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Using recent polarimetric observations of the Crab Nebula in the hard X-ray band by INTEGRAL, we show that the absence of vacuum birefringence effects constrains O(E/M) Lorentz violation in QED to the level |\xi| < 9x10^{-10} at three sigma CL, tightening by more than three orders of magnitude previous constraints. We show that planned X-ray polarimeters have the potential the potential to probe |\xi|~ 10^{-16} by detecting polarization in active galaxies at red-shift ~1., Comment: 4 pages, 3 figures

In this paper we report on the spectral evolution of 4U 1608-522 performed as part of the long Galactic Bulge monitoring with INTEGRAL. The data set include the April 2005 outburst. BeppoSAX archival data (two observations, in 1998 and 2000) have been also analysed and compared with the INTEGRAL ones. Three different spectral states have been identified from the hard Color-Intensity diagram derived from INTEGRAL: the canonical Hard and Soft as well as an Intermediate state. The hard state spectrum is well described by a weak black body component plus a Comptonised plasma component with high electron temperature (kTe~60 keV) extending up to 200 keV without any additional cut-off. The soft spectra are characterized by a cold Comptonised plasma (kTe= 2-3 keV, and 7 keV for the intermediate state) and a strong disk black body component. A reflection component, indicating reflection of the X-ray radiation from the accretion disc, is also present in the soft state revealed by BeppoSAX in 1998. The 2000 BeppoSAX observation revealed the source in quiescent state modelled by a neutron star atmosphere (assuming a neutron star with radius 10 km and mass 1.4 Msun) with an effective temperature, kTeff of 0.1 keV plus a power law component with Gamma~3 detected for the first time for this source. This spectrum can also be modelled with a simple black body compatible with emission originating from a small fraction of the NS surface of radius of 0.4 km., Comment: 8 pages, 6 figures, to be published in ApJ

We report on INTEGRAL observations of the bright black-hole transient GX 339-4 performed during the period August-September 2004. Our data cover three different spectral states, namely Hard/Intermediate State, Soft/Intermediate State and High/Soft State. We investigate the spectral variability of the source across the different spectral states. The hard X-ray spectrum becomes softer during the HIMS-to-SIMS transition, but it hardens when reaching the HSS state. A principal component analysis demonstrates that most of the variability occurs through two independent modes: a pivoting of the spectrum around 6 keV (responsible for 75% of the variance) and an intensity variation of the hard component (responsible for 21%). The pivoting is interpreted as due to changes in the soft cooling photon flux entering the corona, the second mode as fluctuations of the heating rate in the corona. Our spectral analysis of the spectra of GX 339-4 shows a high energy excess with respect to pure thermal Comptonisation models in the HIMS: a non-thermal power-law component seems to be requested by data. In all spectral states joint IBIS, SPI and JEM-X data are well represented by hybrid thermal/non-thermal Comptonisation (EQPAIR). The spectral evolution seems to be predominantly driven by a reduction of the ratio of the electron heating rate to the soft cooling photon flux in the corona, l_h/l_s. The inferred accretion disc soft thermal emission increases by about two orders of magnitude, while the Comptonised luminosity decreases by at most a factor of 3. This confirms that the softening we observed is due to a major increase in the flux of soft cooling photons in the corona associated with a modest reduction of the electron heating rate., Comment: 11 pages, 6 figures, 4 tables; accepted for publication in MNRAS Main Journal

We present the broadband analysis of the powerful quasar 4C04.42 (z=0.965) observed by XMM and INTEGRAL. The 0.2--200 keV spectrum is well reproduced with a hard power-law component ($\Gamma\sim$1.2), augmented by a soft component below 2 keV (observer frame), which is described by a thermal blackbody with temperature kT$\backsimeq$ 0.15 keV. Alternatively, a broken power-law with E$_{break}$=2 keV and $\Delta\Gamma$=0.4 can equally well describe the data. Using archival data we compile the not-simultaneous Spectral Energy Distribution of the source from radio to gamma-ray frequencies. The SED shows two main components: the low frequency one produced by Synchrotron radiation from the electrons moving in the jet and the high energy one produced through external Compton scattering of the electrons with the photon field of the Broad Line Region. Within this scenario the excess emission in the soft-X ray band can be interpreted as due to Bulk Compton radiation of cold electrons. However, some other processes, briefly discussed in the text, can also reproduce the observed bump., Comment: Accepted for pubblication in MNRAS Letters

Context: IGR J16479-4514 is a fast X-ray transient known to display flares lasting typically a few hours. Recently, its counterpart has been identified with a supergiant star, therefore the source can be classified as member of the newly discovered class of Supergiant Fast X-ray Transients (SFXTs), specifically it is the one with the highest duty cycle. Aims: to characterize the quiescent X-ray behaviour of the source and to compare its broad band spectrum to that during fast X-ray flares. Methods: we performed an analysis of IBIS and JEM-X data with OSA 5.1 as well as an analysis of archival Swift/XRT data. Results: we present results from a long term monitoring of IGR J16479-4514 with detailed spectral and timing informations on 19 bright fast X-ray flares, 10 of which newly discovered. We also report for the first time results on the quiescent X-ray emission; the typical luminosity value (about 10^34 erg s^-1) is about 2 orders of magnitude greater than that typical of SFXTs while its broad band X-ray spectrum has a shape very similar to that during fast X-ray transient activity, i.e. a rather steep power law with Gamma=2.6. Conclusions: IGR J16479$-$4514 is characterized by a quiescent X-ray luminosity higher than that typical of other known SFXTs but lower than persistent emission from classical SGXBs. We suggest that such source is a kind of transition object between these two systems, supporting the idea that there is a continuum of behaviours between the class of SFXTs and that of classical persistent SGXBs., Comment: 5 pages, 4 figures, accepted for publication in A&A

During the operational life of BeppoSax, the Wide Field Camera observations covered almost the full sky at different epochs. The energy coverage, wide field of view and imaging capabilities share many commonalities with IBIS, the gamma-ray telescope onboard INTEGRAL satellite. We mosaicked all the available single pointing WFC observation images and then we searched the map for persistent and transient sources as has been done for the IBIS survey. This work represents the first unbiased source list compilation produced in this way from the overall WFC data set., Comment: Contributed talk at the conference "A population explosion: the nature and evolution of X-ray binaries in diverse environments", held in St.Petersburg Beach, Florida (28 October-2 November 2007); R.M.Bandyopadhyay, S.Wachter, D.Gelino, C.R.Gelino, eds

