from that obtained in the Newtonian case, unless the hot source is very close to the black hole. These results are clearly different from standard accretion disk models where the gravitational energy is mainly released in the vicinity of the black hole.

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arXiv1997

Anisotropic illumination of AGN's accretion disk by a non thermal source : I General theory and application to the Newtonian geometry

We present a model of accretion disk where the disk luminosity is entirely due to the reprocessing of hard radiation impinging on the disk. The hard radiation itself is emitted by a hot point source above the disk, that could be physically realized by a strong shock terminating an aborted jet. This hot source contains ultrarelativistic leptons scattering the disk soft photons by Inverse Compton (IC) process. Using a simple formula to describe the IC process in an anisotropic photon field, we derive a self-consistent angular distribution of soft and hard radiation in the Newtonian geometry. The radial profile of the disk effective temperature is also univoquely determined. The high energy spectrum can be calculated for a given lepton distribution. This offers an alternative picture to the standard accretion disk emission law. We discuss the application of this model to Active Galactic Nuclei, either for reproducing individual spectra, or for predicting new scaling laws that fit better the observed statistical properties.

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arXiv1997

High Energy Plasma Physics in Black Hole Environment with Jets

The Compton Gamma Ray Observatory revealed the high energy emission of some Active Galactic Nuclei called "Blazars"; few of them have also been observed by ground Cererenkov arrays. Those sources observed with Whipple Observatory emit gamma rays up to few TeV. From the spectral data and variability measurements, a purely electrodynamical description of this emission can be proposed, provided that it comes from relativistic flares at the beginning of the jet. Because of the source compacity, the creation of electron-positron pairs and their escape from the black hole environment could likely explain the shape of the spectra. A plasma dominated by relativistic electrons (positrons) close to equipartition with the magnetic field is a suitable acceleration medium that can account for both the required high Lorentz factors and the short variability.

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arXiv1996

An anisotropic illumination model of Seyfert I galaxies

We present a new model of accretion disk where the disk luminosity is entirely due to the reprocessing of hard radiation impinging on the disk. The hard radiation itself is emitted by a hot point source above the disk, that could be physically realized by a strong shock terminating an aborted jet. This hot source contains ultra-relativistic leptons scattering the disk soft photons by Inverse Compton (IC) process. Using simple formula to describe the IC process in an anisotropic photon field, we derive a self-consistent solution in the Newtonian geometry, where the angular distribution of soft and hard radiation, and the radial profile of the disk effective temperature are determined in a univocal way. This offers an alternative picture to the standard accretion disk emission law, reproducing individual spectra and predicting new scaling laws that fit better the observed statistical properties. General relativistic calculations are also carried out. It appears that differences with the Newtonian case are weak, unless the hot source is very close to the black hole.

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