arXiv subjects

R. Terrier

Publications and source records attributed to R. Terrier.

At least 37 records · Page 2Linked to original sources

arXiv2015

Study of the VHE diffuse emission in the central 200 pc of our Galaxy with H.E.S.S

The very high energy emission from the Galactic Center Ridge was revealed by the High Energy Stereoscopic System (H.E.S.S.) in 2006, after subtraction of the point sources HESS J1745-290, possibly associated with Sgr A$^\star$, and HESS J1747$-$281, associated with the composite supernova remnant G0.9$+$0.1. The hard spectrum of the Ridge emission and its spatial correlation with the local gas density suggest that the emission is due to collisions of multi-TeV cosmic rays with the dense clouds of interstellar gas present in this region. The much larger H.E.S.S. dataset (250 hrs) that is now available from this region and the improved analysis method dedicated to the detection of faint emission allow us to reconsider the characterization of this gamma-ray emission in the central 200 pc of our Galaxy through a detailed morphology study. To test the various contributions to the total gamma-ray emission, we use a 2D maximum likelihood approach that allows to constrain a phenomenological model of the signal. We discuss the nature of the various components, and their implication on the cosmic-ray distribution in the central region of our Galaxy. Finally, we will reveal an additional source in this region and will discuss its potential nature.

astro-ph.HE↗

arXiv2015

Search for new supernova remnant shells in the Galactic plane with H.E.S.S.

Amongst the population of TeV gamma-ray sources detected with the High Energy Stereoscopic System (H.E.S.S.) in the Galactic plane, clearly identified supernova remnant (SNR) shells constitute a small but precious source class. TeV-selected SNRs are prime candidates for sources of efficient cosmic-ray acceleration. In this work, we present new SNR candidates that have been identified in the entire H.E.S.S. phase I data set of the Galactic plane recorded over the past ten years. Identification with a known SNR shell candidate was successful for one new source, HESS J1534-571. In other cases, TeV-only shell candidates are challenging to firmly identify as SNRs due to their lack of detected non-thermal emission in lower energy bands. We will discuss how these objects may present an important link between young and evolved SNRs, since their shell emission may be dominated by hadronic processes.

astro-ph.HE↗

arXiv2015

The XMM-Newton view of the central degrees of the Milky Way

The deepest XMM-Newton mosaic map of the central 1.5 deg of the Galaxy is presented, including a total of about 1.5 Ms of EPIC-pn cleaned exposures in the central 15" and about 200 ks outside. This compendium presents broad-band X-ray continuum maps, soft X-ray intensity maps, a decomposition into spectral components and a comparison of the X-ray maps with emission at other wavelengths. Newly-discovered extended features, such as supernova remnants (SNRs), superbubbles and X-ray filaments are reported. We provide an atlas of extended features within +-1 degree of Sgr A*. We discover the presence of a coherent X-ray emitting region peaking around G0.1-0.1 and surrounded by the ring of cold, mid-IR-emitting material known from previous work as the "Radio Arc Bubble" and with the addition of the X-ray data now appears to be a candidate superbubble. Sgr A's bipolar lobes show sharp edges, suggesting that they could be the remnant, collimated by the circumnuclear disc, of a SN explosion that created the recently discovered magnetar, SGR J1745-2900. Soft X-ray features, most probably from SNRs, are observed to fill holes in the dust distribution, and to indicate a direct interaction between SN explosions and Galactic center (GC) molecular clouds. We also discover warm plasma at high Galactic latitude, showing a sharp edge to its distribution that correlates with the location of known radio/mid-IR features such as the "GC Lobe". These features might be associated with an inhomogeneous hot "atmosphere" over the GC, perhaps fed by continuous or episodic outflows of mass and energy from the GC region.

astro-ph.HE↗

arXiv2015

Fifteen years of XMM-Newton and Chandra monitoring of Sgr A*: Evidence for a recent increase in the bright flaring rate

