arXiv subjects

R. Walter

Publications and source records attributed to R. Walter.

At least 91 records · Page 5Linked to original sources

arXiv2011

Supergiant X-ray binaries observed by Suzaku

Suzaku observations are presented for the high-mass X-ray binaries IGR J16207-5129 and IGR J17391-3021 (=XTE J1739-302). For IGR J16207-5129, we provide the first X-ray broadband (0.5--60 keV) spectrum from which we confirm a large intrinsic column density (nH = 1.6e23 /cm2), and we constrain the cutoff energy for the first time (Ec = 19 keV). A prolonged (> 30 ks) attenuation of the X-ray flux was observed which we tentatively attribute to an eclipse of the probable neutron star by its massive companion, in a binary system with an orbital period between 4 and 9 days, and inclination angles > 50 degrees. For IGR J17391-3021, we witnessed a transition from quiescence to a low-activity phase punctuated by weak flares whose peak luminosities in the 0.5--10 keV band are only a factor of 5 times that of the pre-flare emission. These micro flares are accompanied by an increase in nH which suggests the accretion of obscuring clumps of wind. We now recognize that these low-activity epochs constitute the most common emission phase for this system, and perhaps in other supergiant fast X-ray transients (SFXTs) as well. We close with an overview of our upcoming program in which Suzaku will provide the first ever observation of an SFXT (IGR J16479-4514) during a binary orbit enabling us to probe the accretion wind at every phase.

astro-ph.HE↗

arXiv2011

IGR J11014-6103: a newly discovered pulsar wind nebula?

Context: IGRJ11014-6103 is one of the still unidentified hard X-ray INTEGRAL sources, reported for the first time in the 4th IBIS/ISGRI catalog. Aims: We investigated the nature of IGR J11014-6103 by carrying out a multiwavelength analysis of the available archival observations performed in the direction of the source. Methods: We present first the results of the timing and spectral analysis of all the X-ray observations of IGR J11014-6103 carried out with ROSAT, ASCA, Einstein, Swift, and XMM-Newton, and then use them to search for possible counterparts to the source in the optical, infra-red, radio and gamma-ray domain. Results: Our analysis revealed that IGR J11014-6103 is comprised of three different X-ray emitting regions: a point-like source, an extended object and a cometary-like "tail" (~4 arcmin). A possible radio counterpart positionally coincident with the source was also identified. Conclusions: Based on these results, we suggest that the emission from IGR J11014-6103 is generated by a pulsar wind nebula produced by a high-velocity pulsar. IGR J11014-6103 might be the first of these systems detected with INTEGRAL IBIS/ISGRI.

astro-ph.HE↗

arXiv2011

XMM-Newton observations of IGRJ18410-0535: The ingestion of a clump by a supergiant fast X-ray transient

IGRJ18410-0535 is a supergiant fast X-ray transients. This subclass of supergiant X-ray binaries typically undergoes few- hour-long outbursts reaching luminosities of 10^(36)-10^(37) erg/s, the occurrence of which has been ascribed to the combined effect of the intense magnetic field and rotation of the compact object hosted in them and/or the presence of dense structures ("clumps") in the wind of their supergiant companion. IGR J18410-0535 was observed for 45 ks by XMM-Newton as part of a program designed to study the quiescent emission of supergiant fast X-ray transients and clarify the origin of their peculiar X-ray variability. We carried out an in-depth spectral and timing analysis of these XMM-Newton data. IGR J18410-0535 underwent a bright X-ray flare that started about 5 ks after the beginning of the observation and lasted for \sim15 ks. Thanks to the capabilities of the instruments on-board XMM-Newton, the whole event could be followed in great detail. The results of our analysis provide strong convincing evidence that the flare was produced by the accretion of matter from a massive clump onto the compact object hosted in this system. By assuming that the clump is spherical and moves at the same velocity as the homogeneous stellar wind, we estimate a mass and radius of Mcl \simeq1.4\times10^(22) g and Rcl \simeq8\times10^(11) cm. These are in qualitative agreement with values expected from theoretical calculations. We found no evidence of pulsations at \sim4.7 s after investigating coherent modulations in the range 3.5 ms-100 s. A reanalysis of the archival ASCA and Swift data of IGR J18410-0535, for which these pulsations were previously detected, revealed that they were likely to be due to a statistical fluctuation and an instrumental effect, respectively.

