| Data Centre for Astrophysics Astronomy Department of the University of Geneva |
ISDC Science Results
Mildly obscured active galaxies and the cosmic X-ray background2016-08-18 | INTEGRAL, HEAVENS, AGN, Diffuse emission We have produced the deepest hard X-ray images of Active Galactic Nuclei, which show that midly obscured sources have a stronger reflection than unobscured objects. Mildly obscured Active Galactic Nuclei contribute massively to the cosmic X-ray background such that a population of Compton thick sources larger than that effectively detected is not required. The stronger reflection suggests that the covering fraction of the gas and dust surrounding their central engines is a key factor shaping their appearance. |
The stellar wind velocity field of HD 775812015-07-07 | INTEGRAL, HEAVENS, X-ray binary The early acceleration of stellar winds in massive stars is poorly constrained. The comparison of X-ray observations of the high-mass X-ray binary Vela X-1 accumulated over 9 years, with hydodynamical simulations allows to constrain the stellar wind velocity field very close to the surface of the super-giant companion. The observed acceletration is more efficient than usually assumed. |
Un-Beamed Tidal Disruption Events at Hard X-Rays2015-05-27 | HEAVENS We have detected nine TDE candidates at hard X-rays during eight years in a sample of ~53'000 galaxies closer than 100 Mpc. The observed hard X-ray fluxes and rate are consistent with the rates observed by XMM-Newton at soft X-rays or in the optical from SDSS observations and also consistent with expectations from simulations. The average hard X-ray spectral shape of our TDE candidates is in addition very similar to that usually observed in active galactic nuclei, pointing towards inverse Compton processes and electrons accelerated in shocks in the accretion flow. |
The high energy spectrum of 3C 2732015-03-12 | AGN, INTEGRAL, Fermi We studied the high energy spectral variability of the bright quasar 3C 273 from ~1 keV up to ~10 GeV using data collected with RXTE-PCA, INTEGRAL and Fermi/LAT and compared them with radio data at 37 GHz collected at the Metsähovi Radio Observatory. Quasi-simultaneous broad band high energy spectra have been built at the epoch of γ ray flares and X-ray flares. Both timing and spectral analysis suggest a two-component scenario, where the X-ray emission is likely dominated by a Seyfert-like component, while the γ ray emission is dominated by a blazar-like component originated in the relativistic jet. |
Origin of the X-ray off-states in Vela X-12014-12-22 | INTEGRAL, X-ray binary, HEAVENS INTEGRAL discovered huge hard X-ray variability and off-sates in Vela X-1 and other high mass X-ray binaries. Hydrodynamic simulations allowed us to discover the likely source for such variations occuring on time scales of hours. These variations are related to oscillations of the accretion rate (with a typical period of ∼ 6800 sec) corresponding to the complex motion of a bow shock forming between the neutron star and the massive companion, moving either towards or away from the neutron star. |
Extraordinary jet shot by a runaway pulsar2014-02-07 | INTEGRAL | Press Release The source IGR J11014-6103, discovered with INTEGRAL, is powered by an isolated pulsar, which is running through our Galaxy at a speed of ~1000 km/s. Thanks to our observations with the Chandra X-ray satellite and with the Australia Radio Telescope Compact Array (ATCA), we found that this pulsar forms a ~15pc (i.e. 40 light-years) long, ballistic jet. This is the first case where a supersonic runaway pulsar is shown to produce a ballistic jet. This discovery opens up several new questions about the formation of pulsars and how do they produce jets. |
Probing the cosmic-ray content of galaxy clusters by stacking Fermi-LAT count maps2013-10-02 | Fermi, VHE, Galaxy cluster The cosmic-ray content of galaxy clusters is expected to produce a gamma-ray signal that could be detected by the Fermi Gamma-ray Space Telescope. We show here that even by stacking the Fermi observations at the position of 53 known galaxy clusters we still cannot detect such an emission. |