In 2005 March 22nd, the INTEGRAL satellite caught a type-I X-ray burst from the unidentified source XMMU J174716.1-281048, serendipitously discovered with XMM-Newton in 2003. Based on the type-I X-ray burst properties, we derived the distance of the object and suggested that the system is undergoing a prolonged accretion episode of many years. We present new data from a Swift/XRT campaign which strengthen this suggestion. AX J1754.2-2754 was an unclassified source reported in the ASCA catalogue of the Galactic Centre survey. INTEGRAL observed a type-I burst from it in 2005, April 16th. Recently, a Swift ToO allowed us to refine the source position and establish its persistent nature., Comment: To be published in AIP Proceedings - Conference 'A Population Explosion: The Nature and Evolution of X-ray Binaries in Diverse Environments', 28 Oct - 2 Nov, St. Petersburg Beach, FL

The 0.5-150 keV broad-band spectra of a sample of nine bright type 1 Seyfert galaxies are analyzed here. These sources have been discovered/detected by INTEGRAL and subsequently observed with XMM--Newton for the first time with high sensitivity below 10 keV. The sample, although small, is representative of the population of type 1 AGN which are now being observed above 20 keV. The intrinsic continuum has been modeled using three different parameterizations: a power-law model, an exponential cut-off power-law and an exponential cut-off power-law with a Compton reflection component. In each model the presence of intrinsic absorption, a soft component and emission line reprocessing features has also been tested. A simple power-law model is a statistically good description of most of the spectra presented here; an FeK line, fully and/or partial covering absorption and a soft spectral component are detected in the majority of the sample sources. The average photon index (< Gamma > = 1.7 $\pm$ 0.2) is consistent, within errors, with the canonical spectral slope often observed in AGN although the photon index distribution peaks in our case at flat Gamma (~ 1.5) values. For four sources, we find a significantly improved fit when the power-law is exponentially cut-off at an energy which is constrained to be below ~ 150 keV. The Compton reflection parameter could be estimated in only two objects of the sample and in both cases is found to be R > 1., Comment: 10 pages, 4 figures, accepted for publication in A&A

Using 8 telescopes in the northern and southern hemispheres, plus archival data from two on-line sky surveys, we performed a systematic optical spectroscopic study of 39 putative counterparts of unidentified or poorly studied INTEGRAL sources in order to determine or at least better assess their nature. This was implemented within the framework of our campaign to reveal the nature of newly-discovered and/or unidentified sources detected by INTEGRAL. Our results show that 29 of these objects are active galactic nuclei (13 of which are of Seyfert 1 type, 15 are Seyfert 2 galaxies and one is possibly a BL Lac object) with redshifts between 0.011 and 0.316, 7 are X-ray binaries (5 with high-mass companions and 2 with low-mass secondaries), one is a magnetic cataclysmic variable, one is a symbiotic star and one is possibly an active star. Thus, the large majority (74%) of the identifications in this sample belongs to the AGN class. When possible, the main physical parameters for these hard X-ray sources were also computed using the multiwavelength information available in the literature. These identifications further underscore the importance of INTEGRAL in studying the hard X-ray spectra 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 still unidentified hard X-ray sources., Comment: 20 pages, 14 figures, 7 tables. Accepted for publication on Astronomy & Astrophysics, main journal. Figures 1-5 are in .jpg format to fit the arXiv uploads size limits

After almost 5 years of operation, ESA's International Gamma-Ray Astrophysics Laboratory (INTEGRAL) Space Observatory has unveiled a new soft Gamma ray sky and produced a remarkable harvest of results, ranging from identification of new high energy sources, to the discovery of dozens of variable sources to the mapping of the Aluminum emission from the Galaxy Plane to the presence of electrons and positrons generating the annihilation line in the Galaxy central radian. INTEGRAL is continuing the deep observations of the Galactic Plane and of the whole sky in the soft Gamma ray range. The new IBIS gamma ray catalogue contains more than 420 sources detected above 20 keV. We present a view of the INTEGRAL high energy sky with particular regard to sources emitting at high energy, including Active Galactic Nuclei (AGN), HESS/MAGIC counterparts and new view of the cosmic gamma ray diffuse background., Comment: Nucl. Instr. and Meth. A, in press. Proc. of Roma International Conference on Astroparticle Physics (RICAP'07)

Aims. We present a 0.2--200 keV broad-band study of absorbed AGN observed with INTEGRAL, XMM-Newton, Chandra and ASCA to investigate the continuum shape and the absorbing/reflecting medium properties. Methods. The sources are selected in the INTEGRAL AGN sample to have a 20--100 keV flux below 8$\times10^{-11}$ $\flux$ (5 mCrab), and are characterized by a 2--10 keV flux in the range (0.8--10)$\times10^{-11}$ $\flux$. The good statistics allow us a detailed study of the intrinsic and reflected continuum components. In particular, the analysis performed on the combined broad-band spectra allow us to investigate the presence of Compton reflection features and high energy cut-off in these objects. Results. The column density of the absorbing gas establishes the Compton thin nature for three sources in which a measure of the absorption was still missing. The Compton thin nature of all the sources in this small sample is also confirmed by the diagnostic ratios F$x/F[OIII]. The Compton reflection components we measure, reflection continuum and iron line, are not immediately compatible with a scenario in which the absorbing and reflecting media are one and the same, i.e. the obscuring torus. A possible solution is that the absorption is more effective than reflection, e.g. under the hypothesis that the absorbing/reflecting medium is not uniform, like a clumpy torus, or that the source is observed through a torus with a very shallow opening angle. The high energy cut-off (a lower limit in two cases) is found in all sources of our sample and the range of values is in good agreement with that found in type 1 Seyfert galaxies. At lower energies there is clear evidence of a soft component (reproduced with a thermal and/or scattering model), in six objects., Comment: Accepted for publication in Astronomy and Astrophysics

We report on the high-redshift blazar identification of a new gamma-ray source, Swift J1656.3-3302, detected with the BAT imager onboard the Swift satellite and the IBIS instrument on the INTEGRAL satellite. Follow-up optical spectroscopy has allowed us to identify the counterpart as an R-band 19 mag source that shows broad Lyman-alpha, Si IV, He II, C IV, and C III] emission lines at redshift z = 2.40+-0.01. Spectral evolution is observed in X-rays when the INTEGRAL/IBIS data are compared to the Swift/BAT results, with the spectrum steepening when the source gets fainter. The 0.7-200 keV X-ray continuum, observed with Swift/XRT and INTEGRAL/IBIS, shows the power law shape typical of radio loud (broad emission line) active galactic nuclei (with a photon index around 1.6) and a hint of spectral curvature below 2 keV, possibly due to intrinsic absorption (N_H about 7e22 cm-2) local to the source. Alternatively, a slope change (of about 1 in terms of photon index) around 2.7 keV can describe the X-ray spectrum equally well. At this redshift, the observed 20-100 keV luminosity of the source is about 1e48 erg s-1 (assuming isotropic emission), making Swift J1656.3-3302 one of the most X-ray luminous blazars. This source is yet another example of a distant gamma-ray loud quasar discovered above 20 keV. It is also the farthest object, among the previously unidentified INTEGRAL sources, whose nature has been determined a posteriori through optical spectroscopy., Comment: 9 pages, 5 figures, 2 tables. Accepted for publication on Astronomy & Astrophysics, main journal

