P. Grandi

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 σ) for nonthermal emission in excess of thermal emission above ~25 keV. The hard excess is very unlikely to be the result of X Comae, the Seyfert 1 galaxy that is present in the field of view of the PDS. A hard spectral tail that is due to inverse Compton scattering on cosmic microwave background 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 μG is derived, while the electron energy density of the emitting electrons is ~7×10−14 ergs cm-3.

The Chandra view of the 3C/FR I sample of low luminosity radio-galaxies

We present results from Chandra observations of the 3C/FR I sample of low luminosity radio-galaxies. We detected a power-law nuclear component in 12 objects out of the 18 with available data. In 4 galaxies we detected nuclear X-ray absorption at a level of N H ∼ (0.2-6) x 10 22 cm -2 . X-ray absorbed sources are associated with the presence of highly inclined dusty disks (or dust filaments projected onto the nuclei) seen in the HST images. This suggests the existence of a flattened X-ray absorber, but of much lower optical depth than in classical obscuring tori. We thus have an unobstructed view toward most FR I nuclei, while absorption plays only a marginal role in the remaining objects. Three pieces of evidence support a jet origin for the X-ray cores: i) the presence of strong correlations between the nuclear luminosities in the radio, optical, and X-ray bands, extending over 4 orders of magnitude and having a much smaller dispersion (∼0.3 dex) when compared to similar trends found for other classes of AGNs, all of which points to a common origin for the emission in the three bands; ii) the close similarity of the broad-band spectral indices with the sub-class of BL Lac objects sharing the same range of extended radio-luminosity, in accord with the FR I/BL Lacs unified model; iii) the presence of a common luminosity evolution of spectral indices in both FR I and BL Lacs. The low luminosities of the X-ray nuclei, regardless of their origin, strengthens the interpretation of low efficiency accretion in low luminosity radio-galaxies.

SIMULTANEOUS X-RAY AND TeV OBSERVATIONS OF A RAPID FLARE FROM MARKARIAN 421

Mrk 421 was observed for about 2 days with BeppoSAX in 1998 April as part of a worldwide multiwavelength campaign. A large, well-defined flare was observed in X-rays. The same flare was observed simultaneously at TeV energies by the Whipple Observatory gamma-ray telescope. These data provide (1) the first evidence that the X-ray and TeV intensities are well correlated on timescales of hours and (2) the first exactly simultaneous X-ray and TeV spectra. The results imply that the X-ray and TeV photons derive from the same region and from the same population of relativistic electrons. The physical parameters deduced from a homogeneous synchrotron self-Compton model for the spectral energy distribution yield electron cooling times close to the observed variability timescales.

Confirmation of Nonthermal Hard X-Ray Excess in the Coma Cluster from Two Epoch Observations

We report the hard X-ray spectrum of the Coma Cluster obtained using the Phoswich Detection System data of two independent BeppoSAX observations performed with a time interval of about 3 yr. In both the spectra a nonthermal excess with respect to the thermal emission is present at a confidence level of ~3.4 σ. The combined spectrum obtained by adding up the two spectra allows a measurement of the excess at the level of ~4.8 σ at energies above 20 keV. The analysis of the full BeppoSAX data set provides a revised nonthermal X-ray flux that is slightly lower than that previously estimated (Fusco-Femiano et al.) and in agreement with that measured by two Rossi X-Ray Timing Explorer observations. The analysis of the offset fields in our Coma observations provides a possible flux determination of the BL Lac object 1ES 1255+244.

Ultraviolet and Multiwavelength Variability of the Blazar 3C 279: Evidence for Thermal Emission

The γ-ray blazar 3C 279 was monitored on a nearly daily basis with IUE, ROSAT, and EGRET for 3 weeks between 1992 December and 1993 January. During this period, the blazar was at a historical minimum at all wavelengths. Here we present the UV data obtained during this 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 (αUV ~ 1). The flattening could represent the lowest energy tail of the inverse Compton component responsible for the X-ray emission, or it could be due to the presence of a thermal component at ~20,000 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 γ-rays are external to the relativistic jet. We further discuss the long-term correlation of the UV flux with the X-ray and γ-ray fluxes obtained at various epochs. All UV archival data are included in the analysis. Both the X-ray and γ-ray fluxes are generally well correlated with the UV flux, with approximately square root and quadratic dependences, respectively.

Hard X-ray emission from the galaxy cluster A2256

After the positive detection by BeppoSAX of hard X-ray radiation up to approximately 80 keV in the Coma Cluster spectrum, we present evidence for nonthermal emission from A2256 in excess of thermal emission at a 4.6 sigma confidence level. In addition to this power-law component, a second nonthermal component already detected by ASCA could be present in the X-ray spectrum of the cluster, which is not surprising given the complex radio morphology of the cluster central region. The spectral index of the hard tail detected by the Phoswich Detection System on board BeppoSAX is marginally consistent with that expected for the inverse Compton model. A value of approximately 0.05 µG is derived for the intracluster magnetic field of the extended radio emission in the northern regions of the cluster, while a higher value of approximately 0.5 µG could be present in the central radio halo, which is likely related to the hard tail detected by ASCA.

