Double-nucleus elliptical MCG-01-12-005 in an X-ray emitting cluster of galaxies

The scenario of galaxy formation is believed to follow a structure that builds up from the bottom, with large galaxies being formed by several merging episodes of smaller ones. In this scenario a number of galaxies can be expected to be seen in the merging phase, with their external regions already mixed, while their nuclei, with stronger self-gravitation, are still recognizable as such. During a photometric monitoring of AGNs in the field of a long-exposure INTEGRAL pointing, we serendipitously found an elliptical galaxy in the center of the X-ray cluster (EXO 0422-086) with two nuclei. We performed surface photometry on our images and those of the SDSS archive and obtained slit spectra of both nuclei. Aperture photometry of the two stellar-like nuclei showed very similar colors in the SDSS image and in our Johnson BVRI images, which is typical of an elliptical galaxy nucleus. The spectra of the nuclei showed the typical absorption lines of an elliptical galaxy without appreciable emission lines. The redshifts derived from each nucleus were equal and fully consistent with the literature value (0.0397). We can therefore exclude the possibility that one of the nuclei is a foreground star or a background AGN and consider this elliptical galaxy as a bona fide example of a galaxy merger.Comment: 4 pages, 3 figures. Accepted on Astronomy and Astrophysic

X-ray spectral evolution of V404 Cygni in the initial phase of the 2015 outburst

The black hole binary GS 2023+338 exhibited an unprecedently bright outburst on June 2015. Since June 17th, the high energy instruments on board INTEGRAL detected an extremely variable emission during both bright and low luminosity phases, with dramatic variations of the hardness ratio on time scales of ~seconds. The analysis of the IBIS and SPI data reveals the presence of hard spectra in the brightest phases, compatible with thermal Comptonization with temperature kTe ~ 40 keV. The seed photons temperature is best fit by kT0 ~ 7 keV, that is too high to be compatible with blackbody emission from the disk. This result is consistent with the seed photons being provided by a different source, that we hypothesize to be a synchrotron driven component in the jet. During the brightest phase of flares, the hardness shows a complex pattern of correlation with flux, with a maximum energy released in the range 40-100 keV. The hard X-ray variability for E > 50 keV is correlated with flux variations in the softer band, showing that the overall source variability cannot originate entirely from absorption, but at least part of it is due to the central accreting source.Comment: 5 pages, 4 figures, accepted for publication in Astrophysical Journal Letter

Disc-Jet coupling in the LMXB 4U1636-53 from INTEGRAL

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 Ap

First INTEGRAL Observations of V404 Cygni during the 2015 Outburst: Spectral Behavior in the 20-650 keV Energy Range

In 2015 June, the source V404 Cygni (= GS2023+38) underwent an extraordinary outburst. We present the results obtained during the first revolution dedicated to this target by the INTEGRAL mission and focus on the spectral behavior in the hard X-ray domain, using both SPI and IBIS instruments. The source exhibits extreme variability and reaches fluxes of several tens of Crab. However, the emission between 20 and 650 keV can be understood in terms of two main components, varying on all the observable timescales, similar to what is observed in the persistent black hole system Cyg X-1. The low-energy component (up to ∼200 keV) presents a rather unusual shape, probably due to the intrinsic source variability. Nonetheless, a satisfactory description is obtained with a Comptonization model, if an unusually hot population of seed photons (kT0 ∼ 7 keV) is introduced. Above this first component, a clear excess extending up to 400-600 keV leads us to investigate a scenario where an additional (cutoff) power law could correspond to the contribution of the jet synchrotron emission, as proposed in Cyg X-1. A search for an annihilation feature did not provide any firm detection, with an upper limit of 2 × 10-4 ph cm-2 s-1 (2σ) for a narrow line centered at 511 keV, on the averaged obtained spectrum. Based on observations with INTEGRAL, an ESA project with instruments and science data center funded by ESA member states (especially the PI countries: Denmark, France, Germany, Italy, Spain, and Switzerland), Czech Republic, and Poland with the participation of Russia and USA

On the Near-infrared Identification of the INTEGRAL Source IGR J16328-4726

The aim of this work is to identify the infrared (IR) counterpart of the Galactic high-mass X-ray binary IGR J16328-4726 discovered by the INTEGRAL satellite, and to derive the extinction and distance to the system. We present new deep sub-arcsecond JHKs imaging and low-resolution near-IR spectroscopy in the 1.5 and 2.4 μm range of IGR J16328-4726. We report the presence of two near-IR stellar sources separated by about 1.″8 at the location of the unresolved 2MASS source J16323791-4723409, previously considered to be the near-IR counterpart of the X-ray source. From the analysis of their near-IR colors and spectra as well as accurate positions, we uniquely identify the true IR counterpart of IGR J16328-4726. Our 1.5-2.4 μm spectrum of this star is consistent with the published classification O8Iafpe. Assuming this, and in combination with new JHKs photometry, a reddening AV = 23.6 ± 0.7 and a distance of 7.2 ± 0.3 kpc from the Sun are derived

The first detection of Compton Reflection in the Low-Mass X-ray Binary 4U1705-44 with INTEGRAL and BeppoSAX

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 Ap

VizieR Online Data Catalog: 8yr INTEGRAL/IBIS soft gamma-ray source obs. (Bird+, 2016)

Here we report an all-sky soft gamma-ray source catalog based on IBIS observations performed during the first 1000 orbits of INTEGRAL. The database for the construction of the source list consists of all good-quality data available, from the launch in 2002, up to the end of 2010. This corresponds to ~110Ms of scientific public observations, with a concentrated coverage on the Galactic Plane and extragalactic deep exposures. This new catalog includes 939 sources above a 4.5σ significance threshold detected in the 17-100keV energy band, of which 120 sources represent previously undiscovered soft gamma-ray emitters. The source positions are determined, mean fluxes are provided in two main energy bands, and these are both reported together with the overall source exposure. Indicative levels of variability are provided, and outburst times and durations are given for transient sources. A comparison is made with previous IBIS catalogs and catalogs from other similar missions. (2 data files)

Spectral variation in the supergiant fast X-ray transient SAX J1818.6-1703 observed by XMM-Newton and INTEGRAL

We present the results of a 30 ks XMM-Newton observation of the supergiant fast X-ray transient (SFXT) SAX J1818.6-1703 - the first in-depth soft X-ray study of this source around periastron. INTEGRAL observations shortly before and after the XMM-Newton observation show the source to be in an atypically active state. Over the course of the XMM-Newton observation, the source shows a dynamic range of ∼100 with a luminosity greater than 1 × 1035 erg s-1 for the majority of the observation. After an ∼6 ks period of low-luminosity (∼1034 erg s-1) emission, SAX J1818.6-1703 enters a phase of fast flaring activity, with flares ∼250 s long, separated by ∼2 ks. The source then enters a larger flare event of higher luminosity and ∼8 ks duration. Spectral analysis revealed evidence for a significant change in spectral shape during the observation with a photon index varying from Γ ∼ 2.5 during the initial low-luminosity emission phase, to Γ ∼ 1.9 through the fast flaring activity, and a significant change to Γ ∼ 0.3 during the main flare. The intrinsic absorbing column density throughout the observation (nH ∼ 5 × 1023 cm-2) is among the highest measured from an SFXT, and together with the XMM-Newton and INTEGRAL luminosities, consistent with the neutron star encountering an unusually dense wind environment around periastron. Although other mechanisms cannot be ruled out, we note that the onset of the brighter flares occurs at 3 × 1035erg s-1, a luminosity consistent with the threshold for the switch from a radiative-dominated to Compton cooling regime in the quasi-spherical settling accretion model