The success of the SWIFT/BAT and INTEGRAL missions has definitely opened a new window for follow-up and deep study of the transient gamma-ray sky. This now appears as the access key to important progresses in the area of cosmological research and deep understanding of the physics of compact objects. To detect in near real-time explosive events like Gamma-Ray bursts, thermonuclear flashes from Neutron Stars and other types of X-ray outbursts we have developed a concept for a wide-field gamma-ray coded mask instrument working in the range 8-200 keV, having a sensitivity of 0.4 ph cm-2 s-1 in 1s (15-150 keV) and arcmin location accuracy over a sky region as wide as 3sr. This scientific requirement can be achieved by means of two large area, high spatial resolution CZT detection planes made of arrays of relatively large (~1cm2) crystals, which are in turn read out as matrices of smaller pixels. To achieve such a wide Field-Of-View the two units can be placed at the sides of a S/C platform serving a payload with a complex of powerful X-ray instruments, as designed for the EDGE mission. The two units will be equipped with powerful signal read out system and data handling electronics, providing accurate on-board reconstruction of the source positions for fast, autonomous target acquisition by the X-ray telescopes., Comment: 9 pages, 7 figures, SPIE Conference on UV, X-ray, and Gamma-Ray Instrumentation for Astronomy, San Diego 26-30 August 2007

We present spectral measurements made in the soft (20-100 keV) gamma-ray band of the region containing the composite supernova remnant G11.2-0.3 and its associated pulsar PSR J1811-1925. Analysis of INTEGRAL/IBIS data allows characterisation of the system above 10 keV. The IBIS spectrum is best fitted by a power law having photon index of 1.8^{+0.4}_{-0.3} and a 20-100 keV flux of 1.5E{-11} erg/cm^2/s. Analysis of archival Chandra data over different energy bands rules out the supernova shell as the site of the soft gamma-ray emission while broad band (1-200 keV) spectral analysis strongly indicates that the INTEGRAL/IBIS photons originate in the central zone of the system which contains both the pulsar and its nebula. The composite X-ray and soft gamma-ray spectrum indicates that the pulsar provides around half of the emission seen in the soft gamma-ray domain; its spectrum is hard with no sign of a cut off up to at least 80 keV. The other half of the emission above 10 keV comes from the PWN; with a power law slope of 1.7 its spectrum is softer than that of the pulsar. From the IBIS/ISGRI mosaics we are able to derive 2 sigma upper limits for the 20-100 keV flux from the location of the nearby TeV source HESS J1809-193 to be 4.8E{-12} erg/cm^2/s. We have also examined the likelihood of an association between PSR J1811-1925 and HESS J1809-193. Although PSR J1811-1925 is the most energetic pulsar in the region, the only one detected above 10 keV and thus a possible source of energy to fuel the TeV fluxes, there is no morphological evidence to support this pairing, making it an unlikely counterpart., Comment: 6 pages, 2 figures, accepted for publication in MNRAS

The X-ray source SAXJ1712.6-3739 is a very weak Low Mass X-ray Binary discovered in 1999 with BeppoSAX and located in the Galactic Center. This region has been deeply investigated by the INTEGRAL satellite with an unprecedented exposure time, giving us an unique opportunity to study the hard X-ray behavior also for weak objects. The spectral results are based on the systematic analysis of all INTEGRAL observations covering the source position performed between February 2003 and October 2006. SAXJ1712.6-3739 did not shows any flux variation along this period as well as compared to previous BeppoSAX observation. Hence, to better constrain the physical parameters we combined both instrument data. Long INTEGRAL monitoring reveals, for the first time, that this X-ray burster is a weak persistent source, displaying a X-ray spectrum extended to high energy and spending most of the time in a low luminosity hard state. The broad-band spectrum is well modeled with a simple Comptonized model with a seed photons temperature of ~0.5keV and an electron temperature of ~24keV. The low mass accretion rate (~2x10^{-10} Msun/yr), the long bursts recurrence time, the small sizes of the region emitting the seed photons consisting with the inner disk radius and the high luminosity ratio in the 40-100keV and 20-40keV band, are all features common to the Ultra Compact source class., Comment: accepted A&A

The microquasar 1E 1740.7-2942 is one of the most appealing source of the Galactic Centre region. The high energy feature detected once by SIGMA has been searched in the last years by INTEGRAL, but never confirmed. Classified as a persistent source, on 2004 it showed a quiescent-like state. In fact for few month 1E 1740.7-2942 was below the detector sensitivity level. We present the long term temporal behaviour of 1E 1740.7-2942 observed by INTEGRAL and RXTE in 2004 and 2005, as well as preliminary results on possible spectral transitions., Comment: 3 pages, proceedings of the 6th INTEGRAL Workshop 'The Obscured Universe', July 2-8 2006, ESA SP-622

The INTEGRAL/IBIS survey was performed collecting all the GPS and GCDE data together with all the available public data . The second catalogue, published in 2006 by Bird et al., is dominated by detection of 113 X-ray binaries, with 38 being high-mass and 67 low-mass. In most systems the compact object is a neutron star, but the sample also contains 4 confirmed Black Holes and 6 LMXB black hole candidates (BHC). There are also, in additional, 6 tentative associations as BHCs based simply on spectral and timing properties. In the sample of 12 sources (BHC and tentatively associated BHC) there are 7 transient sources that went into outbursts during the INTEGRAL survey observations. We present here the monitoring of the time and spectral evolution of these 7 outbursts., Comment: conference proceeding: The 6th INTEGRAL Workshop The Obscured Universe, Moscow, Russia July 2-8, 2006. (ESA SP 622)