astro-ph.HE↗

arXiv2011

INTEGRAL and Swift observations of IGRJ19294+1816 in outburst

IGRJ19294+1816 was discovered by INTEGRAL in 2009 during a bright X-ray outburst and was classified as a possible Be X-ray binary or supergiant fast X-ray transient. On 2010 October 28, the source displayed a second X-ray outburst and a 2 months-long monitoring with Swift was carried out to follow the evolution of the source X-ray flux during the event. We report on the INTEGRAL and Swift observations of the second X-ray outburst observed from IGRJ19294+1816. We detected pulsations in the X-ray emission from the source at \sim12.5 s up to 50 keV. The source X-ray flux decreased smoothly during the two months of observation displaying only marginal spectral changes. Due to the relatively rapid decay of the source X-ray flux, no significant variations of the source spin period across the event could be measured. This prevented a firm confirmation of the previously suggested orbital period of the source at 117 d. This periodicity was also searched by using archival Swift /BAT data. We detected a marginally significant peak in the periodogram and determined the best period at 116.2\pm0.6 days (estimated chance probability of a spurious detection 1%). The smooth decline of the source X-ray flux across the two months of observations after the onset of the second outburst, together with its relatively low value of the spin period and the absence of remarkable changes in the spectral parameters (i.e., the absorption column density), suggests that IGRJ19294+1816 is most likely another member of the Be X-ray binaries discovered by INTEGRAL and not a supergiant fast X-ray transient.

astro-ph.HE↗

arXiv2011

The nature of the absorber in the INTEGRAL highly obscured sgHMXB IGR J17252-3616

INTEGRAL played a key role in discovering obscured sgHMXB in the Galaxy. We used XMM-Newton to perform X-ray wind tomography of a specific of these systems, IGR J17252-3616, featuring eclipses of the accreting pulsar. The X-ray band (0.2-10 keV) reveals vital information on the geometry of the surrounding gas probing simultaneously the absorption and the fluorescence emission. The XMM observations were scheduled to cover as many orbital phases as possible. Timing analysis allows the derivation of an accurate orbital solution and of the system parameters. Spectral analysis revealed remarkable variations of the absorbing column density along the orbit and of the Fe K$α$ fluorescence line around the eclipse. The combination of these observables revealed a highly asymmetric and unprecedentedly extended structure in the stellar wind extending up to 2-3 stellar radii. The observations can be modeled in terms of three independent components: i) the unperturbed stellar wind ii) the contribution of a highly asymmetric hydrodynamic wind tail-like structure and iii) a cusp of material close to the neutron star. These dynamical structures are imaged for the first time in a sgHMXB and explain the source of the high obscuration.

astro-ph.HE↗

arXiv2011

Study of the X-ray/gamma source AX J1910.7+0917 and three newly discovered INTEGRAL sources

AX J1910.7+0917 is a still unidentified source discovered with ASCA and observed more recently with IBIS/ISGRI, mainly noticeable for its rather hard spectrum. We analyzed all the public available data on this source, and we took advantage of the recent improvements performed in the INTEGRAL data analysis software to fully exploit the IBIS/ISGRI data. In the data collected from INTEGRAL, XMM-Newton, Chandra and ASCA the source is clearly variable. The spectrum can be modelled as an absorbed powerlaw (NH~6x10^22 cm^-2, Gamma=1.5) with an iron line at 6.4 keV. The present data still do not allow for a unique classification of the source. 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, where the latter is positionally coincident with the Einstein source 2E 1912.5+1031. For the first two sources we report results obtained from follow-up observations carried out with Swift/XRT.

