ALBERT

Description: We report results on multiband observations from radio to -rays of the two radio-loud narrow-line Seyfert 1 (NLSy1) galaxies PKS 2004–447 and J1548+3511. Both sources show a core–jet structure on parsec scale, while they are unresolved at the arcsecond scale. The high core dominance and the high variability brightness temperature make these NLSy1 galaxies good -ray source candidates. Fermi -Large Area Telescope detected -ray emission only from PKS 2004–447, with a -ray luminosity comparable to that observed in blazars. No -ray emission is observed for J1548+3511. Both sources are variable in X-rays. J1548+3511 shows a hardening of the spectrum during high activity states, while PKS 2004–447 has no spectral variability. A spectral steepening likely related to the soft excess is hinted below 2 keV for J1548+3511, while the X-ray spectra of PKS 2004–447 collected by XMM–Newton in 2012 are described by a single power law without significant soft excess. No additional absorption above the Galactic column density or the presence of an Fe line is detected in the X-ray spectra of both sources.

Description: We report results on multiband observations from radio to -rays of the two radio-loud narrow-line Seyfert 1 (NLSy1) galaxies PKS 2004–447 and J1548+3511. Both sources show a core–jet structure on parsec scale, while they are unresolved at the arcsecond scale. The high core dominance and the high variability brightness temperature make these NLSy1 galaxies good -ray source candidates. Fermi -Large Area Telescope detected -ray emission only from PKS 2004–447, with a -ray luminosity comparable to that observed in blazars. No -ray emission is observed for J1548+3511. Both sources are variable in X-rays. J1548+3511 shows a hardening of the spectrum during high activity states, while PKS 2004–447 has no spectral variability. A spectral steepening likely related to the soft excess is hinted below 2 keV for J1548+3511, while the X-ray spectra of PKS 2004–447 collected by XMM–Newton in 2012 are described by a single power law without significant soft excess. No additional absorption above the Galactic column density or the presence of an Fe line is detected in the X-ray spectra of both sources.

Description: In order to investigate the role of absorption in active galactic nuclei (AGN) with jets, we have studied the column density distribution of a hard X-ray selected sample of radio galaxies, derived from the INTEGRAL /Imager on Board the Integral Satellite (IBIS) and Swift /The Burst Alert Telescope (BAT) AGN catalogues (~7–10 per cent of the total AGN population). The 64 radio galaxies have a typical FR II radio morphology and are characterized by high 20–100 keV luminosities (from 10 42 to 10 46 erg s –1 ) and high Eddington ratios (log L Bol / L Edd typically larger than ~0.01). The observed fraction of absorbed AGN ( N H 〉 10 22  cm –2 ) is around 40 per cent among the total sample, and ~75 per cent among type 2 AGN. The majority of obscured AGN are narrow-line objects, while unobscured AGN are broad-line objects, obeying to the zeroth-order predictions of unified models. A significant anti-correlation between the radio core dominance parameter and the X-ray column density is found. The observed fraction of Compton thick AGN is ~2–3 per cent, in comparison with the 5–7 per cent found in radio-quiet hard X-ray selected AGN. We have estimated the absorption and Compton thick fractions in a hard X-ray sample containing both radio galaxies and non-radio galaxies and therefore affected by the same selection biases. No statistical significant difference was found in the absorption properties of radio galaxies and non-radio galaxies sample. In particular, the Compton thick objects are likely missing in both samples and the fraction of obscured radio galaxies appears to decrease with luminosity as observed in hard X-ray non-radio galaxies.