We present a study of the X-ray flaring activity of Sgr A* during all the 150 XMM-Newton and Chandra observations pointed at the Milky Way center over the last 15 years. This includes the latest XMM-Newton and Chandra campaigns devoted to monitoring the closest approach of the very red Br-Gamma emitting object called G2. The entire dataset analysed extends from September 1999 through November 2014. We employed a Bayesian block analysis to investigate any possible variations in the characteristics (frequency, energetics, peak intensity, duration) of the flaring events that Sgr A* has exhibited since their discovery in 2001. We observe that the total bright-or-very bright flare luminosity of Sgr A* increased between 2013-2014 by a factor of 2-3 (~3.5 sigma significance). We also observe an increase (~99.9% significance) from 0.27+-0.04 to 2.5+-1.0 day^-1 of the bright-or-very bright flaring rate of Sgr A*, starting in late summer 2014, which happens to be about six months after G2's peri-center passage. This might indicate that clustering is a general property of bright flares and that it is associated with a stationary noise process producing flares not uniformly distributed in time (similar to what is observed in other quiescent black holes). If so, the variation in flaring properties would be revealed only now because of the increased monitoring frequency. Alternatively, this may be the first sign of an excess accretion activity induced by the close passage of G2. More observations are necessary to distinguish between these two hypotheses.

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arXiv2015

Fermi/LAT Study of the Cygnus Loop Supernova Remnant: Discovery of a Point-like Source and of Spectral Differences in its gamma-ray emission

The Cygnus Loop is a nearby supernova remnant (SNR) observed across the electromagnetic spectrum. With the analysis of 6 years of Fermi/LAT data we find that, what previous studies had considered a single source, consists of an extended source plus a point-like source south-east of the SNR. The extended gamma-ray emission is well correlated with the thermal X-ray emission of the SNR, and the energy spectrum displays a pronounced maximum at $\sim0.6$\,GeV. However, in a region where the radio emission is strongly and distinctly polarized, the gamma-ray spectrum shows no sign of a break. Therefore, the spatially resolved gamma-ray emission permits the study of different interaction conditions of the SNR and the surrounding medium.

astro-ph.HE↗

arXiv2014

The reflection of two past outbursts of Sagittarius A* observed by Chandra during the last decade

The supermassive black hole at the Galactic center, Sagittarius A*, has experienced periods of higher activity in the past. The reflection of these past outbursts is observed in the molecular material surrounding the black hole but reconstructing its precise lightcurve is difficult since the distribution of the clouds along the line of sight is poorly constrained. Using Chandra high-resolution data collected from 1999 to 2011 we studied both the 6.4 keV and the 4-8 keV emission of the region located between Sgr A* and the Radio Arc, characterizing its variations down to 15" angular scale and 1-year time scale. The emission from the molecular clouds in the region varies significantly, showing either a 2-year peaked emission or 10-year linear variations. This is the first time that such fast variations are measured. Based on the cloud parameters, we conclude that these two behaviors are likely due to two distinct past outbursts of Sgr A* during which its luminosity rose to at least 10^39 erg/s.

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arXiv2014

On the Fe K absorption - accretion state connection in the Galactic center neutron star X-ray binary AX J1745.6-2901

AX J1745.6-2901 is a high-inclination (eclipsing) neutron star Low Mass X-ray Binary (LMXB) located less than ~1.5 arcmin from Sgr A*. Ongoing monitoring campaigns have targeted Sgr A* frequently and these observations also cover AX J1745.6-2901. We present here an X-ray analysis of AX J1745.6-2901 using a large dataset of 38 XMM-Newton observations, including eleven which caught AX J1745.6-2901 in outburst. Fe K absorption is clearly seen when AX J1745.6-2901 is in the soft state, but disappears during the hard state. The variability of these absorption features does not appear to be due to changes in the ionizing continuum. The small Kalpha/Kbeta ratio of the equivalent widths of the Fe xxv and Fe xxvi lines suggests that the column densities and turbulent velocities of the absorbing ionised plasma are in excess of N_H ~ 10^23 cm^-2 and v_turb >~ 500 km s^-1. These findings strongly support a connection between the wind (Fe K absorber) and the accretion state of the binary. These results reveal strong similarities between AX J1745.6-2901 and the eclipsing neutron star LMXB, EXO 0748-676, as well as with high-inclination black hole binaries, where winds (traced by the same Fe K absorption features) are observed only during the accretion-disc-dominated soft states, and disappear during the hard states characterised by jet emission.

