arXiv subjects

R. Walter

Publications and source records attributed to R. Walter.

At least 127 records · Page 7Linked to original sources

arXiv2007

2003--2005 INTEGRAL and XMM-Newton observations of 3C 273

The aim of this paper is to study the evolution of the broadband spectrum of one of the brightest and nearest quasars 3C 273. We analyze the data obtained during quasi-simultaneous INTEGRAL and XMM monitoring of the blazar 3C 273 in 2003--2005 in the UV, X-ray and soft gamma-ray bands and study the results in the context of the long-term evolution of the source. The 0.2-100 keV spectrum of the source is well fitted by a combination of a soft cut-off power law and a hard power law. No improvement of the fit is achieved if one replaces the soft cut-off power law by either a blackbody, or a disk reflection model. During the observation period the source has reached the historically softest state in the hard X-ray domain with a photon index $Γ=1.82\pm 0.01$. Comparing our data with available archived X-ray data from previous years, we find a secular evolution of the source toward softer X-ray emission (the photon index has increased by $ΔΓ\simeq 0.3-0.4$ over the last thirty years). We argue that existing theoretical models have to be significantly modified to account for the observed spectral evolution of the source.

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arXiv2007

The 3rd IBIS/ISGRI soft gamma-ray survey catalog

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.

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arXiv2006

The XMM-Newton/INTEGRAL monitoring campaign of IGR J16318-4848

IGR J16318-4848 is the prototype and one of the more extreme examples of the new class of highly obscured Galactic X-ray sources discovered by INTEGRAL. A monitoring campaign on this source has been carried out by XMM-Newton and INTEGRAL, consisting in three simultaneous observations performed in February, March and August 2004. The long-term variability of the Compton-thick absorption and emission line complexes will be used to probe the properties of the circumstellar matter. A detailed timing and spectral analysis of the three observations is performed, along with the reanalysis of the XMM-Newton observation performed in February 2003. The results are compared with predictions from numerical radiative transfer simulations to derive the parameters of the circumstellar matter. Despite the large flux dynamic range observed (almost a factor 3 between observations performed a few months apart), the source remained bright (suggesting it is a persistent source) and Compton-thick (NH >1.2x10^24 cm-2). Large Equivalent Width (EW) emission lines from Fe Kalpha, Fe Kbeta and Ni Kalpha were present in all spectra. The addition of a Fe Kalpha Compton Shoulder improves the fits, especially in the 2004 observations. Sporadic occurrences of rapid X-ray flux risings were observed in three of the four observations. The Fe Kalpha light curve followed the continuum almost instantaneously, suggesting that the emission lines are produced by illumination of small-scale optically-thick matter around the high-energy continuum source. Using the iron line EW and Compton Shoulder as diagnostic of the geometry of the matter, we suggest that the obscuring matter is in a flattened configuration seen almost edge-on.

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arXiv2006

A Possible Magnetar Nature for IGR J16358-4726

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.

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arXiv2006

SWIFT/XRT follow-up observations of INTEGRAL AGNs

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.

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arXiv2006

Unveiling the nature of INTEGRAL objects through optical spectroscopy. V. Identification and properties of 21 southern hard X-ray sources

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.

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arXiv2006

INTEGRAL and XMM-Newton observations of LSI +61 303

LSI +61 303 is one of the few X-ray binaries with Be star companion from which both radio and high-energy gamma-ray emission have been observed. We present XMM-Newton and INTEGRAL observations which reveal variability of the X-ray spectral index of the system. The X-ray spectrum is hard (photon index ~ 1.5) during the orbital phases of both high and low X-ray flux. However, the spectrum softens at the moment of transition from high to low X-ray state. The spectrum of the system in the hard X-ray band does not reveal the presence of a cut-off (or, at least a spectral break) at 10-60 keV energies, expected if the compact object is an accreting neutron star. The observed spectrum and spectral variability can be explained if the compact object in the system is a rotation powered pulsar.

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arXiv2006

Integral/Ibis Census of the Sky Beyond 100 kev

We report on the first census of INTEGRAL/IBIS detections ($\gtrsim 4σ$ 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.

