M. Gliozzi

Six years of BeppoSAX observations of blazars: A spectral catalog

We present a spectral catalog for blazars based on the BeppoSAX archive. The sample includes 44 High-energy peaked BL Lacs (HBLs), 14 Low-energy peaked BL Lacs (LBLs), and 28 Flat Spectrum Radio Quasars (FSRQs). A total of 168 LECS, MECS, and PDS spectra were analyzed, corresponding to observations taken in the period 1996−2002. The 0.1−50 keV continuum of LBLs and FSRQs is generally fitted by a single power law with Galactic column density. A minority of the observations of LBLs (25%) and FSRQs (15%) is best fitted by more complex models like the broken power law or the continuously curved parabola. These latter models provide also the best description for half of the HBL spectra. Complex models are more frequently required for sources with fluxes F2−10 keV > 10 −11 erg cm −2 s −1 , corresponding to spectra with higher signal-to-noise ratio. As a result, considering sources with flux above this threshold, the percentage of spectra requiring those models increases for all the classes. We note that there is a net separation of X-ray spectral properties between HBLs on one side, and LBLs and FSRQs on the other, the distinction between LBLs and FSRQs is more blurry. This is most likely related to ambiguities in the optical classification of the two classes.

Obscuration and Origin of Nuclear X-Ray Emission in FR I Radio Galaxies

We present X-ray observations of the nuclear region of 25 Fanaroff-Riley type I (FR I) radio galaxies from the 3CRR and B2 catalogs, using data from the Chandra and XMM-Newton archives. We find the presence of a X-ray central compact core (CCCX) in 13/25 sources; in 3/25 sources the detection of a CCCX is uncertain, while in the remaining 9/25 sources no CCCX is found. All the sources are embedded in a diffuse soft X-ray component, generally on kiloparsec scales, which is in agreement with the halo of the host galaxy and/or with the intracluster medium. The X-ray spectra of the cores are described by a power law with photon indices Γ = 1.1-2.6. In eight sources excess absorption over the Galactic value is detected, with rest-frame column densities N ~ 1020-1021 cm-2; thus, we confirm the previous claim, based on optical data, that most FR I radio galaxies lack a standard optically thick torus. We find significant correlations between the X-ray core luminosity and the radio and optical luminosities, suggesting that at least a fraction of the X-ray emission originates in a jet; however, the origin of the X-rays remains ambiguous. If the X-ray emission is entirely attributed to an isotropic, accretion-related component, we find very small Eddington ratios, Lbol/LEdd ~ 10-3 to 10-8, and we calculate the radiative efficiency to be η ~ 10-2 to 10-6 on the basis of the Bondi accretion rates from the spatial analysis. This suggests that radiatively inefficient accretion flows are present in the cores of low-power radio galaxies.

A Chandra Snapshot Survey of Infrared-bright LINERs: A Possible Link Between Star Formation, Active Galactic Nucleus Fueling, and Mass Accretion

We present results from a high-resolution X-ray imaging study of nearby LINERs observed by ACIS on board Chandra. This study complements and extends previous X-ray studies of LINERs, focusing on the underexplored population of nearby dust-enshrouded infrared-bright LINERs. The sample consists of 15 IR-bright LINERs (LFIR/LB > 3), with distances that range from 11 to 26 Mpc. Combining our sample with previous Chandra studies, we find that ~51% (28/55) of the LINERs display compact hard X-ray cores. The nuclear 2-10 keV luminosities of the galaxies in this expanded sample range from ~2 × 1038 to ~2 × 1044 ergs s-1. We find that the most extreme IR-faint LINERs are exclusively active galactic nuclei (AGNs). The fraction of LINERs containing AGNs appears to decrease with IR brightness and increase again at the highest values of LFIR/LB. We find that of the 24 LINERs showing compact nuclear hard X-ray cores in the expanded sample that were observed at Hα wavelengths, only eight actually show evidence of a broad line. Similarly, of the 14 LINERs showing compact nuclear hard X-ray cores with corresponding radio observations, only eight display a compact flat spectrum radio core. These findings emphasize the need for high-resolution X-ray imaging observations in the study of IR-bright LINERs. Finally, we find an intriguing trend in the Eddington ratio versus LFIR and LFIR/LB for the AGN-LINERs in the expanded sample that extends over 7 orders of magnitude in L/LEdd. This correlation may imply a link between black hole growth, as measured by the Eddington ratio, and the star formation rate, as measured by the far-IR luminosity and IR-brightness ratio. If the far-IR luminosity is an indicator of the molecular gas content in our sample of LINERs, our results may further indicate that the mass accretion rate scales with the host galaxys fuel supply. We discuss the potential implications of our results in the framework of black hole growth and AGN fueling in low-luminosity AGNs.

