Broad band spectral properties of Seyfert 1 galaxies observed with BeppoSAX
We will present some results on the broad--band observations of BeppoSAX of the bright Seyfert galaxies NGC 4151 and NGC 5548.
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arXiv subjects
L. Maraschi
Publications and source records attributed to L. Maraschi.
ISO observations of the BL Lac object PKS 2155-304
The Infrared Space Observatory (ISO) observed the BL Lac object PKS 2155-304 16 times from 1996, May 7 to June 8, with both the ISOCAM camera and the ISOPHOT photometer, as part of a more general multiwavelength campaign. Two additional observations were performed on 1996, November 23 and 1997, May 15. This is the first time that there are simultaneous mid and far infrared data in a multiwavelength monitoring of a BL Lac object. We obtained four light curves at 4.0, 14.3, 60 and 90 microns, and a broad-band filter spectrum from 2.8 to 170 micron. No variability was detected in the infrared, although the source was varying at shorter wavelengths. The IR spectrum can be fitted by a single power law with an energy spectral index alfa = 0.40 +/- 0.06 and it can be explained as due to synchrotron emission only, with no noticeable contributions from thermal sources. Using the simultaneous data, we constructed the SED of PKS 2155-304.
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X-ray Rapid Variability of MKN 421
We present a detailed Cross Correlation analysis of the light curves of the X-ray bright BL Lac object Mkn 421 observed with $Beppo$SAX period 1997-1998. Monte Carlo simulations are performed in order to estimate the uncertainties. We found that in one occasion the lag is negative.
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Rapid X-ray Variability of the BL Lacertae Object PKS 2155-304
(Abridged) We present a detailed power density spectrum and cross-correlation analysis of the X-ray light curves of the BL Lac object PKS 2155-304, observed with BeppoSAX in 1997 and 1996, aimed at exploring the rapid variability properties and the inter-band cross correlations in the X-rays. We also perform the same analysis on the (archival) X-ray light curve obtained with ASCA in 1994.
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Ultraviolet and Multiwavelength Variability of the Blazar 3C 279: Evidence for Thermal Emission
The gamma-ray blazar 3C 279 was monitored on a nearly daily basis with IUE, ROSAT and EGRET for three weeks between December 1992 and January 1993. During this period, the blazar was at a historical minimum at all wavelengths. Here we present the UV data obtained during the above multiwavelength campaign. A maximum UV variation of ~50% is detected, while during the same period the X-ray flux varied by no more than 13%. At the lowest UV flux level the average spectrum in the 1230-2700 A interval is unusually flat for this object ( ~1). The flattening could represent the lowest energy tail of the inverse Compton component responsible for the X-ray emission, or could be due to the presence of a thermal component at ~20000 K possibly associated with an accretion disk. The presence of an accretion disk in this blazar object, likely observable only in very low states and otherwise hidden by the beamed, variable synchrotron component, would be consistent with the scenario in which the seed photons for the inverse Compton mechanism producing the gamma-rays are external to the relativistic jet. We further discuss the long term correlation of the UV flux with the X-ray and gamma-ray fluxes obtained at various epochs. All UV archival data are included in the analysis. Both the X- and gamma-ray fluxes are generally well correlated with the UV flux, approximately with square root and quadratic dependences, respectively.
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Hour-Scale Multiwavelength Variability in Markarian 421
Markarian 421 was observed for about four days with BeppoSAX and the Whipple Observatory gamma-ray telescope in April 1998. A pronounced, well-defined, flare with hour-scale variability was observed simultaneously in X-rays and very high energy gamma-rays. These data provide the first evidence that the X-ray and TeV intensities are well correlated on time-scales of hours. While the rise of the flare occurred on a similar time-scale in the two wavebands, the decay of the flare was much more rapid in gamma rays, providing the first clear indication that the X-ray and gamma-ray emission may not be completely correlated in Markarian 421.
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The blazar PKS 0528+134: new results from BeppoSAX observations
BeppoSAX observed 8 times the gamma-ray bright blazar PKS 0528+134 in Feb. and Mar. 1997, during a multiwavelength campaign involving EGRET, onboard CGRO, and ground based telescopes. The source was in its faintest X-ray state observed so far, with an unabsorbed [2-10] keV flux of 2.5E-12 erg/cm2/s. The spectrum can be fitted with a power law, with energy index alpha_X=0.49+-0.07 between 0.1 and 10 keV. The high energy instrument PDS detected a flux in the 15-100 keV band which appears disconnected from the lower X-ray energies, most likely due to a contaminating source. We consider our findings in the context of the overall spectral energy distribution and discuss their implications for synchrotron and inverse Compton models.