We report on the identification of a new soft gamma ray source, namely IGR J22517+2218, detected with IBIS/INTEGRAL. The source, which has an observed 20-100 keV flux of ~4 x10^-11 erg cm-2 s-1, is spatially coincident with MG3 J225155+2217, a quasar at z=3.668. The Swift/XRT 0.5-10 keV continuum is flat (Gamma=1.5) with evidence for a spectral curvature below 1-2 keV either due to intrinsic absorption (NH=3 +/- 2 x 10^22 cm-2) or to a change in slope (Delta Gamma= 0.5). X-ray observations indicate flux variability over a 6 days period which is further supported by a flux mismatch between Swift and INTEGRAL spectra. IGR J22517+2218 is radio loud and has a flat radio spectrum; optically it is a broad line emitting quasar with the atypical property of hosting a narrow line absorption system. The Source Spectral Energy Distribution is unusual compared to blazars of similar type: either it has the synchrotron peak in the X/gamma-ray band (i.e. much higher than generally observed) or the Compton peak in the MeV range (i.e. lower than typically measured). IGR J22517+2218=MG3 J225155+2217 is the second most distant blazar detected above 20 keV and a gamma-ray lighthouse shining from the edge of our Universe., Comment: 4 pages, 4 figures, Accepted for publication in Astrophysical Journal Letters

HESS J1616-508 is one of the brightest emitters in the TeV sky. Recent observations with the IBIS/ISGRI telescope on board the INTEGRAL spacecraft have revealed that a young, nearby and energetic pulsar, PSR J1617-5055, is a powerful emitter of soft gamma-rays in the 20-100 keV domain. In this paper we present an analysis of all available data from the INTEGRAL, Swift, BeppoSAX and XMM-Newton telescopes with a view to assessing the most likely counterpart to the HESS source. We find that the energy source that fuels the X/gamma-ray emissions is derived from the pulsar, both on the basis of the positional morphology, the timing evidence and the energetics of the system. Likewise, the 1.2% of the pulsar's spin down energy loss needed to power the 0.1-10 TeV emission is also fully consistent with other HESS sources known to be associated with pulsars. The relative sizes of the X/gamma-ray and VHE sources are consistent with the expected lifetimes against synchrotron and Compton losses for a single source of parent electrons emitted from the pulsar. We find that no other known object in the vicinity could be reasonably considered as a plausible counterpart to the HESS source. We conclude that there is good evidence to assume that the HESS J1616-508 source is driven by PSR J1617-5055 in which a combination of synchrotron and inverse Compton processes combine to create the observed morphology of a broad-band emitter from keV to TeV energies., Comment: 7 pages, including 5 figures and 1 table. Accepted for publication in MNRAS

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 ApJ

KS 1741-293, discovered in 1989 by the X-ray camera TTM in the Kvant module of the Mir space station and identified as an X-ray burster, has not been detected in the hard X band until the advent of the INTEGRAL observatory. Moreover this source has been recently object of scientific discussion, being also associated to a nearby extended radio source that in principle could be the supernova remnant produced by the accretion induced collapse in the binary system. Our long term monitoring with INTEGRAL, covering the period from February 2003 to May 2005, confirms that KS 1741-293 is transient in soft and hard X band. When the source is active, from a simultaneous JEM-X and IBIS data analysis, we provide a wide band spectrum from 5 to 100 keV, that can be fit by a two component model, a multiple blackbody for the soft emission and a Comptonized or a cut-off power law model for the hard component. Finally, by the detection of two X-ray bursters with JEM-X, we confirm the bursting nature of KS 1741-293, including this source in the class of the hard tailed X-ray bursters., Comment: 7 pages, accepted for publication on MNRAS

We here report on the multiwavelength study which led us to the identification of X-ray source IGR J16194-2810 as a new Symbiotic X-ray Binary (SyXB), that is, a rare type of Low Mass X-ray Binary (LMXB) composed of a M-type giant and a compact object. Using the accurate X-ray position allowed by Swift/XRT data, we pinpointed the optical counterpart, a M2 III star. Besides, the combined use of the spectral information afforded by XRT and INTEGRAL/IBIS shows that the 0.5-200 keV spectrum of this source can be described with an absorbed Comptonization model, usually found in LMXBs and, in particular, in SyXBs. No long-term (days to months) periodicities are detected in the IBIS data. The time coverage afforded by XRT reveals shot-noise variability typical of accreting Galactic X-ray sources, but is not good enough to explore the presence of X-ray short-term (seconds to hours) oscillations in detail. By using the above information, we infer important parameters for this source such as its distance (about 3.7 kpc) and X-ray luminosity (about 1.4e35 erg/s in the 0.5-200 keV band), and we give a description for this system (typical of SyXBs) in which a compact object (possibly a neutron star) accretes from the wind of its M-type giant companion. We also draw some comparisons between IGR J16194-2810 and other sources belonging to this subclass, finding that this object resembles SyXBs 4U 1700+24 and 4U 1954+31., Comment: 9 pages, 4 figures, 3 tables. Accepted for publication on Astronomy & Astrophysics, main journal. Slight changes to match the proof-corrected version: Pdot/P for 4U 1954+31 (in Table 3) corrected; acknowledgements expanded; references updated

Context: The transient hard X-ray source IGR J11321-5311 was discovered by INTEGRAL on June 2005, during observations of the Crux spiral arm. To date, this is the only detection of the source to be reported by any X/gamma-ray mission. Aims: To characterize the behaviour and hence the nature of the source through temporal and spectral IBIS analysis. Methods: Detailed spectral and temporal analysis has been performed using standard INTEGRAL software OSA v.5.1. Results: To date, IGR J11321-5311 has been detected only once. It was active for about 3.5 hours, a short and bright flare lasting about 1.5 hours is evident in the IBIS light curve. It reached a peak flux of about 80 mCrab or 2.2x10E-9 erg cmE-2 sE-1 (20--300 keV),corresponding to a peak luminosity of 1.1x10E37 erg sE-1 (assuming a distance of 6.5 kpc). During the outburst, the source was detected with a significance of 18 sigma (20--300 keV) and 8 sigma (100--300 keV). The spectrum of the total outburst activity (17--300 keV) is best fitted by the sum of a power law (Gamma=0.55+/-0.18) plus a black body (kT=1.0{+0.2}_{-0.3} keV), with no evidence for a break up to 300 keV. A spectral analysis at Science Window level revealed an evident hardening of the spectrum through the outburst. The IBIS data were searched for pulsations with no positive result. Conclusions: The X-ray spectral shape and the flaring behaviour favour the hypothesis that IGR J11321-5311 is an Anomalous X-ray Pulsar, though a different nature can not be firmly rejected at the present stage., Comment: accepted for publication in A&A letter, 4 pages, 6 figures

