Accurate classification of 75 counterparts of objects detected in the 54 month Palermo Swift/BAT hard X-ray catalogue

Through an optical campaign performed at 4 telescopes located in the northern and the southern hemispheres, we have obtained optical spectroscopy for 75 counterparts of unclassified or poorly studied hard X-ray emitting objects detected with Swift/BAT and listed in the 54 month Palermo BAT catalogue. All these objects have also observations taken with Swift/XRT, ROSAT or Chandra satellites which allowed us to reduce the high energy error box and pinpoint the most likely optical counterpart/s. We find that 69 sources in our sample are Active Galactic Nuclei (AGNs); of them, 35 are classified as type 1 (with broad and narrow emission lines), 33 are classified as type 2 (with only narrow emission lines) and one is an high redshift QSO; the remaining 6 objects are galactic cataclysmic variables (CVs). Among type 1 AGNs, 32 are objects of intermediate Seyfert type (1.2-1.9) and one is Narrow Line Seyfert 1 galaxy; for 29 out of 35 type 1 AGNs, we have been able to estimate the central black hole mass and the Eddington ratio. Among type 2 AGNs, two display optical features typical of the LINER class, 3 are classified as transition objects, 1 is a starburst galaxy and 2 are instead X-ray bright, optically normal galaxies. All galaxies classified in this work are relatively nearby objects (0.006 - 0.213) except for one at redshift 1.137.Comment: 19 pages, 5 figures, 6 tables, accepted for publications on Astronomy and Astrophysics, main journal. arXiv admin note: text overlap with arXiv:1206.509

Candidate Tidal Dwarf Galaxies in the Compact Group CG J1720-67.8

This is the second part of a detailed study of the ultracompact group CG J1720-67.8: in the first part we have focused the attention on the three main galaxies of the group and we have identified a number of candidate tidal dwarf galaxies (TDGs). Here we concentrate on these candidate TDGs. Absolute photometry of these objects in BVRJHKs bands confirms their relatively blue colors, as we already expected from the inspection of optical and near-infrared color maps and from the presence of emission-lines in the optical spectra. The physical conditions in such candidate TDGs are investigated through the application of photoionization models, while the optical colors are compared with grids of spectrophotometric evolutionary synthesis models from the literature. Although from our data self-gravitation cannot be proved for these objects, their general properties are consistent with those of other TDG candidates. Additionally we present the photometry of a few ``knots'' detected in the immediate surroundings of CG J1720-67.8 and consider the possibility that these objects might belong to a dwarf population associated with the compact group.Comment: Accepted for publication in the Astrophysical Journa

BL Lacertae identifications in a ROSAT-selected sample of Fermi unidentified objects

The optical spectroscopic followup of 27 sources belonging to a sample of 30 high-energy objects selected by positionally cross correlating the first Fermi/LAT Catalog and the ROSAT All-Sky Survey Bright Source Catalog is presented here. It has been found or confirmed that 25 of them are BL Lacertae objects (BL Lacs), while the remaining two are Galactic cataclysmic variables (CVs). This strongly suggests that the sources in the first group are responsible for the GeV emission detected with Fermi, while the two CVs most likely represent spurious associations. We thus find an 80% a posteriori probability that the sources selected by matching GeV and X-ray catalogs belong to the BL Lac class. We also show suggestions that the BL Lacs selected with this approach are probably high-synchrotron-peaked sources and in turn good candidates for the detection of ultra-high-energy (TeV) photons from them.Comment: 16 pages, 9 figures, 4 tables, one appendix, accepted for publication on A&A, main journal. Tables 1-3 and Figures 2-6 will only be published in the electronic edition of the journa

The AGN nature of 11 out of 12 Swift/RXTE unidentified sources through optical and X-ray spectroscopy

The Swift Burst Alert Telescope (BAT) is performing a high Galactic latitude survey in the 14-195 keV band at a flux limit of ~10^{-11} erg cm^{-2} s^{-1}, leading to the discovery of new high energy sources, most of which have not so far been properly classified. A similar work has also been performed with the RXTE slew survey leading to the discovery of 68 sources detected above 8 keV, many of which are still unclassified. Follow-up observations with the Swift X-ray Telescope (XRT) provide, for many of these objects, source localization with a positional accuracy of few arcsec, thus allowing the search for optical counterparts to be more efficient and reliable. We present the results of optical/X-ray follow-up studies of 11 Swift BAT detections and one AGN detected in the RXTE Slew Survey, aimed at identifying their counterparts and at assessing their nature. These data allowed, for the first time, the optical classification of 8 objects and a distance determination for 3 of them. For another object, a more refined optical classification than that available in the literature is also provided. For the remaining sources, optical spectroscopy provides a characterization of the source near in time to the X-ray measurement. The sample consists of 6 Seyfert 2 galaxies, 5 Seyferts of intermediate type 1.2-1.8, and one object of Galactic nature - an Intermediate Polar (i.e., magnetic) Cataclysmic Variable. Out of the 11 AGNs, 8 (~70%) including 2 Seyferts of type 1.2 and 1.5, are absorbed with NH > 10^{22} cm^{-2}. Up to 3 objects could be Compton thick (i.e. NH > 1.5 x 10^{24} cm^{-2}), but only in one case (Swift J0609.1-8636) does all the observational evidence strongly suggests this possibility.Comment: 50 pages, including 16 figures and 7 tables. Accepted for publication in Ap

