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
P. Grandi
Publications and source records attributed to P. Grandi.
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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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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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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Hard X-Ray Radiation in the Coma Cluster Spectrum
Hard X-ray radiation has been detected for the first time in the Coma cluster by BeppoSAX. Thanks to the unprecedented sensitivity of the Phoswich Detection System (PDS) instrument, the source has been detected up to ~80 keV. There is clear evidence (4.5 sigma) for non-thermal emission in excess of thermal above ~25 keV. The hard excess is very unlikely due to X Comae, the Seyfert 1 galaxy present in the field of view of the PDS. A hard spectral tail due to inverse Compton on CMB photons is predicted in clusters, like Coma, with radio halos. Combining the present results with radio observations, a volume-averaged intracluster magnetic field of ~0.15 micro G is derived, while the electron energy density of the emitting electrons is ~7x10**-14 erg/cm**3.
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BeppoSAX confirms extreme relativistic effects in the X-ray spectrum of MCG-6-30-15
We report the first simultaneous measure of the X-ray broadband (0.1--200 keV) continuum and of the iron K-alpha fluorescent line profile in the Seyfert 1 galaxy MCG-6-30-15. Our data confirms the ASCA detection of a skewed and redshifted line profile (Tanaka et al. 1995). The most straightforward explanation is that the line photons are emitted in the innermost regions of a X-ray illuminated relativistic disk. The line Equivalent Width (~200 eV) is perfectly consistent with the expected value for solar abundances, given the observed amount of Compton reflection. We report also the discovery of a cut-off in the nuclear primary emission at the energy of ~160 keV.
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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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BeppoSAX Observations of Bright Seyfert 2 Galaxies: Measuring the Intrinsic Continuum Emission
We report broad band (0.1-200 keV) X-ray observations, made by BeppoSAX, of a sample of bright Seyfert 2 galaxies: NGC7172, NGC2110, NGC4507, Mkn 3 and NGC7674. These spectra provide a better understanding of the effects of X-ray reprocessing by cold material in the source and allow to put tighter constraints on the various spectral parameters involved. In particular, the data are used to determine, with less ambiguities than in the past, the shape of the intrinsic continuum emission by means of the high energy data. Within the small sample both Compton thin and Compton thick sources are found according to the expectations of the unified theory.
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Quasi-simultaneous observations of 3C 273 by BeppoSAX and ASCA
We report the results of quasi-simultaneous observations of the bright quasar 3C 273 using the BeppoSAX Low-Energy Concentrator Spectrometer (LECS) and the ASCA Solid-State Imaging Spectrometer (SIS). These observations are part of an intercalibration programme and demonstrate good agreement between both instruments in the common energy range 0.5-10 keV. In particular, the absorption feature discovered by BeppoSAX/LECS at $\sim$ 0.5 keV is consistently seen by ASCA/SIS. We present the most accurate measurement yet of the spectral shape of this absorption feature and provide constraints on its physical interpretation. A self-consistent warm absorber model in photoionization equilibrium only gives an adequate spectral fit if a very high velocity inflow is allowed for the warm absorber (v $\sim$ 0.36 c). The ionization parameter and the column density of the warm absorber are then $\sim$ 1 and $\sim$ 1.3e21 cm^-2, respectively. If the differences in best-fit neutral absorption column density between the LECS and each SIS are taken as estimates of the systematic spectral uncertainties, then these are < 0.8e20 and < 2.e20 cm^-2 (at 90 % confidence) for SIS0 and SIS1, respectively.
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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 observations of PKS 0528+134
We report on the BeppoSAX observations of the gamma-ray blazar PKS 0528+134 performed in Feb and Mar 1997, during a multiwavelength campaign involving EGRET and ground based telescopes. The source was in a faint and hard state, with energy spectral index alpha=0.48+-0.04 between 0.1 and 10 keV, and [2-10] keV flux of 2.7E-12 erg/cm2/s. No significant variability was observed. The source was detected in the 20-120 keV band by the PDS, with a flux lying slightly above the extrapolation from lower X-ray energies. Comparing this low state with previous higher states of the source, there is an indication that the X-ray spectrum hardens and the gamma-ray spectrum steepens when the source is fainter.
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High Energy Break and Reflection Features in the Seyfert Galaxy MCG+8-11-11
We present the results from ASCA and OSSE simultaneous observations of the Seyfert 1.5 galaxy MCG+8-11-11 performed in August-September 1995. The ASCA observations indicate a modest flux increase (20%) in 3 days, possibly correlated to a softening of the 0.6-9 keV spectrum. The spectrum is well described by a hard power law (Gamma=1.64) absorbed by a column density slightly larger than the Galactic value, with an iron line at 6.4 keV of EW=400 eV. The simultaneous OSSE data are characterized by a much softer power law with photon index Gamma=3.0, strongly suggesting the presence of a spectral break in the hard X/soft gamma-ray band. A joint fit to OSSE and ASCA data clearly shows an exponential cut-off at about 300 keV, and strong reflection component. MCG+8-11-11 features a spectral break in the underlying continuum unambiguously. This, together with the inferred low compactness of this source, favours thermal or quasi-thermal electron Comptonization in a structured Corona as the leading process of high energy radiation production.
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A BeppoSAX observation of 3C273: broadband spectrum and detection of a low-energy absorption feature
We report the results of a 3C273 observation performed during the Science Verification Phase (SVP) of the BeppoSax satellite. The broad-band spectrum is well represented by a power-law between 1 keV and 200 keV. The spectral slope is flat (Γ~1.6) with a weak emission line at ~ 6.4 keV (rest frame) of EW ~ 30 eV. Below 1 keV, a deviation from a power-law due to an absorption feature plus a soft component is present. This is the first time that a feature in absorption at ~ 0.5 keV (observer frame) is unambiguously detected in 3C273.
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Soft X-ray observations of the complex warm absorber in MCG-6-30-15 with BeppoSAX
We report on soft X-ray observations of the Seyfert 1 galaxy MCG-6-30-15 with the Low and Medium Energy Concentrator Spectrometers on board BeppoSAX. The time averaged 0.1-4 keV spectrum shows evidence for a complex warm absorber. K-edges of highly ionized oxygen (O vii and O viii) can only partly describe the soft X-ray spectrum below 2 keV. A spectral variability study reveales large and rapid changes in absorption at the energy of the O viii K-edge and in the continuum shape and count rate. These fluctuations are not simply correlated.
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Multiwavelength Monitoring of the BL Lacertae Object PKS 2155-304 in May 1994. I. The Ground-Based Campaign
Optical, near-infrared, and radio observations of the BL Lac object PKS2155-304 were obtained simultaneously with a continuous UV/EUV/X-ray monitoring campaign in 1994 May. Further optical observations were gathered throughout most of 1994. The radio, millimeter, and near-infrared data show no strong correlations with the higher energies. The optical light curves exhibit flickering of 0.2-0.3 mag on timescales of 1-2 days, superimposed on longer timescale variations. Rapid variations of ~0.01 mag/min, which, if real, are the fastest seen to date for any BL Lac object. Small (0.2-0.3 mag) increases in the V and R bands occur simultaneously with a flare seen at higher energies. All optical wavebands (UBVRI) track each other well over the period of observation with no detectable delay. For most of the period the average colors remain relatively constant, although there is a tendency for the colors (in particular B-V) to vary more when the source fades. In polarized light, PKS 2155-304 showed strong color dependence and the highest optical polarization (U = 14.3%) ever observed for this source. The polarization variations trace the flares seen in the ultraviolet flux.
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