The X-ray transient XMMU J174716.1-281048 was serendipitously discovered with XMM-Newton in 2003. It lies about 0.9 degrees off the Galactic Centre and its spectrum shows a high absorption (~8 x 10E22 cm^(-2)). Previous X-ray observations of the source field performed in 2000 and 2001 did not detect the source, indicative of a quiescent emission at least two orders of magnitude fainter. The low luminosity during the outburst (~5 x 10E34 erg/s at 8 kpc) indicates that the source is a member of the ``very faint X-ray transients'' class. On 2005 March 22nd the INTEGRAL satellite caught a possible type-I X-ray burst from the new INTEGRAL source IGR J17464-2811, classified as fast X-ray transient. This source was soon found to be positionally coincident, within the uncertainties, with XMMU J174716.1-281048. Here we report data analysis of the X-ray burst observed with the IBIS and JEM-X telescopes and confirm the type-I burst nature. We also re-analysed XMM-Newton and Chandra archival observations of the source field. We discuss the implications of these new findings, particularly related to the source distance as well as the source classification., Comment: 4 pages, 8 figures, accepted for publication in A&A Letters

IGR J18483-0311 is a poorly known transient hard X-ray source discovered by INTEGRAL during observations of the Galactic Center region performed between 23--28 April 2003. Aims: To detect new outbursts from IGR J18483-0311 using INTEGRAL and archival Swift XRT observations and finally to characterize the nature of this source using the optical/near-infrared (NIR) information available through catalogue searches. Results: We report on 5 newly discovered outbursts from IGR J18483-0311 detected by INTEGRAL.For two of them it was possible to constrain a duration of the order of a few days. The strongest outburst reached a peak flux of 120 mCrab (20--100 keV): its broad band JEM--X/ISGRI spectrum (3--50 keV) is best fitted by an absorbed cutoff power law with photon index=1.4+/-0.3, cutoff energy of ~22 keV and Nh ~9x10^22 cm^-2. Timing analysis of INTEGRAL data allowed us to identify periodicities of 18.52 days and 21.0526 seconds which are likely the orbital period of the system and the spin period of the X-ray pulsar respectively. Swift XRT observations of IGR J18483$-$0311 provided a very accurate source position which strongly indicates a highly reddened star in the USNO--B1.0 and 2MASS catalogues as its possible optical/NIR counterpart. Conclusions: The X-ray spectral shape, the periods of 18.52 days and 21.0526 seconds, the high intrinsic absorption, the location in the direction of the Scutum spiral arm and the highly reddened optical object as possible counterpart, all favour the hypothesis that IGR J18483-0311 is a HMXB with a neutron star as compact companion. The system is most likely a Be X-ray binary, but a Supergiant Fast X-ray Transient nature can not be entirely excluded., Comment: accepted for publication in A&A, 10 pages, 17 figures, 4 tables

We report the results of simultaneous multiwavelength observations of the X-ray transient source SWIFT J1753.5-0127 performed with INTEGRAL, RXTE, NTT, REM and VLA on 2005 August 10-12. The source, which underwent an X-ray outburst since 2005 May 30, was observed during the INTEGRAL Target of Opportunity program dedicated to new X-ray novae located in the Galactic Halo. Broad-band spectra and fast timing variability properties of SWIFT J1753.5-0127 are analyzed together with the optical, near infra-red and radio data. We show that the source was significantly detected up to 600 keV with Comptonization parameters and timing properties typical of the so-called Low/Hard State of black hole candidates. We build a spectral energy distribution and we show that SWIFT J1753.5-0127 does not follow the usual radio/X-ray correlation of X-ray binaries in the Low/Hard State. We give estimates of distance and mass. We conclude that SWIFT J1753.5-0127 belongs to the X-ray nova class and that it is likely a black hole candidate transient source of the Galactic Halo which remained in the Low/Hard State during its main outburst. We discuss our results within the context of Comptonization and jet models., Comment: Accepted for publication in ApJ, 25 pages, 4 tables, 11 figures (3 in color)

In this paper we report on the third soft gamma-ray source catalog obtained with the IBIS/ISGRI gamma-ray imager on board the INTEGRAL satellite. The scientific dataset is based on more than 40 Ms of high quality observations performed during the first three and a half years of Core Program and public IBIS/ISGRI observations. Compared to previous IBIS/ISGRI surveys, this catalog includes a substantially increased coverage of extragalactic fields, and comprises more than 400 high-energy sources detected in the energy range 17-100 keV, including both transients and faint persistent objects which can only be revealed with longer exposure times., Comment: Accepted for publication in ApJ Suppl.; 11 pages; 4 figures Minor changes to content

On March 2003, INTEGRAL/IBIS detected an outburst from a new source, IGR J17464-3213, that turned out to be a HEAO-1 transient, namely H1743-322. Its fux decayed below the RXTE PCA sensitivity limit in November 2003. On July 3, 2004 the source was again detected by RXTE/PCA reaching an intensity of 70 mCrab in the 2-10 keV band. Another outburst was observed in August 2005. A multiwavelength observation campaign was performed, during the three outbursts, by INTEGRAL, RXTE and CHANDRA satellites and the radio telescopes VLA and ATCA. INTEGRAL data show a good coverage during the July 2004 and August 2005 outbursts. We show here the spectral and temporal variability study performed on the data collected by INTEGRAL., Comment: 5 pages, 7 figures, accepted for publication in the proceedings of VI Microquasar workshop: Microquasar and Beyond, 18-22 September 2006 in Como (Italy), eds: T. Belloni et al. (2006)

We report on a Chandra grating observation of the recently discovered hard X-ray transient IGR J17497-2821. The observation took place about two weeks after the source discovery at a flux level of about 20 mCrab in the 0.8-8 keV range. We extracted the most precise X-ray position of IGR J17497-2821, RA=17 49 38.037, DEC= -28 21 17.37 (90% uncertainty of 0.6"). We also report on optical and near infra-red photometric follow-up observations based on this position. With the multi-wavelength information at hand, we discuss the possible nature of the source proposing that IGR J17497-2821 is a low-mass X-ray binary, most likely hosting a black hole, with a red giant K-type companion., Comment: 4 pages, 5 figures, Accepted for publication in ApJL

We report on INTEGRAL observations performed during the 2004 outburst of the bright black hole transient GX 339-4. We analysed IBIS and JEM-X public data starting on 9th August and lasting about one month. During this period GX 339-4 showed spectral state transitions. In order to seek for variability patterns, a principal component analysis (PCA) has been used., Comment: 5 pages, 6 figures, accepted for publication in the proceedings of VI Microquasar workshop: Microquasar and Beyond, 18-22 September 2006 in Como (Italy), eds: T. Belloni et al. (2006)