astro-ph.HE↗

arXiv2011

INTEGRAL and the Unified Model of AGN

According to the unified model, the differences observed in different classes of AGN are due to anisotropic obscuration. This implies that in the hard X-rays, at energies where the radiation can pierce the obscuring material, the average spectral characteristics of different classes of AGN should be the same. Here we present a study of the 17-250 keV INTEGRAL IBIS/ISGRI average spectra of Seyfert galaxies, aimed to constrain similarities and possible differences between different classes of AGN. The sample we consists of all the 172 Seyfert galaxies detected by INTEGRAL IBIS/ISGRI at z<0.2 during its first 7 years of observations. Of these, 45 are Seyfert 1s, 31 Seyfert 1.5s, 67 Compton-thin Seyfert 2s, 11 Narrow Line Seyfert 1s, 11 Compton-thick Seyfert 2s, and 9 LINERs. Comparing the average hard X-ray spectra of the different classes we found that, while the spectra of Seyfert 1s, Seyfert 1.5 and NLS1s are consistent, Compton-thin Seyfert 2 show a clear excess in the 20-60 keV band over the spectrum of Seyfert 1s and Seyfert 1.5s. This excess might be due to an intrinsically stronger reflection component or to a clumpy structure of the absorber in Compton-thin Seyfert 2s.

astro-ph.CO↗

arXiv2011

Compton processes in the bright AGN MCG+8-11-11

We present preliminary results on the hard X-ray emission properties of the Seyfert 1.5 galaxy MCG+8-11-11 as observed by INTEGRAL and SWIFT. All the INTEGRAL IBIS/ISGRI data available up to October 2009 have been analyzed together with two SWIFT/XRT snapshot observations performed in August and October 2009, quasi-simultaneously to INTEGRAL pointed observations of MCG+8-11-11. No correlation is observed between the hard X-ray flux and the spectral slope, while the position of the high-energy cut-off is found to have varied during the INTEGRAL observations. This points to a change in the temperature of the Comptonising medium from a minimum value of kT = 30-50 keV to values larger than 100-150 keV. There is no significant detection of Compton reflection, with a 3 sigma upper limit of R < 0.2, and no line has been detected at 112 keV, as previously claimed from HEAT observations (112 keV flux F < 2.4e-4 ph/cm^2/s). The variability behaviour of MCG+8-11-11 is found to be similar to that shown by IC 4329A, with different temperatures of the electron plasma for similar flux levels of the source, while other bright Seyfert galaxies present different variability patterns at hard X-rays, with spectral changes correlated to flux variations (e.g. NGC 4151).

astro-ph.HE↗

arXiv2011

Twelve-hour spikes from the Crab Pevatron

Aims. The Crab nebula displayed a large gamma-ray flare on September 18, 2010. To more closely understand the origin of this phenomenon, we analyze the INTEGRAL (20-500 keV) and FERMI (0.1-300 GeV) data collected almost simultaneously during the flare. Methods. We divide the available data into three different sets, corresponding to the pre-flare period, the flare, and the subsequent quiescence. For each period, we perform timing and spectral analyses to differentiate between the contributions of the pulsar and from the surrounding nebula to the gamma-ray luminosity. Results. No significant variations in the pulse profile and spectral characteristics are detected in the hard X-ray domain. In contrast, we identify three separate enhancements in the gamma-ray flux lasting for about 12 hours and separated by an interval of about two days from each other. The spectral analysis shows that the flux enhancement, confined below ~ 1 GeV, can be modelled by a power-law with a high energy exponential cut-off, where either the cut-off energy or the model normalization increased by a factor of ~ 5 relative to the pre-flare emission. We also confirm that the gamma-ray flare is not pulsed. Conclusions. The timing and spectral analysis indicate that the gamma-ray flare is due to synchrotron emission from a very compact Pevatron located in the region of interaction between the pulsar wind and the surrounding nebula. These are the highest electron energies ever measured in a cosmic accelerator. The spectral properties of the flare are interpreted in the framework of a relativistically moving emitter and/or a harder emitting electron population.