Swings between rotation and accretion power in a millisecond binary pulsar 2013-09-26 | INTEGRAL, X-ray binary | Press Release A new transient source, identified as IGR J18245-2452, was first detected in X-rays on 28 March 2013 by INTEGRAL in the globular cluster M28, which lies in the constellation Sagittarius. Observations by XMM-Newton determined the pulsar's spin period to be 3.9 milliseconds clearly identifying it as an X-ray-bright millisecond pulsar powered by accretion of material from a nearby low-mass star companion. The spin period and other key characteristics was found to match perfectly those of a pulsar in M28 that had been observed in 2006, but only at radio wavelengths. This is then the first ever fast-spinning 'millisecond pulsar' caught in a crucial evolutionary phase, as it swings between emitting pulses of X-rays and radio waves. |
Tidal disruption of a super-Jupiter by a massive black hole2013-04-02 | INTEGRAL | Press Release A strong hard X-ray flare was discovered in the galaxy NGC 4845 by INTEGRAL in January 2011. This emission corresponds to the tidal disruption of a super-Jupiter close to the giant black hole at the center of the galaxy. The disrupted material heated up before falling in the black hole, emitting at high energies. |
Introducing the CTA concept2013-03-01 | VHE The Cherenkov Telescope Array (CTA) is a new observatory for very high-energy (VHE) gamma rays. CTA has ambitions science goals, for which it is necessary to achieve full-sky coverage, to improve the sensitivity by about an order of magnitude, to span about four decades of energy, from a few tens of GeV to above 100 TeV with enhanced angular and energy resolutions over existing VHE gamma-ray observatories. |
The baryon content of galaxy clusters in their entire volume2013-01-07 | Galaxy cluster, Cosmology, Planck | Press Release Galaxy clusters form from the largest inhomogeneities of the early Universe, and thus they are expected to retain their mass (both baryonic and dark matter) throughout cosmic time. We measure for the first time the baryon fraction of galaxy clusters in their entire volume by combining X-ray (ROSAT) and Sunyev-Zel'dovich (Planck) data. As expected, we find that the average baryon fraction reaches the cosmic value Ωb/Ωm in the outer regions, which shows that clusters are suitable targets for cosmological studies. |
Thermodynamic properties of the virial region of galaxy clusters2013-01-07 | Galaxy cluster, Planck Galaxy clusters are still forming in their outer regions through the accretion of smaller structures (galaxies, groups). Using a combined X-ray (ROSAT) and Sunyaev-Zel'dovich (Planck) data analysis, we determine the thermodynamic properties of a sample of 18 galaxy clusters and constrain the current mechanisms of structure formation. We find that in unrelaxed clusters non-thermal effects (cosmic rays, turbulence, magnetic fields) and/or gas clumping play an important role, while relaxed cool-core clusters follow closely the predictions of gravitational collapse. |
Measuring neutron star masses by gravitational deflection2012-09-13 | INTEGRAL, X-ray binary | Press Release Neutron stars are fundamental to probe the physics of matter at the highest density, where even atom nuclei are broken into quarks. New stellar systems, discovered by INTEGRAL, including a neutron star orbiting a massive companion, allow to weight the neutron star in an original manner, by measuring the gravitational deflection of the wind stream generated by the companion star. The gas stream is so dense that it completely absorbs the emission of the neutron star, even X-rays. Only gamma-rays telescopes, such as INTEGRAL, can detect them. |
A stacking method to study the gamma-ray emission of source samples based on the co-adding of Fermi LAT count maps2012-09-13 | Fermi, VHE Objects with low integrated gamma-ray fluxes that are not detected individually may become detectable when their corresponding count maps are added. This paper describes a method that can be used to analyze stacked data obtained with the Fermi Large Area Telescope. |