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arXiv2006

INTEGRAL observations of AGN obscured by the Galactic Plane

In this paper we present INTEGRAL observations of 7 AGNs: two newly discovered type 1 Seyferts, IGR J18027-1455 and IGR J21247+5058, and five well known Seyferts, NGC 6814 (type 1.5), Cyg A (Type 2), MCG-05-23-16 (type 2), ESO 103-G035 (type2) and GRS1734-292. For IGR J18027-1455 and IGR J21247+5058 only INTEGRAL/IBIS data were available, while broadband spectra are presented and discussed for the remaining 5 sources for which either BeppoSAX or ASCA data were used in conjunction with INTEGRAL measurements. In the cases of NGC 6814 and GRS 1734-292, data taken in different periods indicate variability in the flux: in the case of NGC 6814 by a factor of 16 over a period of about 10 years. Although limited in size, our sample can be used to investigate the parameter space of both the photon index and cut-off energy. The mean photon index is 1.8, while the cut-off energy ranges from 30-50 keV to greater than 200 keV; in the particular case of MCG-05-23-16, ESO 103-G035 and GRS 1734-292 the cut-off energy is well constrained at or below 100 keV. We have also tested an enlarged sample, which includes INTEGRAL data of 3 more AGNs, against the correlation found by a number of authors between the photon index and the cut-off energy but have found no evidence for a relation between these two parameters. Our analysis indicates that there is a diversity in cut-off energies in the primary continuum of Seyfert galaxies.

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arXiv2006

Unveiling the nature of INTEGRAL objects through optical spectroscopy. IV. A study of six new hard X-ray sources

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.

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arXiv2006

Orbital Parameters for the X-ray Pulsar IGR J16393-4643

With recent and archival Rossi X-Ray Timing Explorer (RXTE) X-ray measurements of the heavily obscured X-ray pulsar IGR J16393-4643, we carried out a pulse timing analysis to determine the orbital parameters. Assuming a circular orbit, we phase-connected data spanning over 1.5 years. The most likely orbital solution has a projected semi-major axis of 43 +- 2 lt-s and an orbital period of 3.6875 +- 0.0006 days. This implies a mass function of 6.5 +- 1.1 M_sun and confirms that this INTEGRAL source is a High Mass X-ray Binary (HMXB) system. By including eccentricity in the orbital model, we find e < 0.25 at the 2 sigma level. The 3.7 day orbital period and the previously known ~910 s pulse period place the system in the region of the Corbet diagram populated by supergiant wind accretors, and the low eccentricity is also consistent with this type of system. Finally, it should be noted that although the 3.7 day solution is the most likely one, we cannot completely rule out two other solutions with orbital periods of 50.2 and 8.1 days.

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arXiv2005

INTEGRAL/IBIS Extragalactic survey: 20-100 keV selected AGN

Analysis of INTEGRAL Core Program and public Open Time observations performed up to April 2005 provides a sample of 62 active galactic nuclei in the 20-100 keV band above a flux limit of ~1.5x10^-11 erg/cm2/s. Most(42) of the sources in the sample are Seyfert galaxies, almost equally divided between type 1 and 2 objects, 6 are blazars and 14 are still unclassified. Excluding the blazars, the average redshift of our sample is 0.021 while the mean luminosity is Log(L) = 43.45. We find that absorption is present in 65% of the objects with 14% of the total sample due to Compton thick active galaxies. In agreement with both Swift/BAT team results and 2-10 keV studies, the fraction of absorbed objects decreases with the 20-100 keV luminosity. All Seyfert 2s in our sample are absorbed as are 33% of Seyfert 1s. The present data highlight the capability of INTEGRAL to probe the extragalactic gamma-ray sky and to find new and/or absorbed active galaxies.