astro-ph.HE↗

arXiv2014

Variation of the X-ray non-thermal emission in the Arches cloud

The origin of the iron fluorescent line at 6.4 keV from an extended region surrounding the Arches cluster is debated and the non-variability of this emission up to 2009 has favored the low-energy cosmic-ray origin over a possible irradiation by hard X-rays. By probing the variability of the Arches cloud non-thermal emission in the most recent years, including a deep observation in 2012, we intend to discriminate between the two competing scenarios. We perform a spectral fit of XMM-Newton observations collected from 2000 to 2013 in order to build the Arches cloud lightcurve corresponding to both the neutral Fe Kalpha line and the X-ray continuum emissions. We reveal a 30% flux drop in 2012, detected with more than 4 sigma significance for both components. This implies that a large fraction of the studied non-thermal emission is due to the reflection of an X-ray transient source.

astro-ph.HE↗

arXiv2013

Echoes of multiple outbursts of Sagittarius A* revealed by Chandra

The relatively rapid spatial and temporal variability of the X-ray radiation from some molecular clouds near the Galactic center shows that this emission component is due to the reflection of X-rays generated by a source that was luminous in the past, most likely the central supermassive black hole, Sagittarius A*. Studying the evolution of the molecular cloud reflection features is therefore a key element to reconstruct Sgr A*'s past activity. The aim of the present work is to study this emission on small angular scales in order to characterize the source outburst on short time scales. We use Chandra high-resolution data collected from 1999 to 2011 to study the most rapid variations detected so far, those of clouds between 5' and 20' from Sgr A* towards positive longitudes. Our systematic spectral-imaging analysis of the reflection emission, notably of the Fe Kalpha line at 6.4 keV and its associated 4-8 keV continuum, allows us to characterize the variations down to 15" angular scale and 1-year time scale. We reveal for the first time abrupt variations of few years only and in particular a short peaked emission, with a factor of 10 increase followed by a comparable decrease, that propagates along the dense filaments of the Bridge cloud. This 2-year peaked feature contrasts with the slower 10-year linear variations we reveal in all the other molecular structures of the region. Based on column density constraints, we argue that these two different behaviors are unlikely to be due to the same illuminating event. The variations are likely due to a highly variable active phase of Sgr A* sometime within the past few hundred years, characterized by at least two luminous outbursts of a few-year time scale and during which the Sgr A* luminosity went up to at least 10^39 erg/s.

astro-ph.HE↗

arXiv2013

The origin of the 6.4 keV line emission and H$_2$ ionization in the diffuse molecular gas of the Galactic center region

We investigate the origin of the diffuse 6.4 keV line emission recently detected by Suzaku and the source of H_2ionization in the diffuse molecular gas of the Galactic Center (GC) region. We show that Fe atoms and H_2 molecules in the diffuse interstellar medium of the GC are not ionized by the same particles. The Fe atoms are most likely ionized by X-ray photons emitted by Sgr A* during a previous period of flaring activity of the supermassive black hole. The measured longitudinal intensity distribution of the diffuse 6.4 keV line emission is best explained if the past activity of Sgr A$* lasted at least several hundred years and released a mean 2-100 keV luminosity > 10^38} erg s^{-1}. The H_2 molecules of the diffuse gas can not be ionized by photons from Sgr A*, because soft photons are strongly absorbed in the interstellar gas around the central black hole. The molecular hydrogen in the GC region is most likely ionized by low-energy cosmic rays, probably protons rather than electrons, whose contribution into the diffuse 6.4 keV line emission is negligible.

