Search

Aims. We report upper limits to the very high energy flux ($E>100$GeV) of the flat spectrum radio quasar 3C 454.3 ($z=0.859$) derived by the Cherenkov telescope MAGIC during the high states of July/August and November/December 2007. We compare the upper limits derived in both time slots with the available quasi-simultaneous MeV-GeV data from the AGILE γ-ray satellite and interpret the observational results in the context of leptonic emission models.

We present initial results of an ESO-VLT large programme (AMAZE) aimed at determining the evolution of the mass-metallicity relation at z> 3 by means of deep near-IR spectroscopy. Gas metallicities are measured, for an initial sample of nine star forming galaxies at z~ 3.5, by means of optical nebular lines redshifted into the near-IR. Stellar masses are accurately determined by using Spitzer-IRAC data, which sample the rest-frame near-IR stellar light in these distant galaxies. When compared with previous surveys, the mass-metallicity relation inferred at z~ 3.5 shows an evolution much stronger than observed at lower redshifts. The evolution is prominent even in massive galaxies, indicating that z~ 3 is an epoch of major action in terms of star formation and metal enrichment also for massive systems. There are also indications that the metallicity evolution of low mass galaxies is stronger relative to high mass systems, an effect which can be considered the chemical version of the galaxy downsizing. The mass-metallicity relation observed at z~ 3.5 is difficult to reconcile with the predictions of some hierarchical evolutionary models. Such discrepancies suggest that at z> 3 galaxies are assembled mostly with relatively un-evolved sub-units, i.e. small galaxies with low star formation efficiency. The bulk of the star formation and metallicity evolution probably occurs once small galaxies are already assembled into bigger systems.

We discuss XMM-Newtonand BeppoSAX observations of MKN 231, the lowest-redshift Broad Absorption Line (BAL) QSO known so far and one of the best-studied Ultra Luminous Infrared Galaxies. By combining the XMM-Newtonspectral resolution and the high-energy bandpass of BeppoSAX we have been able to study in more detail than previously possible its 0.2–50 keV spectral properties. The BeppoSAX PDS data unveiled, for the first time, a highly absorbed ($N_{\rm H}\sim 2\times 10^{24}$cm-2) power-law component. We find that: a) below 10 keV we are seeing only reprocessed radiation through reflection and/or scattering; b) the intrinsic 2–10 keV luminosity of MKN 231 is $ 1^{+1.0}_{-0.5} \times 10^{44}$erg s-1, i.e. more than an order of magnitude greater than previous measurements; c) the starburst activity significantly contributes to the soft ($E<2$keV) X-ray emission.

We investigate the diffusion of relativistic particles in the presence of a time constant, spatially non-homogeneous magnetic field. We show that the intrinsic dynamical instability of the particle trajectories can provide an important source of non-collisional diffusion even in the absence of time-dependent phenomena and for the simplest possible field structure that still allows microscopic chaos.

We present BeppoSAX observations of the two radio galaxies PKS 1934-63 and S5 1946+708. Strong Iron Kαlines are detected in both objects indicating that the two sources are absorbed by column densities higher than 1024cm-2. 
Combining radio continuum, HI absorption and X-ray data we can constrain the physical state and the dimensions of the absorber. We find that the putative obscuring torus is molecular and located at a distance higher than 20 pc from the center in S5 1946+70. In PKS 1934-63 no radio nucleus has been observed. If this is due to free-free absorption a radius of only a few pc is inferred. Since the Kαlines have been detected only at 90% confidence, we also discuss the implications if they would be not as strong as found from our data. From our results it appears that the combination of hard X-ray data and spatially resolved HI absorption measurements provides a powerful diagnostic for conditions in the absorbing “torus”.