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Spectral Evolution of PKS 2155-304 observed with BeppoSAX during an Active Gamma-ray Phase
We present the results of BeppoSAX observations of PKS 2155-304 during an intense gamma-ray flare. The source was in a high X-ray state. A temporal analysis of the data reveals a tendency of the amplitude of variations to increase with energy, and the presence of a soft lag with a timescale of the order 10^3 s. A curved continuum spectrum, with no evidence of spectral features, extends up to ~50 keV, while there is indication of a flatter component emerging at higher energies, consistent with the interpretation of the broad band spectral energy distribution (SED) as due to synchrotron self-Compton (SSC) emission from a single region. Notably, the fitting of the SED with such a model is consistent with an interpretation of the detected soft lag as due to radiative cooling, supporting the idea that radiation losses play an important role in variability. The observed shifts of the SED peaks between the lowest and highest flux levels can be accounted for by an increase of the break energy in the relativistic particle spectrum. The model predicts emission at TeV energies in good agreement with the recently reported detection.
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Correlated variability of Mkn 421 at X-ray and TeV wavelengths on timescales of hours
Mkn 421 was observed for about two days with BeppoSAX, prior to and partly overlapping the start of a 1 week continuous exposure with ASCA in April 1998, as part of a world-wide multiwavelength campaign. A pronounced, well defined, flare observed in X-rays was also observed simultaneously at TeV energies by the Whipple Observatory's 10 m gamma-ray telescope. These data provide the first evidence that the X-ray and TeV intensities are well correlated on time-scales of hours.
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Progress in understanding Blazars from BeppoSAX observations
Results obtained with BeppoSAX observations of blazars within various collaborative programs are presented. The spectral similarity "paradigm", whereby the spectral energy distributions of blazars follow a sequence, leading to a unified view of the whole population, is briefly illustrated. We concentrate on recent observations of flares and associated spectral variability for three objects at the "blue" end of the spectral sequence, namely PKS 2155-304, Mkn 421 and Mkn 501. The results are discussed in terms of a general analytic synchrotron self-Compton interpretation of the overall spectrum. The physical parameters of the quasi-stationary emission region can be derived with some confidence, while the variability mechanism(s) must be complex.
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BeppoSAX Observations of the Radio Galaxy Centaurus A
We present preliminary results from two observations of the radio galaxy Centaurus A performed by the BeppoSAX satellite. We did not detect any spectral variation of the nuclear continuum in spite of the long-term flux change (by a factor 1.3) between the two observations. At both epochs, the nuclear point-like emission was well fitted with a strongly absorbed power law with an exponential cutoff at high energies (E_cutoff>200 keV). We also observed a significant flux variation of the iron line between the two observations. The flux of the line and of the continuum changed in the opposite sense. The line is more intense at the first epoch, when the nuclear source was at the lower intensity level. The implied delay between the continuum and line variations strongly suggests that the cold material responsible for the iron line production is not located very near to the primary X-ray source. There is also evidence that the line profile changed between the two epochs, being broader and slightly blueshifted when the source was fainter. It is possible that the emission feature is a blend of cold and ionized iron lines produced in separate regions surrounding the nuclear source.
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Fast X-ray Variability of PKS 2155-304: A Cross Correlation Analysis
PKS 2155-304 is one of the prototypes of BL Lac objects. Its spectral energy distribution is known from the radio to the TeV band and has two peaks, one in the soft X-rays and the other in the TeV region. These may be interpreted within the synchrotron self-Compton model, which attributes the first peak to synchrotron radiation of electrons, which also produce the second peak via Compton scattering of the same synchrotron photons. The emission is relativistically boosted in the observer direction. A way of testing the model and characterizing its geometrical and kinematical properties is through the study of correlated variability in different bands. Here we report on the search and accurate study of lags in the X-ray light curves obtained with the BeppoSAX satellite during the performance verification phase in November 1996 and compare the results with the findings of a similar analysis applied to the November 1997 BeppoSAX and 1994 ASCA data.
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Reprocessing and Variable Cold Absorption in the Broad-Line Radio Galaxy 3C390.3
A BeppoSAX observation of the BLRG 3C390.3 is reported. For the first time, both the 6.4 keV iron line and a strong reflection hump, produced by the illumination of the primary X-ray emission on cold matter, are detected in this source. The 0.1-100 keV continuum is modeled by an absorbed power law (Γ~ 1.8) reflected at high energies by material with a fairly large covering factor (Ω/2π~1). The iron line is intrinsically narrow ($σ=73(+207,-73) eV) and has an equivalent width of ~140$ eV. We discuss the results in the context of current models for AGNs and suggest that the primary X-ray power law continuum is probably produced by a hot inner flow, while the reprocessed radiation comes from an outer cold thin disk, and/or from a thick torus at even larger radii. Further observations with BeppoSAX could distinguish between the latter two cases. Beamed radiation associated to the radio jet is unlikely to contribute significantly to the X-ray emission. Finally, an historical study of the column density NH, also reported here, shows that the absorption along the line of sight changes in time. The NH time variability, which is not correlated with that of the primary continuum, seems to imply variations of the geometry of the absorber rather than variations in the ionization state of the gas.