astro-ph.HE↗

arXiv2010

Suzaku captures a possible eclipse in IGR J16207-5129 and identifies a weak-flaring state in IGR J17391-3021

We present the results from analyses of Suzaku observations of the supergiant X-ray binaries IGR J16207-5129 and IGR J17391-3021. For IGR J16207-5129, we provide the first broadband (0.5--60 keV) spectrum from which we confirm a large intrinsic column density (nH = 16e22 /cm2), and constrain the cutoff energy for the first time (Ec = 19 keV). We observed a prolonged (> 30 ks) attenuation of the X-ray flux which we tentatively attribute to an eclipse of the probable neutron star by its massive companion. For IGR J17391-3021, we witnessed a transition from quiescence to a low-activity phase punctuated by weak flares whose peak luminosities in the 0.5--10 keV band are only a factor of 5 times that of the pre-flare emission. The weak flaring is accompanied by an increase in the absorbing column which suggests the accretion of obscuring clumps of wind. Placing this observation in the context of the recent Swift monitoring campaign, we now recognize that these low-activity epochs constitute the most common emission phase for this system, and perhaps in other SFXTs as well.

astro-ph.HE↗

arXiv2010

AX J1910.7+0917 and three newly discovered INTEGRAL sources

We take advantage of the high sensitivity of the IBIS/ISGRI telescope and the improvements in the data analysis software to investigate the nature of the still poorly known X-ray source AX J1910.7+0917, and search for close-by previously undetected objects. We analyze all publicly available INTEGRAL data of AX J1910.7+0917, together with a number of archival observations that were carried out in the direction of the source with Chandra, XMM-Newton, and ASCA. 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; the latter is positionally coincident with the Einstein source 2E 1912.5+1031. For the first two sources, we also report the results of follow-up observations carried out with Swift/XRT. AX J1910.7+0917 features a clear variability in the X-rays. Its spectrum can be well described with an absorbed (N_H~6x10^(22) cm^(-2)) power-law (Γ~1.5) model plus an iron line at ~6.4 keV. We also obtained a refined position and report on possible infrared counterparts. The present data do not allow for a unique classification of the sources. Based on the property of its X-ray emission and the analysis of a likely infrared counterpart, we investigate different possibilities for the nature of AX J1910.7+0917.

astro-ph.HE↗

arXiv2010

Suzaku observes weak flares from IGR J17391-3021 representing a common low-activity state in this SFXT

We present an analysis of a 37-ks observation of the supergiant fast X-ray transient (SFXT) IGR J17391-3021 (=XTE J1739-302) gathered with Suzaku. The source evolved from quiescence to a low-activity level culminating in three weak flares lasting ~3 ks each in which the peak luminosity is only a factor of 5 times that of the pre-flare luminosity. The minimum observed luminosity was 1.3x10^33 erg/s (d/2.7 kpc)^2 in the 0.5--10 keV range. The weak flares are accompanied by significant changes in the spectral parameters including a column density (nH = (4.1+-0.5)x10^22 /cm^2) that is ~2--9 times the absorption measured during quiescence. Accretion of obscuring clumps of stellar wind material can explain both the small flares and the increase in nH. Placing this observation in the context of the recent Swift monitoring campaign, we find that weak-flaring episodes, or at least epochs of enhanced activity just above the quiescent level but well below the moderately bright or high-luminosity outbursts, represent more than 60+-5% of all observations in the 0.5--10keV energy range making this the most common state in the emission behavior of IGR J17391-3021