We present the XMM-Newtonobservation of the merging system IRAS 19254-7245, also known as The Superantennae, whose southern nucleus is classified as a Seyfert 2 galaxy. The XMM-Newtondata have allowed us to perform a detailed X-ray imaging and spectral analysis of this system. We clearly detect, for the first time in this system, a strong ($EW \sim 1.4$keV) Fe emission line at $6.49\pm 0.1$keV (rest-frame). The X-ray spectrum requires a soft thermal component ($kT\sim0.9$keV; $L_{0.5-2}\sim 4\times10^{41}$erg s-1), likely associated with the starburst, and a hard power-law continuum above 2 keV (observed $L_{2-10}\sim 4\times10^{42}$erg s-1). We confirm the flatness of this latter component, already noted in previous ASCA data. This flatness, together with the detection of the strong Fe-Kaline and other broad band indicators, suggest the presence of a Compton-thick AGN with intrinsic luminosity ?1044erg s-1. We show that a Compton-thick model can perfectly reproduce the X-ray spectral properties of this object.

Based on the most recent QSO ultraviolet spectra, the covering factor of the clouds of the Broad Line Region (BLR) is about 30% , or larger. This value would imply that in at least 30% of the QSOs our line of sight crosses one, or more, BLR clouds and, in the latter case, the UV spectrum should show a sharp Ly-edge in absorption. This Ly-edge in absorption is never observed. This paradox is solved if, as suggested by various authors, the BLR is flattened and the dusty gas in the outer parts, on the same plane, prevents the observation along the lines of sight passing through the BLR clouds. The objects observed edge-on (with respect to the flattened BLR) would be classified as obscured QSOs or, within the framework of the unified model, type 2 QSOs. The covering factor of the BLR constrains the fraction of obscured QSOs to be QSO2/QSO1 > 0.5. This lower limit is already high with respect to the number of candidate type 2 QSOs claimed so far. We discuss this constraint in relation to recent AGN surveys.

Quasars selected in optical surveys by means of their blue or UV excess are known to be strong emitters in the X-rays, except for Broad Absorption Line (BAL) objects. In this paper we study the X-ray emission of quasars selected through their emission line spectrum rather than their blue excess. X-ray data are obtained by cross-correlating two optical samples (the Hamburg Survey Catalogue and the Palomar Transit Catalogue) with the WGA catalogue of ROSAT observations. We find that a significant fraction of objects are strongly underluminous in the X-rays. We discuss the physical nature of these sources and we propose an interpretation for their optical and X-ray properties based on a dust-poor and gas-rich absorber. Our results suggest the existence of a population of AGNs having a type 1 optical spectrum and a type 2 X-ray emission.

We present new, quasi-simultaneous hard X-ray and near-IR spectra of the ultraluminous infrared galaxy IRAS 05189-2524, and discuss them together with archival and literature data. The 1.9 Seyfert nucleus is Compton-thin. The near-IR broad lines are seen in transmission, similarly to the X-rays, and the medium along the line of sight has an $A_V/N_{\rm H}$ratio definitely lower than Galactic. The increase in obscuration observed at the latter epoch has $\Delta A_V/ \Delta N_{\rm H}$also less than Galactic, supporting a correlation between the properties of the obscuring matter and its proximity to the center. The measured AVis compatible with the broad component of $\rm H\alpha$being seen in transmission, as opposed to scattering, so that most of the observed polarization must be due to dichroism. The bolometric luminosity of the AGN, deduced from the X-ray and line luminosities, falls definitely short of accounting for the IR luminosity of the galaxy, consistent with its coolish infrared color.

We present synthesis models of the X-ray background where the available X-ray observational constraints are used to derive information on the AGN population properties. We show the need for luminous X-ray absorbed AGNs, the QSO2s, in reproducing the 2-10 keV source counts at relatively bright fluxes. We compare a model where the evolution of absorbed AGNs is faster than that of unabsorbed ones, with a standard model where absorbed and unabsorbed AGNs evolve at the same rate. It is found that an increase by a factor of ∼2 from $z=0$to $z\sim1.3$in the ratio between absorbed and unabsorbed AGNs would provide a significant improvement in the data description. Finally, we make predictions on the AGNs to be observed in deep X-ray surveys which contain information on the AGN space density at high redshift.