BeppoSAX View of Radio-loud Active Galactic Nuclei

A systematic analysis of a large sample of radio-loud active galactic nuclei (AGNs) available in the BeppoSAX public archive has been performed. The sample includes 3 narrow-line radio galaxies (NLRGs), 10 broad-line radio galaxies (BLRGs), 6 steep-spectrum radio quasars (SSRQs), and 16 flat-spectrum radio quasars (FSRQs). According to the unified models, these classes correspond to objects with increasing viewing angles. As expected, the presence of a nonthermal beamed component emerges clearly in FSRQs. This class shows in fact a featureless continuum (with the exception of 3C 273) and a significantly flatter average spectral slope. However, traces of a nonthermal Doppler-enhanced radiation are elusive in the other classes. We find that the iron line equivalent widths (EWs) are generally weaker in radio-loud AGNs than in Seyfert 1 galaxies, and we confirm the presence of an X-ray Baldwin effect; that is, a decrease of EW with the 2-10 keV luminosity from Seyfert galaxies to BLRGs and quasars. Since the EW-L2-10 keV anticorrelation is present also in radio-quiet AGNs alone, this effect cannot be ascribed entirely to a strongly beamed jet component. Possible alternative interpretations are explored.

XIPE: the X-ray imaging polarimetry explorer

Abstract X-ray polarimetry, sometimes alone, and sometimes coupled to spectral and temporal variability measurements and to imaging, allows a wealth of physical phenomena in astrophysics to be studied. X-ray polarimetry investigates the acceleration process, for example, including those typical of magnetic reconnection in solar flares, but also emission in the strong magnetic fields of neutron stars and white dwarfs. It detects scattering in asymmetric structures such as accretion disks and columns, and in the so-called molecular torus and ionization cones. In addition, it allows fundamental physics in regimes of gravity and of magnetic field intensity not accessible to experiments on the Earth to be probed. Finally, models that describe fundamental interactions (e.g. quantum gravity and the extension of the Standard Model) can be tested. We describe in this paper the X-ray Imaging Polarimetry Explorer (XIPE), proposed in June 2012 to the first ESA call for a small mission with a launch in 2017. The proposal was, unfortunately, not selected. To be compliant with this schedule, we designed the payload mostly with existing items. The XIPE proposal takes advantage of the completed phase A of POLARIX for an ASI small mission program that was cancelled, but is different in many aspects: the detectors, the presence of a solar flare polarimeter and photometer and the use of a light platform derived by a mass production for a cluster of satellites. XIPE is composed of two out of the three existing JET-X telescopes with two Gas Pixel Detectors (GPD) filled with a He-DME mixture at their focus. Two additional GPDs filled with a 3-bar Ar-DME mixture always face the Sun to detect polarization from solar flares. The Minimum Detectable Polarization of a 1 mCrab source reaches 14 % in the 2–10 keV band in 105 s for pointed observations, and 0.6 % for an X10 class solar flare in the 15–35 keV energy band. The imaging capability is 24 arcsec Half Energy Width (HEW) in a Field of View of 14.7 arcmin × 14.7 arcmin. The spectral resolution is 20 % at 6 keV and the time resolution is 8 μs. The imaging capabilities of the JET-X optics and of the GPD have been demonstrated by a recent calibration campaign at PANTER X-ray test facility of the Max-Planck-Institut für extraterrestrische Physik (MPE, Germany). XIPE takes advantage of a low-earth equatorial orbit with Malindi as down-link station and of a Mission Operation Center (MOC) at INPE (Brazil). The data policy is organized with a Core Program that comprises three months of Science Verification Phase and 25 % of net observing time in the following 2 years. A competitive Guest Observer program covers the remaining 75 % of the net observing time.

The Broadband Spectrum of 3C 120 Observed by BeppoSAX

We study a broadband X-ray observation by BeppoSAX of the broad-line radio galaxy 3C 120. The primary X-ray continuum is well described by a power law with the photon index of Γ ~ 1.85 ± 0.05 and a high-energy break or a cutoff. The obtained e-folding energy of ~100-300 keV corresponds to kT ~ 50 keV in thermal-Compton models. A soft X-ray excess is found at a statistical significance of 98%. Our physical best-fit model of the excess is optically thin emission from an extended plasma (with a luminosity of ~2% of the total X-ray luminosity), which interpretation is supported by an independent finding of an extended soft X-ray halo in 3C 120. We find a moderate Compton reflection component together with a moderately broad Fe Kα line with an equivalent width (~102 eV) fully consistent with the strength of reflection, indicating their origin in an optically thick accretion disk extending relatively close to the central black hole. We observed strong spectral variability during our 2 day observation with Γ varying from ~1.7 to ~2 and correlated with the soft X-ray flux. The variability is very well modeled by thermal Comptonization in a hot plasma irradiated by a variable UV flux. Both a hot patchy corona outflowing with a mildly relativistic velocity away from a cold disk and a hot inner accretion flow overlapping with the cold disk are viable geometries. The presence of an outflow in the first case and an overlap between the hot and cold phases in the second case are required by energy balance and the observed strength of Compton reflection.

Gamma-loud Quasars: A View with BeppoSAX

We present BeppoSAX observations of three γ-ray-emitting quasars, namely, 0836+710, 1510-089, and 2230+114. The three objects have been detected up to ~100 keV showing extremely flat power-law spectra above 2 keV (energy index α2-10 = 0.3-0.5). The soft X-ray spectrum of 0836+710 implies either an absorption column density higher than the Galactic one or an intrinsically very hard slope (α0.1-1 = -0.2) below 1 keV. 1510-089 shows a soft excess, with the low-energy spectrum steeper (α0.1-1 = 1.6) than the high-energy power law. The results are discussed in the framework of current inverse Compton models for the high-energy emission of flat-spectrum radio quasars and are used to estimate the physical quantities in the jet-emitting region and to shed light on the energy transport mechanisms in jets. Finally we discuss the estimates of the jet luminosity in the context of the Blandford & Znajek mechanism for jet production.