Description: Using the recent INTEGRAL /IBIS and Swift /BAT surveys we have extracted a sample of 64 confirmed plus three candidate radio galaxies selected in the soft gamma-ray band. The sample covers all optical classes and is dominated by objects showing a Fanaroff–Riley type II radio morphology; a large fraction (70 per cent) of the sample is made of ‘radiative mode’ or high-excitation radio galaxies. We measured the source size on images from the NRAO VLA Sky Survey, the Faint Images of the Radio Sky at twenty-cm and the Sydney University Molonglo Sky Survey images and have compared our findings with data in the literature obtaining a good match. We surprisingly found that the soft gamma-ray selection favours the detection of large size radio galaxies: 60 per cent of objects in the sample have size greater than 0.4 Mpc while around 22 per cent reach dimension above 0.7 Mpc at which point they are classified as giant radio galaxies (GRGs), the largest and most energetic single entities in the Universe. Their fraction among soft gamma-ray selected radio galaxies is significantly larger than typically found in radio surveys, where only a few per cent of objects (1–6 per cent) are GRGs. This may partly be due to observational biases affecting radio surveys more than soft gamma-ray surveys, thus disfavouring the detection of GRGs at lower frequencies. The main reasons and/or conditions leading to the formation of these large radio structures are still unclear with many parameters such as high jet power, long activity time and surrounding environment all playing a role; the first two may be linked to the type of active galactic nucleus discussed in this work and partly explain the high fraction of GRGs found in the present sample. Our result suggests that high energy surveys may be a more efficient way than radio surveys to find these peculiar objects.

Description: The presence of megaparsec-scale radio haloes in galaxy clusters has already been established by many observations over the last two decades. The emerging explanation for the formation of these giant sources of diffuse synchrotron radio emission is that they trace turbulent regions in the intracluster medium, where particles are trapped and accelerated during cluster mergers. Our current observational knowledge is, however, mainly limited to massive systems. Here we present observations of a sample of 14 mass-selected galaxy clusters, i.e. M 500  〉 4 x 10 14 M , in the Southern hemisphere, aimed to study the occurrence of radio haloes in low-mass clusters and test the correlation between the radio halo power at 1.4 GHz P 1.4 and the cluster mass M 500 . Our observations were performed with the 7-element Karoo Array Telescope at 1.86 GHz. We found three candidates to host diffuse cluster-scale emission and derived upper limits at the level of 0.6–1.9 x 10 24 Watt Hz –1 for ~50 per cent of the clusters in the sample, significantly increasing the number of clusters with radio halo information in the considered mass range. Our results confirm that bright radio haloes in less massive galaxy clusters are statistically rare.

Description: In this paper, we report the discovery and detailed radio/X-ray analysis of a peculiar giant radio galaxy (GRG) detected by INTEGRAL , IGR J14488–4008. The source has been recently classified as a Seyfert 1.2 galaxy at redshift 0.123; the radio data denote the source to be a type II Fanaroff–Riley radio galaxy, with a linear projected size exceeding 1.5 Mpc, clearly assigning IGR J14488–4008 to the class of GRG. In the X-rays, the source shows a remarkable spectrum, characterized by absorption by ionized elements, a characteristic so far found in only other four broad-line radio galaxies.

Description: Multifrequency Very Long Baseline Array (VLBA) observations were performed to study the radio morphology and synchrotron spectra of four high-frequency peaking radio sources. They are resolved into several compact components and the radio emission is dominated by hotspots/lobes. The core region is detected unambiguously in J1335+5844 and J1735+5049. The spectra of the main source components peak above 3 GHz. Assuming that the spectral peak is produced by synchrotron self-absorption, we estimate the magnetic field directly from observable quantities: in half of the components it agrees with the equipartition field, while in the others the difference exceeds an order of magnitude. By comparing the physical properties of the targets with those of larger objects, we found that the luminosity increases with linear size for sources smaller than a few kpc, while it decreases for larger objects. The asymmetric sources J1335+5844 and J1735+5049 suggest that the ambient medium is inhomogeneous and is able to influence the evolution of the radio emission even during its first stages. The core luminosity increases with linear size for sources up to a few kpc, while it seems constant for larger sources, suggesting an evolution independent of source total luminosity.