We present observational evidences that dust in the circumnuclear region of AGNs has different properties than in the Galactic diffuse interstellar medium. By comparing the reddening of optical and infrared broad lines and the X-ray absorbing column density we find that the $E_{B-V}/N_{\rm H}$ratio is nearly always lower than Galactic by a factor ranging from ∼3 up to ∼100. Other observational results indicate that the $A_V/N_{\rm H}$ratio is significantly lower than Galactic in various classes of AGNs including intermediate type 1.8-1.9 Seyferts, hard X-ray selected and radio selected quasars, broad absorption line QSOs and grism selected QSOs. The lack of prominent absorption features at 9.7 μm (silicates) and at 2175 Å (carbon dip) in the spectra of Seyfert 2s and of reddened Seyfert 1s, respectively, add further evidence for dust in the circumnuclear region of AGNs being different from Galactic. These observational results indicate that the dust composition in the circumnuclear region of AGNs could be dominated by large grains, which make the extinction curve flatter, featureless and are responsible for the reduction of the $E_{B-V}/N_{\rm H}$and $A_V/N_{\rm H}$ratios. Regardless of the physical origin of these phenomena, the reduced dust absorption with respect to what expected from the gaseous column density should warn about a mismatch between the optical and the X-ray classification of the active galactic nuclei in terms of their obscuration.

We present initial results of an ESO-VLT large programme (AMAZE) aimed at determining the evolution of the mass-metallicity relation at z > 3 by means of deep near-IR spectroscopy. Gas metallicities are measured, for an initial sample of nine star forming galaxies at z ~ 3.5, by means of optical nebular lines redshifted into the near-IR. Stellar masses are accurately determined by using Spitzer-IRAC?data, which sample the rest-frame near-IR stellar light in these distant galaxies. When compared with previous surveys, the mass-metallicity relation inferred at z ~ 3.5 shows an evolution much stronger than observed at lower redshifts. The evolution is prominent even in massive galaxies, indicating that z ~ 3 is an epoch of major action in terms of star formation and metal enrichment also for massive systems. There are also indications that the metallicity evolution of low mass galaxies is stronger relative to high mass systems, an effect which can be considered the chemical version of the galaxy downsizing. The mass-metallicity relation observed at z ~ 3.5 is difficult to reconcile with the predictions of some hierarchical evolutionary models. Such discrepancies suggest that at z > 3?galaxies are assembled mostly with relatively un-evolved sub-units, i.e. small galaxies with low star formation efficiency. The bulk of the star formation and metallicity evolution probably occurs once small galaxies are already assembled into bigger systems.

Context.Although many radio-loud quasars and galaxies have been observed in X-rays, systematic studies of well defined samples are rare.Aims.We investigate the X-ray properties of the most luminous radio sources in the 3CR?catalogue, in order to assess whether they are similar to the most luminous radio-quiet quasars, for instance in the X-ray normalization with respect to the optical luminosity, or in the distribution of the absorption column density.Methods.We have selected the (optically identified) 3CR?radio sources whose 178-MHz monochromatic luminosity lies in the highest factor-of-three bin. The 4?most luminous objects had already been observed in X-rays. Of the remaining 16, we observed 8 randomly chosen ones with XMM-Newton, with the only requirement that half were of type?1 and half of type?2 according to the optical identification.Results.All targets were detected. The optical-to-X-ray spectral index, ?ox, can be computed only for the type?1s and, in agreement with previous studies, is found to be flatter than in radio-quiet quasars of similar luminosity. However, the Compton-thin type?2s have an absorption-corrected X-ray luminosity systematically lower than the type?1s, by a factor which makes them consistent with the radio-quiet ?ox. Within the limited statistics, the Compton-thick objects seem to have a reflected component more luminous than the Compton-thin ones.Conclusions.The extra X-ray component observed in type?1 radio-loud quasars is beamed for intrinsic causes, and is not collimated by the absorbing torus as is the case for the (intrinsically isotropic) disk emission. The extra component can be associated with a relativistic outflow, provided that the flow opening angle and the Doppler beaming factor are ~1/5-1/7?radians.