Fermi/LAT observations of 1ES 0229+200: implications for extragalactic magnetic fields and background light2012-01-25 | Fermi, VHE New data from the Fermi/LAT instrument allow us to measure the GeV spectrum of a prominent TeV blazar 1ES 0229+200. Being one of the hardest TeV sources on the sky, 1ES 0229+200 provides the best constraints for the Extragalactic magnetic field and Extragalactic background light. The new observations permit us to measure these two quantities simultaneously. |
The environment of Weak Emission-Line Quasars2011-11-17 | AGN We analyse the ultraviolet and X-ray properties of 83 weak emission-line quasars (WLQ) and ~240 ordinary quasars to answer whether the Baldwin effect is responsible for the unusual weakness of emission-lines in WLQ or not. We find that this weakness is rather caused by a low covering factor of the broad line region in WLQ than by their extremely soft ionizing continuum. |
The gas distribution in galaxy cluster outer regions2011-11-01 | Galaxy cluster We analyze archival ROSAT observations for a sample of 31 galaxy clusters to study the spatial distribution of the gas out to the virial radius of these objects, and compare this with state-of-the-art numerical simulations. We find that the mean observed density profiles are flatter than predicted by numerical simulations including gravitational collapse only, which indicates that non-gravitational processes (gas cooling, clumping, AGN feedback) cannot be neglected even in regions with a very low density. |
Multiple synchrotron self-Compton modeling of gamma-ray flares in 3C 2792011-09-27 | AGN We test and constrain a new theoretical model for the high-energy emission of blazars. We fit the model parameters to longterm lightcurves and to radio-to-gamma-ray spectra of 3C 279. It turns out that the modeling of shock waves in a jet emitting synchrotron self-Compton radiation including multiple-order scatterings is a promising alternative to the usual external-Compton emission models. |
Search for high energy γ-ray emission from galaxies of the Local Group with Fermi/LAT2011-09-19 | Fermi High-energy γ-ray emission is searched for in galaxies from the Local Group using Fermi/LAT. No significant signal is found in the data, however the study shows imminent detection is expected from M 33 and M 83. |
Reflection in Seyfert galaxies and the unified model of AGN2011-08-02 | INTEGRAL, AGN, Diffuse emission | Press Release From an extremely long hard X-ray look at AGN we found unexpected differences amongst different classes of objects. These differences could be explained by supermassive black holes living in different environments, and might help shading light on the origin of the peak of the cosmic X-ray background. |
IGR J11014-6103: a newly discovered pulsar wind nebula?2011-07-19 | INTEGRAL To study the unidentified source IGR J11014-6103, we analyzed archival data from X-rays to the radio energy range spanning 30 years of observation. The analysis in soft X-rays revealed a 4-arcmin cometary-like tail with two brighter objects at one end of the extended emission. Based on the results of our spectral and timing analysis, we suggest that the source might be a pulsar wind nebula produced by a spinning neutron star (pulsar) advancing at high-velocity through the interstellar medium. These system are rare, and if confirmed, this would be the first detected with INTEGRAL. |
Stochastic Acceleration and the Evolution of Spectral Distributions in SSC Sources: A Self Consistent Modeling of Blazars' Flares 2011-07-12 | AGN, VHE We investigated the role of the stochastic acceleration on the evolution of the particle energy distribution in relativistic jet, taking into account radiative losses in a SSC scenario. We find that in the acceleration dominated stage the distribution is well described by a log-parabolic shape, and that the predicted trends among synchrotron curvature (bs), peak energy (Es), and peak luminosities (Ls), match those observed in a sample of 6 TeV detected BL Lacs. |