The soft X-ray and soft gamma observations of the new discovered TeV sources by HESS and MAGIC are crucial to discriminate between various emitting scenarios and to fully understand their nature. The INTEGRAL Observatory has regularly observed the entire galactic plane during the first 1000 day in orbit providing a survey in the 20-100 keV range resulted in a soft gamma-ray sky populated with more than 200 sources. In the case of HESS J1813-178 INTEGRAL found the hard X-ray counterpart IGR J18135-1751 and Swift/XRT Telescope performed follow-up observations on this source. Here we present the soft/hard X-ray spectral study. We reported on the INTEGRAL observation of LS 5039, the first microquasar that have been observed by HESS up to now., Comment: Advances of Space Research (AdSpR, Elsevier) devoted to results presented in Session E1.4 of the COSPAR 2006 assembly in Beijing, edited by Jacco Vink and Parviz Ghavamian

We present data from INTEGRAL and BeppoSAX satellites showing spectral state transitions of the neutron-star, atoll-type, low-mass X-ray binary 1705-44. Its energy spectrum can be described as the sum of one or two blackbody components, a 6.4-keV Fe line, and a component due to thermal Comptonization. In addition, and for the first time in this source, we find a strong signature of Compton reflection, presumably due to illumination of the optically-thick accretion disk by the Comptonization spectrum. The two blackbody components, which the soft-state data require, presumably arise from both the disk and the neutron-star surface. The Comptonization probably takes place in a hot inner flow irradiated by some of the blackbody photons. The spectral transitions are shown to be associated with variations in the bolometric luminosity, most likely proportional to the accretion rate. Indipendentely from the spectral state, we also see changes in the temperature of the Comptonizing electrons and the strength of Compton reflection., Comment: accepted ApJ

Context: AXJ1845.0-0433 was discovered by ASCA in 1993 during fast outburst activity characterized by several flares on short timescales. Up to now, the source was not detected again by any X-ray mission. Its optical counterpart is suggested to be an O9.5I supergiant star, which is the only remarkable object found inside the ASCA error box. Aims: To detect and characterize new fast outbursts of AXJ1845.0-0433 and confirm its supergiant HMXB nature, using INTEGRAL and archival Swift XRT observations. Methods: We performed an analysis of INTEGRAL IBIS and JEM-X data with OSA 5.1 as well as an analysis of archive Swift XRT data. Results: We report on fast flaring activity from the source on timescales of a few tens of minutes, the first to be reported since its discovery in 1993. Two outbursts have been detected by INTEGRAL (Apr 2005 and Apr 2006) with similar peak fluxes and peak luminosities of 80 mCrab and 9.3X10^35 erg s^-1 (20--40 keV), respectively. Two other outbursts were detected by Swift XRT on Nov 2005 and Mar 2006. The refined Swift XRT position of AXJ1845.0-0433 confirms its association with the supergiant star previously proposed as optical counterpart. Conclusions: Our INTEGRAL and Swift results fully confirm the supergiant HMXB nature of AXJ1845.0-0433 which can therefore be classified as a supergiant fast X-ray transient. Moreover they provide for the first time evidence of its recurrent fast transient behaviour., Comment: 5 pages,5 figures, 1 table, accepted for publication in A&A

We present detailed spectral and timing analysis of the hard x-ray transient IGR J16358-4726 using multi-satellite archival observations. A study of the source flux time history over 6 years, suggests that lower luminosity transient outbursts can be occuring in intervals of at most 1 year. Joint spectral fits of the higher luminosity outburst using simultaneous Chandra/ACIS and INTEGRAL/ISGRI data reveal a spectrum well described by an absorbed power law model with a high energy cut-off plus an Fe line. We detected the 1.6 hour pulsations initially reported using Chandra/ACIS also in the INTEGRAL/ISGRI light curve and in subsequent XMM-Newton observations. Using the INTEGRAL data we identified a spin up of 94 s (dP/dt = 1.6E-4), which strongly points to a neutron star nature for IGR J16358-4726. Assuming that the spin up is due to disc accretion, we estimate that the source magnetic field ranges between 10^13 - 10^15 G, depending on its distance, possibly supporting a magnetar nature for IGR J16358-4726., Comment: Accepted for publication in ApJ, 34 pgs, 14 figures, 4 tables

In five years of operation, data from INTEGRAL has been used to discover a large number of gamma-ray sources, a substantial fraction of which have turned out to be active galactic nuclei (AGN). Recently Bassani et al. (2006) have presented a sample of around 60 AGNs of which some still lack optical identification or information in the X-ray band. In this work we present X-ray data for 8 objects acquired with the XRT telescope on-board Swift satellite. The XRT positioning has allowed us to identify the optical counterparts and to classify their types through follow-up measurements. Analysis of these data has also provided information on their spectra below 10 keV., Comment: 4 pages, submitted proceeding of the 6th INTEGRAL Workshop "The Obscured Universe" July 2-8, 2006 Space Research Institute Moscow, Russia

\integral and \sax observations of the neutron-star LMXB 4U~1705--44 have been analysed to deeply investigate the spectral state transitions nature. Its energy spectrum can be described as the sum of one or two blackbody, a 6.4-keV Fe line and a component due to thermal Comptonization. For the first time in this source, we find a strong signature of Compton reflection, presumably due to illumination of the optically-thick accretion disk by the Comptonized spectrum. Detection of two blackbody component in the soft states could originate in the disk and the neutron-star surface, and the Comptonized component arises from a hot inner flow with the seed photons coming from the disk and/or the neutron-star surface. The spectral transitions are shown to be associated with variations in the accretion rate, which changes in turn the temperature of the Comptonizing electrons and the strength of Compton reflection., Comment: The 6th INTEGRAL Workshop 2006. The 6th INTEGRAL Workshop 2006, submitted

The 4-200 keV spectral and temporal behaviour of the low mass X-ray binary 4U 1820-30 has been studied with INTEGRAL during 2003-2005. This source as been observed in both the soft (banana) and hard (island) spectral states. A high energy tail above 50 keV in the hard state has been revealed for the first time. This places the source in the category of X-ray bursters showing high-energy emission. The tail can be modeled as a soft power law component, with the photon index of ~ 2.4, on top of thermal Comptonization emission from a plasma with the electron temperature of kT_e ~ 6 keV and optical depth of $\tau ~ 4. Alternatively, but at a lower goodness of the fit, the hard-state broad band spectrum can be accounted for by emission from a hybrid, thermal-nonthermal, plasma. During the observations, the source spent most of the time in the soft state, as previously reported and the $\ge$4 keV spectra can be represented by thermal Comptonization with kT_e ~ 3 keV and $\tau ~ 6-7., Comment: 4 pages, 3 figures, submitted proceeding of the 6th INTEGRAL Workshop "The Obscured Universe" July 2-8, 2006 Space Research Institute Moscow, Russia