THE LINK BETWEEN STAR FORMATION AND ACCRETION IN LINERs: A COMPARISON WITH OTHER ACTIVE GALACTIC NUCLEUS SUBCLASSES

We present archival high-resolution X-ray imaging observations of 25 nearby LINERs observed by ACIS on board Chandra. This sample builds on our previously published proprietary and archival X-ray observations and includes the complete set of LINERs with published black hole masses and FIR luminosities that have been observed by Chandra. Of the 82 LINERs observed by Chandra, 41 (50%) display hard nuclear cores consistent with an AGN. The nuclear 2-10 keV luminosities of these AGN-LINERs range from ~2 × 1038 to ~1 × 1044 ergs s-1. Reinforcing our previous work, we find a significant correlation between the Eddington ratio, Lbol/LEdd, and the FIR luminosity, LFIR, as well as the IR brightness ratio, LFIR/LB, in the host galaxy of AGN-LINERs that extends over 7 orders of magnitude in Lbol/LEdd. Combining our AGN-LINER sample with galaxies from other AGN subclasses, we find that this correlation is reinforced in a sample of 129 AGNs, extending over almost 9 orders of magnitude in Lbol/LEdd. Using archival and previously published observations of the 6.2 μm PAH feature from ISO, we find that it is unlikely that dust heating by the AGN dominates the FIR luminosity in our sample of AGNs. Our results may therefore imply a fundamental link between the mass accretion rate (), as measured by the Eddington ratio, and the star formation rate (SFR), as measured by the FIR luminosity. Apart from the overall correlation, we find that the different AGN subclasses occupy distinct regions in the LFIR and Lbol/LEdd plane. Assuming a constant radiative efficiency for accretion, our results may imply a variation in the SFR/ ratio as a function of AGN activity level, a result that may have significant consequences for our understanding of galaxy formation and black hole growth.

Multiwavelength Monitoring of the Narrow-Line Seyfert 1 Galaxy Arakelian 564. III. Optical Observations and the Optical-UV-X-Ray Connection

We present the results of a 2 yr long optical monitoring program of the narrow-line Seyfert 1 galaxy Ark 564. The majority of this monitoring project was also covered by X-ray observations (RXTE), and for a period of ~50 days, we observed the galaxy in UV (HST) and X-rays (RXTE and ASCA) simultaneously with the ground-based observations. Rapid and large-amplitude variations seen in the X-ray band, on a daily and hourly timescale, were not detected at optical and UV wavelengths, which in turn exhibited much lower variability either on short (1 day) or long (several months) timescales. The only significant optical variations can be described as two 2-4 day events with ~10% flux variations. We detect no significant optical line variations and thus cannot infer a reverberation size for the broad-line region. Similarly, the large X-ray variations seem to vanish when the light curve is smoothed over a period of 30 days. The UV continuum follows the X-rays with a lag of ~0.4 days, and the optical band lags the UV band by ~2 days. No significant correlation was found between the entire X-ray data set and the optical band. Focusing on a 20 day interval around the strongest optical event we detect a significant X-ray-optical correlation with similar events seen in the UV and X-rays. Our data are consistent with reprocessing models on the grounds of the energy emitted in this single event. However, several large X-ray flares produced no corresponding optical emission.