Context. The Milagro experiment has announced the discovery of an excess flux of TeV cosmic rays from the general direction of the heliotail, also close to the Galactic anticenter.Aims. We investigate the hypothesis that the excess cosmic rays were produced in the SN?explosion that gave birth to the Geminga pulsar.Methods. The assumptions underlying our proposed scenario are that the Geminga supernova occurred about 3.4?105?years ago (as indicated by the spin down timescale), that a burst of cosmic rays was injected with total energy ~ 1049?erg (i.e., about?1% of a typical SN output), and that the Geminga pulsar was born with a positive radial velocity of 100?200?km?s-1.Results. We find that our hypothesis is consistent with the available information. In a first variant (likely oversimplified), the cosmic rays have diffused according to the Bohm prescription (i.e., with a diffusion coefficient on the order of c?rL, with?c the speed of light and rLthe Larmor radius). An alternative scheme assumes that diffusion only occurred initially, and the final propagation to the Sun was a free streaming in a diverging magnetic field.Conclusions. If the observed cosmic ray excess does indeed arise from the Geminga SN explosion, the long?sought ?smoking gun? connecting cosmic rays with supernovae would finally be at hand. It could be said that, while looking for the ?smoking gun?, we were hit by the bullets themselves.

Context.3C 279, the first quasar discovered to emit VHE γ-rays by the MAGIC telescope in 2006, was reobserved by MAGIC in January 2007 during a major optical flare and from December 2008 to April 2009 following an alert from the Fermi space telescope on an exceptionally high γ-ray state.Aims.The January 2007 observations resulted in a detection on January 16 with significance 5.4σ, corresponding to a F(>150 GeV) (3.8 ± 0.8) × 10-11ph cm-2s-1while the overall data sample does not show significant signal. The December 2008–April 2009 observations did not detect the source. We study the multiwavelength behaviour of the source at the epochs of MAGIC observations, collecting quasi-simultaneous data at optical and X-ray frequencies and for 2009 also γ-ray data from Fermi.Methods.We study the light curves and spectral energy distribution of the source. The spectral energy distributions of three observing epochs (including the February 2006, which has been previously published) are modelled with one-zone inverse Compton models and the emission on January 16, 2007 also with two zone model and with a lepto-hadronic model.Results.We find that the VHE γ-ray emission detected in 2006 and 2007 challenges standard one-zone model, based on relativistic electrons in a jet scattering broad line region photons, while the other studied models fit the observed spectral energy distribution more satisfactorily.

Context.Part of the very high energy γ-ray radiation coming from extragalactic sources is absorbed through the pair production process on the extragalactic background light photons. Extragalactic magnetic fields alter the trajectories of these cascade pairs and, in turn, convert cosmic background photons to γ-ray energies by inverse Compton scattering. These secondary photons can form an extended halo around bright VHE sources.

Context. Wide-range spectral coverage of blazar-type active galactic nuclei is of paramount importance for understanding the particle acceleration mechanisms assumed to take place in their jets. The Major Atmospheric Gamma Imaging Cerenkov (MAGIC) telescope participated in three multiwavelength (MWL) campaigns, observing the blazar Markarian (Mkn) 421 during the nights of April 28 and 29, 2006, and June 14, 2006.Aims. We analyzed the corresponding MAGIC very-high energy observations during 9 nights from April 22 to 30, 2006 and on June 14, 2006. We inferred light curves with sub-day resolution and night-by-night energy spectra.Methods. MAGIC detects γ-rays by observing extended air showers in the atmosphere. The obtained air-shower images were analyzed using the standard MAGIC analysis chain.Results. A strong γ-ray signal was detected from Mkn 421 on all observation nights. The flux (E>250 GeV) varied on night-by-night basis between (0.92±0.11) × 10-10cm-2s-1(0.57 Crab units) and (3.21±0.15) × 10-10cm-2s-1(2.0 Crab units) in April 2006. There is a clear indication for intra-night variability with a doubling time of 36±$10_{\rm stat}$min on the night of April 29, 2006, establishing once more rapid flux variability for this object. For all individual nights γ-ray spectra could be inferred, with power-law indices ranging from 1.66 to 2.47. We did not find statistically significant correlations between the spectral index and the flux state for individual nights. During the June 2006 campaign, a flux substantially lower than the one measured by the Whipple 10-m telescope four days later was found. Using a log-parabolic power law fit we deduced for some data sets the location of the spectral peak in the very-high energy regime. Our results confirm the indications of rising peak energy with increasing flux, as expected in leptonic acceleration models.