astro-ph.HE↗

arXiv2013

Acceleration of cosmic rays and gamma-ray emission from supernova remnants in the Galaxy

Galactic cosmic rays are believed to be accelerated at supernova remnant shocks. Though very popular and robust, this conjecture still needs a conclusive proof. The strongest support to this idea is probably the fact that supernova remnants are observed in gamma-rays, which are indeed expected as the result of the hadronic interactions between the cosmic rays accelerated at the shock and the ambient gas. However, also leptonic processes can, in most cases, explain the observed gamma-ray emission. This implies that the detections in gamma rays do not necessarily mean that supernova remnants accelerate cosmic ray protons. To overcome this degeneracy, the multi-wavelength emission (from radio to gamma rays) from individual supernova remnants has been studied and in a few cases it has been possible to ascribe the gamma-ray emission to one of the two processes (hadronic or leptonic). Here we adopt a different approach and, instead of a case-by-case study we aim for a population study and we compute the number of supernova remnants which are expected to be seen in TeV gamma rays above a given flux under the assumption that these objects indeed are the sources of cosmic rays. The predictions found here match well with current observational results, thus providing a novel consistency check for the supernova remnant paradigm for the origin of galactic cosmic rays. Moreover, hints are presented for the fact that particle spectra significantly steeper than E^-2 are produced at supernova remnants. Finally, we expect that several of the supernova remnants detected by H.E.S.S. in the survey of the galactic plane should exhibit a gamma-ray emission dominated by hadronic processes (i.e. neutral pion decay). The fraction of the detected remnants for which the leptonic emission dominates over the hadronic one depends on the assumed values of the physical parameters and can be as high as roughly a half.

astro-ph.HE↗

arXiv2012

A new nearby pulsar wind nebula overlapping the RX J0852.0-4622 supernova remnant

Energetic pulsars can be embedded in a nebula of relativistic leptons which is powered by the dissipation of the rotational energy of the pulsar. The object PSR J0855-4644 is an energetic and fast-spinning pulsar (Edot = 1.1x10^36 erg/s, P=65 ms) discovered near the South-East rim of the supernova remnant (SNR) RX J0852.0-4622 (aka Vela Jr) by the Parkes multibeam survey. The position of the pulsar is in spatial coincidence with an enhancement in X-rays and TeV gamma-rays, which could be due to its putative pulsar wind nebula (PWN). The purpose of this study is to search for diffuse non-thermal X-ray emission around PSR J0855-4644 to test for the presence of a PWN and to estimate the distance to the pulsar. An X-ray observation was carried out with the XMM-Newton satellite to constrain the properties of the pulsar and its nebula. The absorption column density derived in X-rays from the pulsar and from different regions of the rim of the SNR was compared with the absorption derived from the atomic (HI) and molecular (12CO) gas distribution along the corresponding lines of sight to estimate the distance of the pulsar and of the SNR. The observation has revealed the X-ray counterpart of the pulsar together with surrounding extended emission thus confirming the existence of a PWN. The comparison of column densities provided an upper limit to the distance of the pulsar PSR J0855-4644 and the SNR RX J0852.0-4622 (d<900 pc). Although both objects are at compatible distances, we rule out that the pulsar and the SNR are associated. With this revised distance, PSR J0855-4644 is the second most energetic pulsar, after the Vela pulsar, within a radius of 1 kpc and could therefore contribute to the local cosmic-ray e-/e+ spectrum.