XMM-Newton observations of IGRJ18410-0535: The ingestion of a clump by a supergiant fast X-ray transient2011-06-28 | INTEGRAL, X-ray binary | Press Release With a stroke of luck, we caught the supergiant fast X-ray transient IGRJ18410-0535 undergoing a bright X-ray flare by using ESA's XMM-Newton X-ray observatory. This outburst of X-rays, which lasted about four hours, was due to a sudden increase in the rate at which the neutron star was accreting matter from its companion, a blue supergiant star. By monitoring this phenomenon in unprecedented detail, the data provide the first, substantive evidence to explain such luminosity variations in this type of binary system; the flare appears to be due to the ingestion of a massive clump of matter by the neutron star. |
AX J1910.7+0917 and three newly discovered INTEGRAL sources2011-01-11 | INTEGRAL, X-ray binary We investigated the nature of the still poorly known X-ray source AX J1910.7+0917 and searched for closeby previously undetected objects, making use of the high sensitivity of the IBIS/ISGRI telescope and the improvements in the data analysis software. We analyzed all publicly available data from INTEGRAL, XMM-Newton, Chandra and ASCA and we discussed the different possibilities for the nature of AX J1910.7+0917. In the IBIS/ISGRI field-of-view around AX J1910.7+0917, we discovered three new sources: IGR J19173+0747, IGR J19294+1327 and IGR J19149+1036. For two of them we report the results of follow-up observations carried out with Swift/XRT. |
12 hours spikes from the Crab Pevatron2010-12-15 | INTEGRAL, Fermi, VHE | Press Release The Crab nebula featured 3 gamma-ray spikes in the GeV band in September 2010. These γ-ray flares are due to synchrotron emission from a very compact Pevatron located closer to the pulsar than the equatorial termination shock between the supersonic wind and the surrounding nebula. The spectral and timing properties of the flare are interpreted in the framework of a relativistically moving emitter. The flare duration is of the order of the synchrotron cooling time scale for electrons at an energy larger than 1015 eV, the highest electron energy ever measured in a cosmic accelerator. |
Eta Carinae: a very large hadron collider2010-12-13 | Fermi, INTEGRAL | Press Release Eta Carinae system consists of two massive stars with dense supersonic winds. Observation with Fermi/LAT suggests that collisions of high energetic hadrons take place in the colliding winds region. |
X-ray wind tomography of the highly absorbed HMXB 2010-12-08 | X-ray binary, INTEGRAL We observed the highly absorbed HMXB IGR J17252-3616 along the orbit with XMM-Newton. Our analysis suggests highly asymmetric (and extended) structures trailing the neutron star and slower wind terminal velocities (~400 km/s) than observed in classical systems. If confirmed, it may turn out that half of the persistent sgHMXB have low stellar wind speeds. |
Seyfert 2 galaxies in the GeV band: jets and starburst2010-10-18 | Fermi, AGN, INTEGRAL We present Fermi/LAT data analysis of two unusual suspects in the GeV band, NGC 1068 and NGC 4945, which are both composite starburst/active galactic nuclei objects. We show that the GeV emission of NGC 4945 could be interpreted in terms of starburst activity, while the one of NGC 1068 should account for a dominant contribution from the central AGN. |
Shock-in-jet model for quasars and microquasars2010-10-06 | AGN, X-ray binary We present the detailed equations for a physical modeling of the synchrotron emission of shock waves propagating in a relativistic jet. We illustrate this by fitting a series of model outbursts to a flaring episode observed in the radio and infrared emission of the microquasar Cyg X-3 and derive the physical conditions in the jet flow. |
INTEGRAL discovery of a new transient source: IGR J16374-50432010-08-24 | INTEGRAL, X-ray binary A new transient hard X-ray source was discovered by INTEGRAL in the large field of view of IBIS/ISGRI during observations of another source, RX J1713.7-3946. The new source called IGR J16374-5043 produced a bright flare on 22 August 2010 lasting about a day. |