Context. Wide-range spectral coverage of blazar-type active galactic nuclei is of paramount importance for understanding the particle acceleration mechanisms assumed to take place in their jets. The Major Atmospheric Gamma Imaging Cerenkov (MAGIC) telescope participated in three multiwavelength (MWL) campaigns, observing the blazar Markarian (Mkn) 421 during the nights of April 28 and 29, 2006, and June 14, 2006.

Context. Gamma-ray bursts are cosmological sources emitting radiation from the gamma-rays to the radio band. Substantial observational efforts have been devoted to the study of gamma-ray bursts during the prompt phase, i.e. the initial burst of high-energy radiation, and during the long-lasting afterglows. In spite of many successes in interpreting these phenomena, there are still several open key questions about the fundamental emission processes, their energetics and the environment.Aims. Independently of specific gamma-ray burst theoretical recipes, spectra in the GeV/TeV range are predicted to be remarkably simple, being satisfactorily modeled with power-laws, and therefore offer a very valuable tool to probe the extragalactic background light distribution. Furthermore, the simple detection of a component at very-high energies, i.e. at ~100 GeV, would solve the ambiguity about the importance of various possible emission processes, which provide barely distinguishable scenarios at lower energies.Methods. We used the results of the MAGIC telescope observation of the moderate resdhift (z~ 0.76) GRB 080430 at energies above about 80 GeV, to evaluate the perspective for late-afterglow observations with ground based GeV/TeV telescopes.Results. We obtained an upper limit of F95% CL= 5.5 × 10-11erg cm-2s-1for the very-high energy emission of GRB 080430, which cannot set further constraints on the theoretical scenarios proposed for this object also due to the difficulties in modeling the low-energy afterglow. Nonetheless, our observations show that Cherenkov telescopes have already reached the required sensitivity to detect the GeV/TeV emission of GRBs at moderate redshift (z≲ 0.8), provided the observations are carried out at early times, close to the onset of their afterglow phase.

We use a long (300 ks), continuous SuzakuX-ray observation of the active nucleus in NGC 1365 to investigate the structure of the circumnuclear broad line region (BLR) clouds through their occultation of the X-ray source. The variations of the absorbing column density and of the covering factor indicate that the clouds surrounding the black hole are far from having a spherical geometry (as sometimes assumed), instead they have a strongly elongated and cometary shape, with a dense head (n~ 1011cm-3) and an expanding, dissolving tail. We infer that the cometary tails must be longer than a few times 1013cm and their opening angle must be smaller than a few degrees. We suggest that the cometary shape may be a common feature of BLR clouds in general, but which has been difficult to recognize observationally so far. The cometary shape may originate from shocks and hydrodynamical instabilities generated by the supersonic motion of the BLR clouds into the intracloud medium. As a consequence of the mass loss into their tail, we infer that the BLR clouds probably have a lifetime of only a few months, implying that they must be continuously replenished. We also find a large, puzzling discrepancy (two orders of magnitude) between the mass of the BLR inferred from the properties of the absorbing clouds and the mass of the BLR inferred from photoionization models; we discuss the possible solutions to this discrepancy.

Context. Gamma-ray bursts are cosmological sources emitting radiation from the gamma-rays to the radio band. Substantial observational efforts have been devoted to the study of gamma-ray bursts during the prompt phase, i.e. the initial burst of high-energy radiation, and during the long-lasting afterglows. In spite of many successes in interpreting these phenomena, there are still several open key questions about the fundamental emission processes, their energetics and the environment.