Description: We present SDSS J143244.91+301435.3, a new case of a radio-loud narrow-line Seyfert 1 (RL NLS1) with a relatively high radio power ( P 1.4 GHz = 2.1 10 25  W Hz –1 ) and large radio-loudness parameter ( R 1.4 = 600 ± 100). The radio source is compact with a linear size below ~1.4 kpc but, in contrast to most of the RL NLS1 discovered so far with such a high R 1.4 , its radio spectrum is very steep (α = 0.93, S    –α ) and does not support a ‘blazar-like’ nature. Both the small mass of the central supermassive black hole and the high accretion rate relative to the Eddington limit estimated for this object (3.2 10 7  M and 0.27, respectively, with a formal error of ~0.4 dex for both quantities) are typical of the NLS1 class. Through modelling the spectral energy distribution of the source, we have found that the galaxy hosting SDSS J143244.91+301435.3 is undergoing quite intense star formation (SFR = 50 M  yr –1 ), which, however, is expected to contribute only marginally (~1 per cent) to the observed radio emission. The radio properties of SDSS J143244.91+301435.3 are remarkably similar to those of compact steep-spectrum (CSS) radio sources, a class of active galactic nuclei (AGN) mostly composed of young radio galaxies. This may suggest a direct link between these two classes of AGN, with CSS sources possibly representing the misaligned version (the so-called ‘parent population’) of RL NLS1 showing blazar characteristics.

Description: We report results of a multiband monitoring campaign of the flat spectrum radio quasar TXS 0536+145 at redshift 2.69. This source was detected during a very high -ray activity state in 2012 March by the Large Area Telescope on board Fermi , becoming the -ray flaring blazar at the highest redshift detected so far. At the peak of the flare the source reached an apparent isotropic -ray luminosity of 6.6 x 10 49 erg s –1 which is comparable to the values achieved by the most luminous blazars. This activity triggered radio-to-X-rays monitoring observations by Swift , Very Long Baseline Array, European VLBI Network, and Medicina single-dish telescope. Significant variability was observed from radio to X-rays supporting the identification of the -ray source with TXS 0536+145. Both the radio and -ray light curves show a similar behaviour, with the -rays leading the radio variability with a time lag of about 4-6 months. The luminosity increase is associated with a flattening of the radio spectrum. No new superluminal component associated with the flare was detected in high-resolution parsec-scale radio images. During the flare the -ray spectrum seems to deviate from a power law, showing a curvature that was not present during the average activity state. The -ray properties of TXS 0536+145 are consistent with those shown by the high-redshift -ray blazar population.

Description: We analysed a deep Chandra observation (334 ks) of the galaxy cluster Abell 115 and detected a shock cospatial with the radio relic. The X-ray surface brightness profile across the shock region presents a discontinuity, corresponding to a density compression factor $\mathcal {C}=2.0\pm 0.1$ , leading to a Mach number $\mathcal {M}=1.7\pm 0.1$ ( $\mathcal {M}=1.4{\rm -}2$ including systematics). Temperatures measured in the upstream and downstream regions are consistent with what expected for such a shock: $T_{\rm u}=4.3^{+1.0}_{-0.6}\:\rm {keV}$ and $T_{\rm d}=7.9^{+1.4}_{-1.1}\:\rm {keV}$ , respectively, implying a Mach number $\mathcal {M}=1.8^{+0.5}_{-0.4}$ . So far, only few other shocks discovered in galaxy clusters are consistently detected from both density and temperature jumps. The spatial coincidence between this discontinuity and the radio relic edge strongly supports the view that shocks play a crucial role in powering these synchrotron sources. We suggest that the relic is originated by shock re-acceleration of relativistic electrons rather than acceleration from the thermal pool. The position and curvature of the shock and the associated relic are consistent with an off-axis merger with unequal mass ratio where the shock is expected to bend around the core of the less massive cluster.