astro-ph.HE↗

arXiv2010

X-Ray Wind Tomography of the highly absorbed HMXB IGR J17252-3616

Our goal is to understand the specificities of highly absorbed sgHMXB and in particular of the companion stellar wind, thought to be responsible for the strong absorption. We have monitored IGR J17252-3616, a highly absorbed system featuring eclipses, with XMM-Newton to study the vari- ability of the column density and of the Fe Kα emission line along the orbit and during the eclipses. We also built a 3D model of the structure of the stellar wind to reproduce the observed variability. We first derived a refined orbital solution built from INTEGRAL, RXTE and XMM data. The XMM monitoring campaign revealed significant variation of intrinsic absorbing column density along the orbit and of the Fe Kα line equivalent width around the eclipses. The origin of the soft X-ray absorption is modeled with an dense and extended hydrodynamical tail, trailing the neutron star. This structure extends along most of the orbit, indicating that the stellar wind is strongly disrupted by the neutron star. The variability of the absorbing column density suggests that the terminal velocity of the wind is smaller (~400 km/s) than observed in classical systems. This can also explain the much stronger density perturbation inferred from the observations. Most of the Fe Kα emission is generated in the most inner region of the hydrodynamical tail. This region, that extends over a few accretion radii, is ionized and does not contribute to the soft X-ray absorption. We have built a qualitative model of the stellar wind of IGR J17252-3616 that can represent the observations and suggest that highly absorbed systems have a lower wind velocity than classical sgHMXB. This proposal could be tested with de- tailed numerical simulations and high-resolution infrared/optical observations. If confirmed, it may turn out that half of the persistent sgHMXB have low stellar wind speeds.

astro-ph.HE↗

arXiv2010

η Carinae: a very large hadron collider

Eta Carinae is the colliding wind binary with the largest mass loss rate in our Galaxy and the only one in which hard X-ray emission has been detected. Eta Carinae is therefore a primary candidate to search for particle acceleration by probing its gamma-ray emission. We used the first 21 months of Fermi/LAT data to extract gamma-ray (0.2-100 GeV) images, spectra and light-curves, then combined them with multi-wavelength observations to model the non-thermal spectral energy distribution. A bright gamma-ray source is detected at the position of eta Carinae. Its flux at a few 100 MeV corresponds very well to the extrapolation of the hard X-ray spectrum towards higher energies. The spectral energy distribution features two distinct components. The first one extends over the keV to GeV energy range, and features an exponential cutoff at ~ 1 GeV. It can be understood as inverse Compton scattering of ultraviolet photons by electrons accelerated up to gamma~1E4 in the colliding wind region. The expected synchrotron emission is compatible with the existing upper limit on the non-thermal radio emission. The second component is a hard gamma-ray tail detected above 20 GeV. It could be explained by pi0-decay of accelerated hadrons interacting with the dense stellar wind. The ratio between the fluxes of the pi0 and inverse Compton components is roughly as predicted by simulations of colliding wind binaries. This hard gamma-ray tail can only be understood if emitted close to the wind collision region. The energy transferred to the accelerated particles (~5% of the collision mechanical energy) is comparable to that of the thermal X-ray emission. We have measured the electron spectrum responsible for the keV to GeV emission and detected an evidence of hadronic acceleration in eta Carinae. These observations are thus in good agreement with the colliding wind scenario suggested for eta Carinae.

astro-ph.HE↗

arXiv2010

HESS J1632-478: an energetic relic

HESS J1632-478 is an extended and still unidentified TeV source in the galactic plane. In order to identify the source of the very high energy emission and to constrain its spectral energy distribution, we used a deep observation of the field obtained with XMM-Newton together with data from Molonglo, Spitzer and Fermi to detect counterparts at other wavelengths. The flux density emitted by HESS J1632-478 peaks at very high energies and is more than 20 times weaker at all other wavelengths probed. The source spectrum features two large prominent bumps with the synchrotron emission peaking in the ultraviolet and the external inverse Compton emission peaking in the TeV. HESS J1632-478 is an energetic pulsar wind nebula with an age of the order of 10^4 years. Its bolometric (mostly GeV-TeV) luminosity reaches 10% of the current pulsar spin down power. The synchrotron nebula has a size of 1 pc and contains an unresolved point-like X-ray source, probably the pulsar with its wind termination shock.