astro-ph.HE↗

arXiv2012

Observational signatures of particle acceleration in supernova remnants

We evaluate the current status of supernova remnants as the sources of Galactic cosmic rays. We summarize observations of supernova remnants, covering the whole electromagnetic spectrum and describe what these obser- vations tell us about the acceleration processes by high Mach number shock fronts. We discuss the shock modification by cosmic rays, the shape and maximum energy of the cosmic-ray spectrum and the total energy budget of cosmic rays in and surrounding supernova remnants. Additionally, we discuss problems with supernova remnants as main sources of Galactic cosmic rays, as well as alternative sources.

astro-ph.HE↗

arXiv2011

First high-resolution radio study of the Supernova Remnant G338.3-0.0 associated with the gamma-ray source HESS J1640-465

We report on a multifrequency radio study of the supernova remnant (SNR) G338.3-0.0, in positional coincidence with the TeV source HESS J1640-465. To carry out this research we observed the SNR G338.3-0.0 using the Giant Metrewave Radio Telescope (GMRT) at 235, 610, and 1280 MHz. We also reprocessed archival data from the Australia Telescope Compact Array (ATCA) at 1290 and 2300 MHz. Also we conducted a search for radio pulsations towards a central point-like source, using the GMRT antennas at 610 and 1280 MHz. The molecular material in the region of the SNR was investigated based on observations made with the NANTEN telescope in the 12^CO (J=1-0) emission line. The new radio observations revealed a remnant with a bilateral morphology, which at 235 MHz has the western wing completely attenuated because of absorption due to foreground ionized gas. The quality of the new images allows us to provide accurate estimates for the total radio flux density of the whole SNR at different radio frequencies. From the new and existing flux density estimates between 235 and 5000 MHz we derived for the whole remnant a spectral index alpha=-0.51 +/-0.06 with a local free-free continuum optical depth at 235 MHz tau_{235}=0.9+/-0.3. No radio pulsations were detected towards the only radio point-like source within the HESS error circle. We derived upper limits of 2.0 and 1.0 mJy at 610 and 1280 MHz, respectively, for the pulsed flux towards this source. No radio counterpart was found for the pulsar wind nebula discovered in X-rays. The inspection of the interstellar molecular gas towards G338.3-0.0 and surroundings revealed that there is not any associated dense cloud that might explain a hadronic origin for the TeV detection.

astro-ph.GA↗

arXiv2011

A new nearby PWN overlapping the Vela Jr SNR

PSR J0855-4644 is an energetic pulsar (Edot = 1.1x10^36 erg/s, P=65 ms) discovered near the South-East rim of the supernova remnant (SNR) RX J0852.0-4622 (aka Vela Jr) by the Parkes Multibeam Survey. The position of the pulsar is in spatial coincidence with an enhancement in X-rays and TeV gamma-rays, which could represent its pulsar wind nebula (PWN). We have revealed with an XMM-Newton observation the X-ray counterpart of the pulsar together with a surrounding extended emission thus confirming the suggestion of a PWN. The comparison of the absorption column density derived in X-rays from the pulsar with 12CO observations (tracing the dense gas) is used to derive an upper limit to the distance of the pulsar (d< 900 pc) and to discuss a possible association of the pulsar with the Vela Jr SNR. This new distance estimate implies that the pulsar is nearby and could therefore significantly contribute to the observed spectrum of cosmic-ray leptons (e-/e+).