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arXiv2005

INTEGRAL and XMM-Newton observations of the X-ray pulsar IGR J16320-4751/AX J1631.9-4752

We report on observations of the X-ray pulsar IGR J16320-4751 (a.k.a. AX J1631.9-4752) performed simultaneously with INTEGRAL and XMM-Newton. We refine the source position and identify the most likely infrared counterpart. Our simultaneous coverage allows us to confirm the presence of X-ray pulsations at ~1300 s, that we detect above 20 keV with INTEGRAL for the first time. The pulse fraction is consistent with being constant with energy, which is compatible with a model of polar accretion by a pulsar. We study the spectral properties of IGR J16320-4751 during two major periods occurring during the simultaneous coverage with both satellites, namely a flare and a non-flare period. We detect the presence of a narrow 6.4 keV iron line in both periods. The presence of such a feature is typical of supergiant wind accretors such as Vela X-1 or GX 301-2. We inspect the spectral variations with respect to the pulse phase during the non-flare period, and show that the pulse is solely due to variations of the X-ray flux emitted by the source and not to variations of the spectral parameters. Our results are therefore compatible with the source being a pulsar in a High Mass X-ray Binary. We detect a soft excess appearing in the spectra as a blackbody with a temperature of ~0.07 keV. We discuss the origin of the X-ray emission in IGR J16320-4751: while the hard X-rays are likely the result of Compton emission produced in the close vicinity of the pulsar, based on energy argument we suggest that the soft excess is likely the emission by a collisionally energised cloud in which the compact object is embedded.

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arXiv2005

Unveiling the nature of INTEGRAL objects through optical spectroscopy. II. The nature of four unidentified sources

We present the results of our optical spectrophotometric campaign ongoing at the Astronomical Observatory of Bologna in Loiano (Italy) on hard X-ray sources detected by INTEGRAL. We observed spectroscopically the putative optical counterparts of four more INTEGRAL sources, IGR J12391-1610, IGR J18406-0539, 2E 1853.7+1534 and IGR J19473+4452. These data have allowed us to determine their nature, finding that IGR J12391-1610 (=LEDA 170194) and IGR J19473+4452 are Seyfert 2 galaxies at redshifts z = 0.036 and z = 0.053, respectively, IGR J18406-0539 (=SS 406) is a Be massive X-ray binary located at about 1.1 kpc from Earth, and 2E 1853.7+1534 is a Type 1 Seyfert galaxy with z = 0.084. Physical parameters for these objects are also evaluated by collecting and discussing the available multiwavelength information. The determination of the extragalactic nature of a substantial fraction of sources inside the INTEGRAL surveys underlines the importance of hard X-ray observations for the study of background Active Galactic Nuclei located beyond the `Zone of Avoidance' of the Galactic Plane.

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arXiv2005

IGR J17252-3616: an accreting pulsar observed by INTEGRAL and XMM-Newton

The discovery of the X-ray source IGR J17252-3616 by INTEGRAL was reported on 9 February 2004. Regular monitoring by INTEGRAL shows that IGR J17252-3616 is a persistent hard X-ray source with an average count rate of 0.96 counts/s (~6.4 mCrab) in the 20-60 keV energy band. A follow-up observation with XMM-Newton, which was performed on 21 March 21 2004, showed that the source is located at R.A.(2000.0)=17h25m11.4 and Dec.=-36degr16'58.6" with an uncertainty of 4". The only infra-red counterpart to be found within the XMM-Newton error circle was 2MASS J17251139-3616575, which has a Ks-band magnitude of 10.7 and is located 1" away from the XMM-Newton position. The analysis of the combined INTEGRAL and XMM-Newton observations shows that the source is a binary X-ray pulsar with a spin period of 413.7 s and an orbital period of 9.72 days. The spectrum can be fitted with a flat power law plus an energy cut off (Gamma~0.02,Ecut~8.2 keV) or a Comptonized model (kTe~5.5 keV, tau~7.8). The spectrum also indicates a large hydrogen column density of Nh~15x1e22 atoms/cm-2 suggesting an intrinsic absorption. The Fe Kalpha line at 6.4 keV is clearly detected. Phase-resolved spectroscopy does not show any variation in the continuum except the total emitted flux. The absorption is constant along the pulse phase. This source can be associated with EXO 1722-363 as both systems show common timing and spectral features. The observations suggest that the source is a wind-fed accreting pulsar accompanied by a supergiant star.