We use a long (300 ks), continuous SuzakuX-ray observation of the active nucleus in NGC 1365 to investigate the structure of the circumnuclear broad line region (BLR) clouds through their occultation of the X-ray source. The variations of the absorbing column density and of the covering factor indicate that the clouds surrounding the black hole are far from having a spherical geometry (as sometimes assumed), instead they have a strongly elongated and cometary shape, with a dense head (n~ 1011cm-3) and an expanding, dissolving tail. We infer that the cometary tails must be longer than a few times 1013cm and their opening angle must be smaller than a few degrees. We suggest that the cometary shape may be a common feature of BLR clouds in general, but which has been difficult to recognize observationally so far. The cometary shape may originate from shocks and hydrodynamical instabilities generated by the supersonic motion of the BLR clouds into the intracloud medium. As a consequence of the mass loss into their tail, we infer that the BLR clouds probably have a lifetime of only a few months, implying that they must be continuously replenished. We also find a large, puzzling discrepancy (two orders of magnitude) between the mass of the BLR inferred from the properties of the absorbing clouds and the mass of the BLR inferred from photoionization models; we discuss the possible solutions to this discrepancy.

The blazar PG 1553+113 is a well known TeV γ-ray emitter. In this paper we determine its spectral energy distribution through simultaneous multi-frequency data to study its emission processes. An extensive campaign was carried out between March and April 2008, where optical, X-ray, high-energy (HE) γ-ray, and very-high-energy (VHE) γ-ray data were obtained with the KVA, Abastumani, REM, RossiXTE/ASM, AGILE and MAGIC telescopes, respectively. We combine the data to derive the source's spectral energy distribution and interpret its double-peaked shape within the framework of a synchrotron self-Compton model.

The blazar PG 1553+113 is a well known TeV γ-ray emitter. In this paper we determine its spectral energy distribution through simultaneous multi-frequency data to study its emission processes. An extensive campaign was carried out between March and April 2008, where optical, X-ray, high-energy (HE) γ-ray, and very-high-energy (VHE) γ-ray data were obtained with the KVA, Abastumani, REM, RossiXTE/ASM, AGILE and MAGIC telescopes, respectively. We combine the data to derive the source's spectral energy distribution and interpret its double-peaked shape within the framework of a synchrotron self-Compton model.

Context. The Milagro hot spot A, close to the Galactic anticenter direction, has been tentatively attributed to cosmic rays from a local reservoir (at a distance ≈100 pc), freely streaming along diverging and smooth magnetic field lines. This is at variance with the geometry of the ≈kpc scale Galactic magnetic field, which is known to be aligned with the spiral arms.Aims. We investigate the information available on the geometry of the magnetic field on the scales (≈100 pc) of relevance here.Methods. The magnetic field immediately upstream of the heliosphere has been investigated by previous authors by modeling the interaction of this field with the solar wind. At larger distances, we use the dispersion measure and the rotation measure of nearby pulsars (especially towards the third Galactic quadrant). Additional information about the local field towards the North Polar Spur is taken from previous studies of the diffuse radio emission and the polarization of starlight.Results. The asymmetry of the heliosphere with respect to the incoming interstellar medium implies a magnetic field almost orthogonal to the local spiral arm, in the general direction of hot spot A, but more to the south. This is in good agreement with the nearby pulsar data on the one side, and the North Polar Spur data on the other.Conclusions. The local magnetic field on scales of ≈100 parsecs around the Sun seems to be oriented so as to provide a direct connection between the Solar System and a possible site of the Geminga supernova; the residual angular difference and the shape and orientation of the Milagro hot spot can be attributed to the field trailing in the wake of the heliosphere.

Context. The Milagro hot spot A, close to the Galactic anticenter direction, has been tentatively attributed to cosmic rays from a local reservoir (at a distance ≈100 pc), freely streaming along diverging and smooth magnetic field lines. This is at variance with the geometry of the ≈kpc scale Galactic magnetic field, which is known to be aligned with the spiral arms.