astro-ph.HE↗

arXiv2011

The Hard X-ray Emission of Cen A

The radio galaxy Cen A has been detected all the way up to the TeV energy range. This raises the question about the dominant emission mechanisms in the high-energy domain. Spectral analysis allows us to put constraints on the possible emission processes. Here we study the hard X-ray emission as measured by INTEGRAL in the 3-1000 keV energy range, in order to distinguish between a thermal and non-thermal inverse Compton process. The hard X-ray spectrum of Cen A shows a significant cut-off at energies Ec = 434 (+106 -73) keV with an underlying power law of photon index 1.73 +- 0.02. A more physical model of thermal Comptonisation (compPS) gives a plasma temperature of kT = 206+-62 keV within the optically thin corona with Compton parameter y = 0.42 (+0.09 -0.06). The reflection component is significant at the 1.9 sigma level with R = 0.12 (+0.09 -0.10), and a reflection strength R>0.3 can be excluded on a 3 sigma level. Time resolved spectral studies show that the flux, absorption, and spectral slope varied in the range f(3-30 keV) = (1.2 - 9.2)e-10 erg/cm**2/s, NH = (7 - 16)e22 1/cm**2, and photon index 1.75 - 1.87. Extending the cut-off power law or the Comptonisation model to the gamma-ray range shows that they cannot account for the high-energy emission. On the other hand, also a broken or curved power law model can represent the data, therefore a non-thermal origin of the X-ray to GeV emission cannot be ruled out. The analysis of the SPI data provides no sign of significant emission from the radio lobes and gives a 3 sigma upper limit of f(40-1000 keV) < 0.0011 ph/cm**2/s. While gamma-rays, as detected by CGRO and Fermi, are caused by non-thermal (jet) processes, the main process in the hard X-ray emission of Cen A is still not unambiguously determined, being either dominated by thermal inverse Compton emission, or by non-thermal emission from the base of the jet.

astro-ph.CO↗

arXiv2011

The COSPIX mission: focusing on the energetic and obscured Universe

Tracing the formation and evolution of all supermassive black holes, including the obscured ones, understanding how black holes influence their surroundings and how matter behaves under extreme conditions, are recognized as key science objectives to be addressed by the next generation of instruments. These are the main goals of the COSPIX proposal, made to ESA in December 2010 in the context of its call for selection of the M3 mission. In addition, COSPIX, will also provide key measurements on the non thermal Universe, particularly in relation to the question of the acceleration of particles, as well as on many other fundamental questions as for example the energetic particle content of clusters of galaxies. COSPIX is proposed as an observatory operating from 0.3 to more than 100 keV. The payload features a single long focal length focusing telescope offering an effective area close to ten times larger than any scheduled focusing mission at 30 keV, an angular resolution better than 20 arcseconds in hard X-rays, and polarimetric capabilities within the same focal plane instrumentation. In this paper, we describe the science objectives of the mission, its baseline design, and its performances, as proposed to ESA.

astro-ph.IM↗

arXiv2011

Concurrent X-ray, near-infrared, sub-millimeter, and GeV gamma-ray observations of Sgr A*

The radiative counterpart of the supermassive black hole at the Galactic center (GC), Sgr A*, is subject to frequent flares visible simultaneously in X-rays and near-infrared (NIR). Often, enhanced radio variability from centimeter to sub-millimeter wavelengths is observed to follow these X-ray/NIR eruptions. We present here a multi-wavelength campaign carried out in April 2009, with the aim of characterizing this broadband flaring activity. Concurrent data from the XMM-Newton/EPIC (2-10 keV), VLT/NACO (2.1 microns, 3.8 microns), APEX/LABOCA (870 microns), and Fermi/LAT (0.1-200 GeV) instruments are employed to derive light curves and spectral energy distributions of new flares from Sgr A*. We detected two relatively bright NIR flares both associated with weak X-ray activity, one of which was followed by a strong sub-mm outburst 200 min later. Photometric spectral information on a NIR flare was obtained for the first time with NACO giving a power-law photon index alpha=-0.4\pm0.3. The first attempt to detect flaring activity from the Fermi GC source 1FGL J1745.6-2900 is also reported. NIR, X-ray, and sub-mm flares are finally modeled in the context of non-thermal emission processes. It is found that the simplest scenario involving a single expanding plasmoid releasing synchrotron NIR/sub-mm and synchrotron self-Compton X-ray radiation is inadequate to reproduce the data, but suggestions to reconcile the basic elements of the theory and the observations are proposed.

astro-ph.HE↗