astro-ph.SR↗

arXiv2010

Extreme flux states of NGC 4151 observed with INTEGRAL

We present a comprehensive spectral analysis of all INTEGRAL data obtained so far for the X-ray--bright Seyfert galaxy NGC 4151. We also use all contemporaneous data from RXTE, XMM, Swift and Suzaku. We find a linear correlation between the medium and hard-energy X-ray fluxes measured by INTEGRAL, which indicates an almost constant spectral index over six years. The majority of INTEGRAL observations were made when the source was either at a very bright or very dim hard--X-ray state. We find that thermal Comptonization models applied to the bright state yields the plasma temperature of 50--70 keV and its optical depth of 1.3--2.6, depending on the assumed source geometry. For the dim state, these parameters are in the ranges of 180--230 keV and 0.3--0.7, respectively. The Compton parameter is y = 1 for all the spectra, indicating a stable geometry. Using this result, we can determine the reflection effective solid angles associated with the close and distant reprocessing media as = 0.3 x 2pi and 0.2 x 2pi, respectively. The plasma energy balance, the weak disc reflection and a comparison of the UV fluxes illuminating the plasma to the observed ones are all consistent with an inner hot accretion surrounded by an outer cold disc. The disc truncation radius can be determined from an approximate equipartition between the observed UV and X-ray emission, and from the fitted disc blackbody model, as 15 gravitational radii. Alternatively, our results can be explained by a mildly relativistic coronal outflow.

astro-ph.CO↗

arXiv2010

INTEGRAL hard X-ray spectra of the cosmic X-ray background and Galactic ridge emission

We derive the spectra of the cosmic X-ray background (CXB) and of the Galactic ridge X-ray emission (GRXE) in the ~20-200 keV range from the data of the IBIS instrument aboard the INTEGRAL satellite obtained during the four dedicated Earth-occultation observations of early 2006. We analyse the modulation of the IBIS/ISGRI detector counts induced by the passage of the Earth through the field of view of the instrument. Unlike previous studies, we do not fix the spectral shape of the various contributions, but model instead their spatial distribution and derive for each of them the expected modulation of the detector counts. The spectra of the diffuse emission components are obtained by fitting the normalizations of the model lightcurves to the observed modulation in different energy bins. The obtained CXB spectrum is consistent with the historic HEAO-1 results and falls slightly below the spectrum derived with Swift/BAT. A 10% higher normalization of the CXB cannot be completely excluded, but it would imply an unrealistically high albedo of the Earth. The derived spectrum of the GRXE confirms the presence of a minimum around 80 keV with improved statistics and yields an estimate of ~0.6 M_Sun for the average mass of white dwarfs in the Galaxy. The analysis also provides updated normalizations for the spectra of the Earth's albedo and the cosmic-ray induced atmospheric emission.

astro-ph.CO↗

arXiv2009

Exceptional flaring activity of the anomalous X-ray pulsar 1E 1547.0-5408

(Abridged) We studied an exceptional period of activity of the anomalous X-ray pulsar 1E 1547.0-5408 in January 2009, during which about 200 bursts were detected by INTEGRAL. The major activity episode happened when the source was outside the field of view of all the INTEGRAL instruments. But we were still able to study the properties of 84 bursts detected simultaneously by the anti-coincidence shield of the spectrometer SPI and by the detector of the imager ISGRI. We find that the luminosity of the 22 January 2009 bursts of 1E 1547.0-5408 was > 1e42 erg/s. This luminosity is comparable to that of the bursts of soft gamma repeaters (SGR) and is at least two orders of magnitude larger than the luminosity of the previously reported bursts from AXPs. Similarly to the SGR bursts, the brightest bursts of 1E 1547.0-5408 consist of a short spike of ~100 ms duration with a hard spectrum, followed by a softer extended tail of 1-10 s duration, which occasionally exhibits pulsations with the source spin period of ~2 s. The observation of AXP bursts with luminosities comparable to the one of SGR bursts strengthens the conjecture that AXPs and SGRs are different representatives of one and the same source type.

astro-ph.HE↗