THE INCIDENCE OF ACTIVE GALACTIC NUCLEI IN PURE DISK GALAXIES: THE SPITZER VIEW

Using the Spitzer telescope, we have conducted a high-resolution spectroscopic study of 18 bulgeless (Hubble type of Sd or Sdm) galaxies that show no definitive signatures of nuclear activity in their optical spectra. This is the first systematic mid-infrared search for weak or hidden active galactic nuclei (AGNs) in a statistically significant sample of bulgeless (Sd/Sdm) disk galaxies. Based on the detection of the high-ionization [NeV] 14.3 µm line, we report the discovery of an AGN in one out of the 18 galaxies in the sample. This galaxy, NGC 4178, is a nearby edge-on Sd galaxy, which likely hosts a prominent nuclear star cluster (NSC). The bolometric luminosity of the AGN inferred from the [NeV] line luminosity is � 8×10 41 ergs s −1 . This is almost two orders of magnitude greater than the luminosity of the AGN in NGC 4395, the best studied AGN in a bulgeless disk galaxy. Assuming that the AGN in NGC 4178 is radiating below the Eddington limit, the lower mass limit for the black hole is � 6×10 3 M⊙. The fact that none of the other galaxies in the sample shows any evidence for an AGN demonstrates that while the AGN detection rate based on mid-infrared diagnostics is high (30-40%) in optically quiescent galaxies with pseudobulges or weak classical bulges (Hubble type Sbc and Sc), it drops drastically in Sd/Sdm galaxies. Our observations therefore confirm that AGNs in completely bulgeless disk galaxies are not hidden in the optical but truly are rare. Of the three Sd galaxies with AGNs known so far, all have prominent NSCs, suggesting that in the absence of a well-defined bulge, the galaxy must possess a NSC in order to host an AGN. On the other hand, while the presence of a NSC appears to be a requirement for hosting an AGN in bulgeless galaxies, neither the properties of the NSC nor those of the host galaxy appear exceptional in late-type AGN host galaxies. The recipe for forming and growing a central black hole in a bulgeless galaxy therefore remains unknown. Subject headings: Galaxies: Active— Galaxies: black hole physics – dark matter – galaxies: spiral: Galaxies — Infrared: Galaxies

STRUCTURE OF THE ACCRETION FLOW IN BROAD-LINE RADIO GALAXIES: THE CASE OF 3C 390.3

We present XMM-Newton and Suzaku observations of the broad-line radio galaxy 3C 390.3 acquired in 2004 October and 2006 December, respectively. An archival Swift BAT spectrum from the 9 month survey is also analyzed, as well as an optical spectrum simultaneous to XMM-Newton. At soft X-rays, no absorption features are detected in the Reflection Grating Spectrometer spectrum of 3C 390.3; a narrow emission line is found at 0.564 keV, most likely originating in the narrow-line region. Both the EPIC and XIS data sets confirm the presence of an Fe Kα emission line at 6.4 keV with equivalent width (EW) = 40 eV. The Fe Kα line has a width FWHM ∼ 8800 km s^−1, consistent within a factor of 2 with the width of the double-peaked Hα line, suggesting an origin from the broad-line region. The data show for the first time a weak, broad bump extending from 5 to 7 keV. When fitted with a Gaussian, its centroid energy is 6.6 keV in the source’s rest frame with FWHM of 43,000 km s^−1 and EW of 50 eV; its most likely interpretation is emission from He-like Fe (Fe xxv), suggesting the presence of an ionized medium in the inner regions of 3C 390.3. The broadband 0.5–100 keV continuum is well described by a single power law with photon index Γ = 1.6 and cutoff energy 157 keV, plus cold reflection with strength R = 0.5. In addition, ionized reflection is required to account for the 6.6 keV bump in the broadband continuum, yielding an ionization parameter ξ ∼ 2700 erg cm s^−1; the inner radius of the ionized reflector is constrained to be larger than 20r_G, although this result depends on the assumed emissivity profile of the disk. If true, we argue that the lack of broad Fe K emission from within 20r_G indicates that the innermost regions of the disk in 3C 390.3 are obscured and/or poorly illuminated. While the spectral energy distribution (SED) of 3C 390.3 is generally dominated by accretionrelated continuum, during accretion low states the jet can significantly contribute in the optical to X-ray bands via synchrotron self-Compton emission. The Compton component is expected to extend to and peak at GeV gamma rays where it will be detected with the Fermi Gamma-Ray Space Telescope during its first few years of operation.