MUSE observations of the giant low surface brightness galaxy Malin 1: Numerous HII regions, star formation rate, metallicity, and dust attenuation

Giant low-surface brightness (GLSB) galaxies are an extreme class of objects with very faint and extended gas-rich disks. Malin 1 is the largest GLSB galaxy known to date, but its formation is still poorly understood. We use VLT/MUSE IFU spectroscopic observations of Malin 1 to reveal, for the first time, the presence of H$\alpha$ emission distributed across numerous regions along its disk, up to radial distances of $\sim$100 kpc. We made an estimate of the dust attenuation using the Balmer decrement and found that Malin 1 has a mean H$\alpha$ attenuation of 0.36 mag. We observe a steep decline in the star formation rate surface density ($\Sigma_{\rm SFR}$) within the inner 20 kpc, followed by a shallow decline in the extended disk. Similarly, the gas phase metallicity we estimated shows a steep gradient in the inner 20 kpc, followed by a flattening of the metallicity in the extended disk with a relatively high value of $\sim$0.6 $Z_{\odot}$. We found that the normalized abundance gradient of the inner disk is similar to values found in normal galaxies but with an extreme value in the extended disk. A comparison of the star formation rate surface density and gas surface density shows that, unlike normal disk galaxies or other LSBs, Malin 1 exhibits a very low star formation efficiency. Owing to the detection of emission lines over a large part of the disk of Malin 1, this work sheds light on the star formation processes in this unique galaxy, highlighting its extended star-forming disk, dust attenuation, almost flat metallicity distribution in the outer disk, and exceptionally low star-formation efficiency. Our findings contribute to a more detailed understanding of the formation of the giant disk of Malin 1 and also constrain possible proposed scenarios on the nature of GLSB galaxies in general.Comment: 12 pages, 9 figures, accepted for publication in A&

Unveiling the nature of INTEGRAL objects through optical spectroscopy. VII. Identification of 20 Galactic and extragalactic hard X-ray sources

Within the framework of our program of assessment of the nature of unidentified or poorly known INTEGRAL sources, we present here spectroscopy of optical objects, selected through positional cross-correlation with soft X-ray detections (afforded with satellites such as Swift, ROSAT, Chandra and/or XMM-Newton) as putative counterparts of hard X-ray sources detected with the IBIS instrument onboard INTEGRAL. Using 6 telescopes of various sizes and archival data from two on-line spectroscopic surveys we are able to identify, either for the first time or independent of other groups, the nature of 20 INTEGRAL hard X-ray sources. Our results indicate that: 11 of these objects are active galactic nuclei (AGNs) at redshifts between 0.014 and 0.978, 7 of which display broad emission lines, 2 show narrow emission lines only, and 2 have unremarkable or no emission lines (thus are likely Compton thick AGNs); 5 are cataclysmic variables (CVs), 4 of which are (possibly magnetic) dwarf novae and one is a symbiotic star; and 4 are Galactic X-ray binaries (3 with high-mass companions and one with a low-mass secondary). It is thus again found that the majority of these sources are AGNs or magnetic CVs, confirming our previous findings. When possible, the main physical parameters for these hard X-ray sources are also computed using the multiwavelength information available in the literature. These identifications support the importance of INTEGRAL in the study of the hard X-ray spectrum of all classes of X-ray emitting objects, and the effectiveness of a strategy of multi-catalogue cross-correlation plus optical spectroscopy to securely pinpoint the actual nature of unidentified hard X-ray sources.Comment: 16 pages, 8 figures, 5 tables. Accepted for publication on Astronomy & Astrophysics, main journal. Slight changes made to match the proof-corrected version; references adde

Normal Globular Cluster Systems in Massive Low Surface Brightness Galaxies

We present the results of a study of the globular cluster systems of 6 massive spiral galaxies, originally cataloged as low surface brightness galaxies but here shown to span a wide range of central surface brightness values, including two intermediate to low surface brightness galaxies. We used the Advanced Camera for Surveys on board HST to obtain photometry in the F475W and F775W bands and select sources with photometric and morphological properties consistent with those of globular clusters. A total of 206 candidates were identified in our target galaxies. From a direct comparison with the Galactic globular cluster system we derive specific frequency values for each galaxy that are in the expected range for late-type galaxies. We show that the globular cluster candidates in all galaxies have properties consistent with globular cluster systems of previously studied galaxies in terms of luminosity, sizes and color. We establish the presence of globular clusters in the two intermediate to low surface brightness galaxies in our sample and show that their properties do not have any significant deviation from the behavior observed in the other sample galaxies. Our results are broadly consistent with a scenario in which low surface brightness galaxies follow roughly the same evolutionary history as normal (i.e. high surface) brightness galaxies except at a much lower rate, but require the presence of an initial period of star formation intense enough to allow the formation of massive star clusters.Comment: 14 pages, 6 figures. AJ accepte