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Constraints on the Physical Parameters of TeV Blazars
We consider the constraints on the physical parameters of a homogeneous SSC model that can be derived from the spectral shape and variability of TeV blazars. Assuming that the relativistic electron spectrum is a broken power law, where the break energy $γ_b$ is a free parameter, we write the analytical formulae that allow to connect the physical parameters of the model to observable quantities. The constraints can be summarized in a plane where the coordinates are the Doppler factor and the magnetic field. The consistency between the break energy and the balance between cooling and escape and the interpretation of the soft photon lags measured in some sources as radiative cooling times are treated as additional independent constraints. We apply themethod to the case of three well known blazars, PKS 2155-304, Mrk 421 and Mrk 501.
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BeppoSAX Observations of PKS 2155-304 during an active Gamma-ray State
PKS 2155-304 was observed with BeppoSAX in November 1997 for 64 ksec (total elapsed time 33.5 hours) and, for the first time, simultaneously in gamma-rays with EGRET on board the Compton Gamma Ray Observatory and with the ground based TeV telescope CANGAROO, during a phase of high brightness in the X-ray band. The LECS and MECS light curves show a pronounced flare (with an excursion of a factor 3.5 between min and max), with evidence of spectral hardening at maximum intensity. The source is weakly detected by the PDS in the 12-100 keV band with no significant evidence of variability. The broad band X-ray data from Beppo SAX are compared with the gamma-ray results and discussed in the framework of homogenous synchrotron self Compton models.
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A theoretical unifying scheme for gamma-ray bright blazars
The phenomenology of gamma-ray bright blazars can be accounted for by a sequence in the source power and intensity of the diffuse radiation field surrounding the relativistic jet. Correspondingly, the equilibrium particle distribution peaks at different energies. This leads to a trend in the observed properties: an increase of the observed power corresponds to: 1) a decrease in the frequencies of the synchrotron and inverse Compton peaks; 2) an increase in the ratio of the powers of the high and low energy spectral components. Objects along this sequence would be observationally classified respectively as high frequency BL Lac objects, low frequency BL Lac objects, highly polarized quasars and lowly polarized quasars. The proposed scheme is based on the correlations among the physical parameters derived in the present paper by applying to 51 gamma ray loud blazars two of the most accepted scenarios for the broad band emission of blazars, namely the synchrotron self--Compton and external Compton models, and explains the observational trends presented by Fossati et al. (1998) in a companion paper, dealing with the spectral energy distributions of all blazars. This gives us confidence that our scheme applies to all blazars as a class.
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Ultraviolet Isotropic Emission from the Blazar 3C 279
Archival IUE SWP and HST FOS spectra show the presence of a relatively strong, broad Lyman alpha emission line superposed onto the UV spectral continuum of the blazar 3C 279. As opposed to a factor ~50 variation of the continuum flux during eight years, the emission line did not exhibit significant intensity changes. Simultaneous IUE SWP and LWP spectra of 3C 279 in low emission state are fitted by power-laws of index alpha_nu ~1 (f_nu ~ nu^{-alpha_nu}), significantly flatter than measured during higher states. Our observations suggest that the Lyman alpha line is not powered by the beamed, anisotropic synchrotron radiation which produces the observed continuum in 3C 279, but rather by an unbeamed component characterized by slower and lower amplitude variability. The latter may account for the UV continuum observed during the very low state of January 1993.
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BeppoSax Spectral Survey of soft X-ray selected BL Lacs
We present X-ray spectra obtained with BeppoSAX (Satellite per Astronomia X) of 10 BL Lac objects, selected from the Einstein Medium Sensitivity and Slew Surveys. We find that in about half of the objects a fit in the 0.1-10 keV range with a single power law and free absorption yields values of NH larger than the Galactic ones. In most of these cases, however, broken power law fits with NH fixed at the Galactic values yield an alternative, better description of the data and indicate a steepening of the spectrum with increasing energy. One object (1ES1101-232) is detected up to ~100 keV. Its spectral energy distribution (SED) peaks in the medium energy X-ray band. For each object we compute the peak frequency of the SED from multifrequency data. The spectral indices alpha_x in the 2-10 keV band (Fnu prop to nu^{-alpha_x}) are smaller (i.e. flatter spectrum) for objects with higher peak frequencies. We therefore confirm and extend to higher energies the behavior already known for X-ray selected BL Lac objects in the ROSAT band. We do not find spectral indices smaller than 1; however, the flat distribution of alpha_x and the correlation between alpha_x and peak frequency found from our data suggest that a number of objects may exist, which in the quiescent status have flatter spectrum and peak frequency in the hard X-ray range.
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