Long-Term X-Ray and TeV Variability of Mrk 501

We present X-ray observations of the nearby TeV blazar Mrk 501 obtained with RXTE during 1997, 1998, 1999, 2000, and 2004. The goal of this study is twofold: (1) to characterize the long-term X-ray flux and spectral variability of the source with a model-independent analysis, and (2) to investigate the X-ray and TeV correlation on long timescales (>days). Significant spectral variations were observed during all the observations along with long-term timescale correlations between the X-ray colors and the count rate. Specifically, on long timescales, a typical blazar behavior is observed with the spectrum hardening when the source brightens, and the fractional variability correlating with the energy band. A similar spectral trend is observed also in the majority of the individual flares. The spectral and temporal variability properties appear to be markedly different compared to those of non-jet-dominated radio-loud and radio-quiet AGNs monitored with RXTE. To investigate the X-ray-TeV correlation on long timescales we compared RXTE monitoring data with HEGRA and Whipple historical light curves. We confirm the presence of a direct correlation between X-ray and gamma-ray emissions, which appears to be stronger when the source is brighter. The analysis of individual flares shows that the X-ray-TeV correlated activity is heterogeneous, in the sense that it might be both linear and nonlinear, and some X-ray flares seem to be lacking the TeV counterpart. However, more sensitive TeV observations are necessary to confirm these findings, and to put tighter constraints on jet models.

THE JET-DISK CONNECTION IN AGNs: CHANDRA AND XMM-NEWTON OBSERVATIONS OF THREE POWERFUL RADIO-LOUD QUASARS

The connection between the accretion process that powers AGNs and the formation of jets is still poorly understood. Here we tackle this issue using new, deep Chandra and XMM-Newton observations of the cores of three powerful radio-loud quasars, 1136-135, 1150+497 (Chandra), and 0723+679 (XMM-Newton), in the redshift range z = 0.3-0.8. These sources are known from our previous Chandra snapshot survey to have kiloparsec-scale X-ray jets. In 1136-135 and 1150+497, evidence is found for the presence of diffuse thermal X-ray emission around the cores, on scales of 40-50 kpc and with luminosity L(0.3-2 keV) ~ 1043 ergs s-1, suggesting thermal emission from the host galaxy or a galaxy group. The X-ray continua of the cores in the three sources are described by an upward-curved (concave) broken power law, with photon indices Γsoft ~ 1.8-2.1 and Γhard ~ 1.7 below and above ≈2 keV, respectively. There is evidence for an unresolved Fe Kα line with EW ~ 70 eV in the three quasars. The spectral energy distributions of the sources can be well described by a mix of jet and disk emission, with the jet dominating the radio and hard X-rays (via synchrotron and external Compton radiation) and the disk dominating the optical/UV through soft X-rays. A comparison of the kiloparsec-scale with the parsec-scale jet powers shows that the two agree within a factor of 2, confirming previous findings for γ-ray blazars. This suggests that the power channeled into the jet is stable on timescales of the order of 104-105 yr and that a negligible fraction of the kinetic power of the jet is dissipiated along its path from the innermost regions of the AGN to the hundreds of kiloparsec scales.

On the origin of the X-rays and the nature of accretion in NGC 4261

We report on the X-ray properties of the radio galaxy NGC 4261, combining information from the XMM, Chandra, and SAX satellites. Goals of this study are to investigate the origin of the X-rays from this low-power radio galaxy and the nature of the accretion process onto the central black hole. The X-ray spectrum of the nuclear source extending up to 100--150 keV is well described by a partially covered (covering factor