HESS J1632-478: An Energetic Relic2010-07-04 | VHE, Fermi The unidentified very high energy source HESS J1632-478 is very likely an old (104 years) Pulsar Wind Nebula with a size of several parsecs containing very high energy electrons cooling slowly mostly through Inverse Compton scattering. Its spectrum is made of a faint synchrotron component (from the radio to the X-rays) emitted by electrons accelerated up to very high energies and of a bright high-energy gamma-ray component emitted by inverse Compton scatering against infrared photons. |
Pulsed thermal emission from the accreting pulsar XMMU J054134.7-6825502010-06-29 | X-ray binary In August 2007, the X-ray binary pulsar XMMU J054134.7-682550 made a giant type II outburst, reaching the Eddington limit. The reflection of the hard X-ray, emitted close to the neutron star, on the inner part of the accretion disk allowed the determination of the geometry of the broadened inner disk (width of 75 km). The thermal emission from the disk could also be detected during the outburst. |
INTEGRAL and RXTE observations of XTE J1946+274 in outburst2010-06-23 | INTEGRAL, X-ray binary During the observations of Cyg X-1 in satellite Rev. 0938 (PI Wilms), INTEGRAL caught the Be/X-ray binary transient XTE J1946+274 in outburst (flux of ~135 mCrab, 20-40 keV range). This is the first outburst of the source since 2001 after a long quiescence period. |
Multi-zone warm and cold clumpy absorbers in 3 Seyfert galaxies2010-05-10 | AGN, INTEGRAL We performed the spectral analysis of 3 Seyfert galaxies presenting evidence of a strong soft excess below 2 keV. We showed how this soft excess can be related only to a clumpy structure of the absorbers, and does not need any additional emission component. |
Evidence for strong extragalactic magnetic fields from Fermi observations of TeV blazars2010-04-04 | Fermi, AGN, VHE | Press Release FERMI/LAT observations of distant blazars together with theoretical modeling provide us with the first clue about the parameters of primordial magnetic field, survived up to nowadays in the voids of the Large-Scale Structure. |
An exceptionally bright outburst from the HMXB XTE J1855-0262010-03-16 | INTEGRAL, X-ray binary On 14 March 2010, INTEGRAL observed three exceptionally bright flares from the high-Mass X-ray Binary XTE J1855-026. The first flare lasted only about 2 minutes and reached a peak intensity of almost that of the Crab nebula. |
X-ray power law spectra in active galactic nuclei2010-01-23 | AGN We discuss X-ray spectral properties in AGN within the framework of clumpy accretion flows in which shocks between accreting elements account for the observed radiation. We obtain that steeper spectra are associated with shorter characteristic timescales, in agreement with the recently discovered "spectral-timing" correlation. |
INTEGRAL spectra of the cosmic X-ray background and Galactic ridge emission2010-01-14 | INTEGRAL, Diffuse emission, AGN, X-ray binary We reanalyzed the INTEGRAL Earth occultation observations of early 2006 to derive IBIS spectra of the cosmic X-ray background (CXB) and of the Galactic ridge X-ray emission (GRXE) in the ~20-200 keV range. The potential of such observations is demonstrated by the state-of-the-art hard X-ray spectra we derived for three fundamental components: the CXB, the GRXE and the Earth emission. |
GRB 090817: INTEGRAL/OMC's unlucky gamma-ray burst2009-08-18 | INTEGRAL, GRB On 17 August 2009, INTEGRAL caught another long Gamma-Ray Burst (GRB) in the field of view of IBIS/ISGRI. This one was special as it was also in the smaller field of view of the X-ray monitor JEM-X 1 and (almost) of the optical camera OMC. |
Discovery of IGR J17497-2821 : a new X-ray nova2006-11-27 | INTEGRAL, X-ray binary | Press Release INTEGRAL discovered a hard X-ray transient on 17 Sep. 2006 near the Galactic Centre. Infrared and X-ray follow-up observations pin-point the source location and show that it is a new X-ray nova and likely a black-hole candidate. |
Public INTEGRAL TOO observation of V 0332+53 in outburst2005-01-12 | INTEGRAL, X-ray binary The High-Mass X-ray Binary V 0332+53 (EXO 0331+530), underwent a dramatic outburst at the end of 2004, and was a Target of Opportunity (TOO) for a public INTEGRAL observation conducted on 6-10 January 2005. Three cyclotron lines are clearly detected in the spectrum. |