We report INTEGRAL/IBIS results on temporal and spectral behaviour of the Low Mass X-ray Binary XB 1832-330 located in the globular cluster NGC 6652. During the 2003-2005 monitoring of the Galactic Centre, the source shows a weak flux variability and an everage flux in the 20-150 keV of about 2*10^{-10} erg s^{-1} cm^{-2}. The overall energy spectrum extends up to 150 keV and is well described by a Comptonization model with an electrons temperature of $\sim$ 22 keV and a plasma optical depth of 1.8. We thus confirm the persistent nature of this burtser., Comment: 3 pages, 4 figures, submitted proceeding of the 6th INTEGRAL Workshop "The Obscured Universe" July 2-8, 2006 Space Research Institute Moscow, Russia

Recent INTEGRAL observations and archival BeppoSAX data have been analyzed to deeply investigate the hard X-ray behavior of the neutron star, atoll type, low mass X-ray Binary 4U1636-53. Our investigation in three different periods outline three corresponding different sates. Infact, the source was detected in both the canonical high and low state and moreover in one occasion \integral spectrum shows, for first time in this source, a hard tail dominating the emission above 30 keV. This spectrum is fitted as the sum of a Comptonized component similar to soft state and a power-law component ($\Gamma=2.76$), indicating the presence of a non thermal electron distribution of velocities., Comment: The 6th INTEGRAL Workshop 2006. The 6th INTEGRAL Workshop 2006, submitted

We study the 4-200 keV spectral and temporal behaviour of the low mass X-ray binary 4U 1820-30 with INTEGRAL during 2003-2005. This source as been observed in both the soft (banana) and hard (island) spectral states. A high energy tail, above 50 keV, in the hard state has been observed for the first time. This places the source in the category of X-ray bursters showing high-energy emission. The tail can be modeled as a soft power law component, with the photon index of ~2.4, on top of thermal Comptonization emission from a plasma with the electron temperature of kT_e~6 keV and optical depth of \tau~4. Alternatively, but at a lower goodness of the fit, the hard-state broad band spectrum can be accounted for by emission from a hybrid, thermal-nonthermal, plasma. During this monitoring the source spent most of the time in the soft state, usual for this source, and the >~4 keV spectra are represented by thermal Comptonization with kT_e~3 keV and \tau~6-7., Comment: 14 pages, 4 figures, accepted for publication by ApJ

Optical spectroscopic identification of the nature of 21 unidentified southern hard X-ray objects is reported here in the framework of our campaign aimed at determining the nature of newly-discovered and/or unidentified sources detected by INTEGRAL. Our results show that 5 of these objects are magnetic Cataclysmic Variables (CVs), 4 are High-Mass X-ray Binaries (HMXBs; one of which is in the Large Magellanic Cloud) and 12 are Active Galactic Nuclei (AGNs). When feasible, the main physical parameters for these hard X-ray sources are also computed using the multiwavelength information available in the literature. These identifications further underscore the importance of INTEGRAL in the study of the hard X-ray spectrum of AGNs, HMXBs and CVs, and the usefulness of a strategy of catalogues cross-correlation plus optical spectroscopy to securely pinpoint the actual nature of the X-ray sources detected with INTEGRAL., Comment: 14 pages, 7 figures, 6 tables, accepted for publication on A&A, main journal. Figures 1 and 2 and Table 1 will only appear on the on-line version of the paper

We present here simultaneous INTEGRAL/RXTE observations of Sco X-1, and in particular a study of the hard X-ray emission of the source and its correlation with the position in the Z-track of the X-ray color-color diagram. We find that the hard X-ray (above about 30 keV) emission of Sco X-1 is dominated by a power-law component with a photon index of ~3. The flux in the power-law component slightly decreases when the source moves in the color-color diagram in the sense of increasing inferred mass accretion rate from the horizontal branch to the normal branch/flaring branch vertex. It becomes not significantly detectable in the flaring branch, where its flux has decreased by about an order of magnitude. These results present close analogies to the behavior of GX 17+2, one of so-called Sco-like Z sources. Finally, the hard power law in the spectrum of Sco X-1 does not show any evidence of a high energy cutoff up to 100 - 200 keV, strongly suggesting a non-thermal origin of this component., Comment: 5 pages including 3 figures. Accepted for publication by ApJ Letters

We study the spectrum of the cosmic X-ray background (CXB) in energy range $\sim$5-100 keV. Early in 2006 the INTEGRAL observatory performed a series of four 30ksec observations with the Earth disk crossing the field of view of the instruments. The modulation of the aperture flux due to occultation of extragalactic objects by the Earth disk was used to obtain the spectrum of the Cosmic X-ray Background(CXB). Various sources of contamination were evaluated, including compact sources, Galactic Ridge emission, CXB reflection by the Earth atmosphere, cosmic ray induced emission by the Earth atmosphere and the Earth auroral emission. The spectrum of the cosmic X-ray background in the energy band 5-100 keV is obtained. The shape of the spectrum is consistent with that obtained previously by the HEAO-1 observatory, while the normalization is $\sim$10% higher. This difference in normalization can (at least partly) be traced to the different assumptions on the absolute flux from the Crab Nebulae. The increase relative to the earlier adopted value of the absolute flux of the CXB near the energy of maximum luminosity (20-50 keV) has direct implications for the energy release of supermassive black holes in the Universe and their growth at the epoch of the CXB origin., Comment: A&A, 14 pages, 12 figues

We report on the first census of INTEGRAL/IBIS detections ($\gtrsim 4\sigma$ significance) above 100 keV based on the Core Program and public Open time observations up to April 2005. There are 49 sources detected in the 100-150 keV band of which 14 are also seen in the 150-300 keV range. The low energy sample is dominated by X-ray binary systems of both low and high mass, but also includes 10 active galaxies. Of the binary systems that are detected above 150 keV, more than 50% are associated with black hole candidates, often reported as microquasars. The present survey results are then used to construct LogN-LogS curves for galactic and extragalactic objects in the 100-150 keV band: above a 1 mCrab sensitivity limit we expect that around 200 galactic sources and almost 350 active galaxies populate the sky above 100 keV. While the contribution of individual point sources to the total Galactic emission has been estimated to be around 70-80% between 100-300 keV, we find that active galaxies detected above 1 mCrab account for only about 3% of the cosmic hard X-ray background in the 100-150 keV band., Comment: accepted ApJ