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arXiv2005

Is the INTEGRAL/IBIS source IGR J17204-3554 a gamma-ray emitting galaxy hidden behind the molecular cloud NGC 6334 ?

We report on the identification of a soft gamma-ray source, IGR J17204-3554, detected with the IBIS imager on board the INTEGRAL satellite. The source has a 20-100 keV flux of ~3x10^-11 erg cm^-2 s^-1 and is spatially coincident with NGC 6334, a molecular cloud located in the Sagittarius arm of the Milky Way. Diffuse X-ray emission has been reported from this region by ASCA and interpreted as coming from five far-infrared cores located in the cloud. However, the combined ASCA spectrum with a 9 keV temperature was difficult to explain in terms of emission from young pre-main sequence stars known to be embedded in the star forming regions. Detection of gamma-rays makes this interpretation even more unrealistic and suggests the presence of a high energy source in or behind the cloud. Follow up observations with Swift and archival Chandra data allow us to disentangle the NGC6334 enigma by locating an extragalactic object with the proper spectral characteristics to explain the gamma-ray emission. The combined Chandra/IBIS spectrum is well fitted by an absorbed power law with index 1.2+/-0.1, NH=1.4+/-0.1x10^23 cm^-2 and an unabsorbed 2-10 keV flux of 0.5x10^-11 erg cm^-2 s^-1. This column density is in excess of the galactic value implying that we are detecting a background galaxy concealed by the molecular cloud and further hidden by material located either in the galaxy itself or between IGR J17204-3554 and the cloud.

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arXiv2005

IGR J16393-4643: a new heavily-obscured X-ray pulsar

An analysis of the high-energy emission from IGR J16393-4643 (=AX J1639.0-4642) is presented using data from INTEGRAL and XMM-Newton. The source is persistent in the 20-40 keV band at an average flux of 5.1x10^-11 ergs/cm2/s, with variations in intensity by at least an order of magnitude. A pulse period of 912.0+/-0.1 s was discovered in the ISGRI and EPIC light curves. The source spectrum is a strongly-absorbed (nH=(2.5+/-0.2)x10^23 atoms/cm2) power law that features a high-energy cutoff above 10 keV. Two iron emission lines at 6.4 and 7.1 keV, an iron absorption edge >7.1 keV, and a soft excess emission of 7x10^-15 ergs/cm2/s between 0.5-2 keV, are detected in the EPIC spectrum. The shape of the spectrum does not change with the pulse. Its persistence, pulsation, and spectrum place IGR J16393-4643 among the class of heavily-absorbed HMXBs. The improved position from EPIC is R.A. (J2000)=16:39:05.4 and Dec.=-46:42:12 (4" uncertainty) which is compatible with that of 2MASS J16390535-4642137.

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arXiv2005

Using the ROSAT Catalogues to find counterparts for the second IBIS/ISGRI Survey Sources

The second IBIS/ISGRI survey has produced a catalogue containing 209 hard X-ray sources visible down to a flux limit of around 1 milliCrab. The point source location accuracy of typically 1-3 arcminutes has allowed the counterparts for most of these sources to be found at other wavelengths. In order to help identify the remaining objects, we have used the cross-correlation recently found between the ISGRI catalogue and the ROSAT All Sky Survey Bright Source Catalogue. In this way, for ISGRI sources which have a counterpart in soft X-rays, we can use the much smaller ROSAT error box to search for identifications. For this second survey, we find 114 associations with the number expected by chance to be ~2. Of these sources, 8 are in the list of unidentified objects and, using the smaller ROSAT error boxes, we can find tentative counterparts for five of them. We have performed the same analysis for the ROSAT Faint Source Catalogue, finding a further nine associations with ISGRI unidentified sources from a total of 29 correlations, and, notwithstanding the poorer location accuracy of these sources and higher chance coincidence possibility, we have managed to find a counterpart for another source. Finally, we have used the ROSAT HRI catalogue to search the ISGRI error boxes and find 5 more X-ray objects, of which two are neither in the bright or faint source catalogues, and for which we have managed to find optical/near infrared associations. This makes a total of 19 objects with X-ray counterparts for which we have found possible identifications for nine, most of which are extragalactic.

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