In the last years, the persistent source 1E 1743.1-2843 has been observed in the X-rays, but never above 20 keV. In previous works, it was stressed that a possible high energy emission could give further indications on the accreting object nature which remains still unknown. We present here more than two years of 1E 1743.1-2843 monitoring with INTEGRAL/IBIS as well as public XMM-Newton and Chandra X-ray observations. The temporal study in the 20-40 keV band shows a rather constant flux on few months time scale. Based on this result we have performed the broad-band spectral analysis using EPIC/IBIS non simultaneous data and ACIS-I/IBIS data collected during 2004. In ~2 Ms, we report a detection of 6 sigma in the energy range 35-70 keV. The first broad-band study (2-70 keV) shows a steep slope (~3) and a black body temperature of 1.7 keV. Combining spectral parameters and discussion about the luminosity evaluations for different possible distances, our conclusions are in favour of a LMXB system with a neutron star at distance higher than the Galactic Centre, even though a firm conclusion can not be stated., Comment: 4 pages, 3 figures, accepted for publication in A&A

In order to provide a firm identification of the newly discovered Galactic TeV sources, a search for counterparts in a broad band from soft X-ray to soft gamma-rays is crucial as data in these wavebands allow us to distinguish between different types of suggested models (for example leptonic versus hadronic) and, in turn, to disentangle their nature. In this paper, we report the results of a set of follow-up observations performed by the Swift/X-Ray Telescope (XRT) on seven sources recently discovered by HESS, in the range from few hundred GeV to about 10 TeV, during the inner Galaxy survey (Aharonian et al. 2006). In all, but one case, we detect X-ray sources inside or close-by the extended TeV emitting region. All these putative X-ray counterparts have accurate arc-second location and are consistent with being point sources. The main result of our search is the discovery that three of them are located at the center of the diffuse radio emission of the supernova remnants, which have been spatially associated to these TeV objects. HESS J1640-465, HESS J1834-087 and HESS J1813-178 show this evidence, suggestive of a possible Pulsar Wind Nebula association., Comment: 22 pages, 6 figures, accepted for publication on The Astrophysical Journal

We report on the spectral analysis results of the neutron star, atoll type, low mass X-ray Binary 4U1636-53 observed by INTEGRAL and BeppoSAX satellites. Spectral behavior in three different epochs corresponding to three different spectral states has been deeply investigated. Two data set spectra show a continuum well described by one or two soft blackbody plus a Comptonized components with changes in the Comptonizing electrons and black body temperature and the accretion rates, which are typical of the spectral transitions from high to low state. In one occasion INTEGRAL spectrum shows, for first time in this source, a hard tail dominating the emission above 30 keV. The total spectrum is fitted as the sum of a Comptonized component similar to soft state and a power-law component (Gamma=2.76), indicating the presence of a non thermal electron distribution of velocities. In this case, a comparison with hard tails detected in soft states from neutron stars systems and some black hole binaries suggests that a similar mechanism could originate these components in both cases., Comment: 6 pages, 4 figures, 2 tables. accepted ApJ

The recent completion and operation of the High Energy Stereoscopic System, an array of ground based imaging Cherenkov telescopes, has provided a survey with unprecedented sensitivity of the inner part of the Galaxy and revealed a new population of very high energy gamma-rays sources emitting at E$>$100 GeV. Most of them were reported to have no known radio or X-ray counterpart and hypothesised to be representative of a new class of dark nucleonic cosmic sources. In fact, very high energy gamma-rays with energies E $>$ 10$^{11}$ eV are the best proof of non-thermal processes in the universe and provide a direct in-site view of matter-radiation interaction at energies by far greater than producible in ground accelerators. At lower energy INTEGRAL has regularly observed the entire galactic plane during the first 1000 day in orbit providing a survey in the 20-100 keV range resulted in a soft gamma-ray sky populated with more than 200 sources, most of them being galactic binaries, either BHC or NS. Very recently, the INTEGRAL new source IGR J18135-1751 has been identified as the soft gamma-ray counterpart of HESS J1813-178 and AXJ1838.0-0655 as the X/gamma-ray counterpart of HESS J1837-069. Detection of non thermal radio, X and gamma-ray emission from these TeV sources is very important to discriminate between various emitting scenarios and, in turn, to fully understand their nature. The implications of these new findings in the high energy Galactic population will be addressed., Comment: 9 pages, 8 figures, Workshop - Focusing Telescopes in Nuclear Astrophysics - September 12 - 15, 2005, Espace St. Jacques, Bonifacio, Corsica, France, "Experimental Astronomy", in press

We present further results from our ongoing optical spectrophotometric campaign at the Astronomical Observatory of Bologna in Loiano (Italy) on unidentified hard X-ray sources detected by INTEGRAL. We observed spectroscopically the putative optical counterparts of the INTEGRAL sources IGR J00234+6141, IGR J01583+6713, IGR J06074+2205, IGR J13091+1137 and IGR J20286+2544. We find that the first two are Galactic objects, namely a Cataclysmic Variable at a distance of about 300 pc and a Be/X transient High-Mass X-ray Binary (HMXB) located at about 6.4 kpc, respectively, whereas the last one is identified with MCG +04-48-002, a Starburst/HII galaxy at redshift z = 0.013 hiding a Seyfert 2 nucleus. We identify IGR J13091+1137 as the (likely Seyfert 2 type) active nucleus of galaxy NGC 4992, which we classify as an X-ray Bright, Optically Normal Galaxy; this is the first example of this type of object to be detected by INTEGRAL, and one of the closest of this class. We moreover confirm the possible Be/X nature of IGR J06074+2205, and we estimate it to be at a distance of about 1 kpc. We also reexamine the spectrum of the z = 0.087 elliptical radio galaxy PKS 0352-686, the possible counterpart of the INTEGRAL source IGR J03532-6829, and we find that it is a BL Lac. Physical parameters for these sources are also evaluated by discussing our findings in the context of the available multiwavelength information. These identifications further stress the importance of INTEGRAL in the study of the hard X-ray spectrum of Active Galactic Nuclei, HMXBs and Cataclysmic Variables., Comment: 11 pages, 3 figures, 2 tables, accepted for publication on A&A, main journal. The quality of Fig. 1 was degraded to fit the arXiv uploads size limits. Revised version matches the A&A corrected proofs