R. Terrier

Echoes of multiple outbursts of Sagittarius A⋆ revealed by Chandra

Context. 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⋆. Aims. Studying the evolution of the molecular cloud reflection features is therefore a key element in reconstructing Sgrxa0u2009xa0A⋆’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. Methods. 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 Sgrxa0u2009xa0A⋆ toward positive longitudes. Our systematic spectral-imaging analysis of the reflection emission, notably of the Fe Kα line at 6.4 keV and its associated 4–8 keV continuum, allows us to characterize the variations down to a 15′′ angular scale and a 1-year time scale. Results. We reveal for the first time abrupt variations of only a few years and, in particular, a short peaked emission, with a factor of 10 increase followed by a comparable decrease, which propagates along the dense filaments of one 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. Conclusions. The variations likely stem from a highly variable active phase of Sgrxa0u2009xa0A⋆ which occurred sometime within the past few hundred years, and is characterized by at least two luminous outbursts with typical time scales of a few years and during which the Sgrxa0u2009xa0A⋆ luminosity went up to at least 1039 ergu2009s-1.

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 centre over the last 15 years. This includes the latest XMM-Newton and Chandra campaigns devoted to monitoring the closest approach of the very red Brγ emitting object called G2. The entire data set analysed extends from 1999 September through 2014 November. 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 and 2014 by a factor of 2-3 (∼3.5σ significance). We also observe an increase (∼99.9u2009peru2009cent significance) from 0.27 ± 0.04 to 2.5 ± 1.0 d−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 G2s pericentre 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.

The hard X-ray emission of Centaurus A

Context. 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. Aims: Spectral analysis allows us to put constraints on the possible emission processes. Here we study the hard X-ray emission, in order to distinguish between a thermal and a non-thermal inverse Compton process. Methods: Using hard X-ray data provided by INTEGRAL, we determined the cut-off of the power-law spectrum in the hard X-ray domain (3-1000 keV). In addition, INTEGRAL data are used to study the spectral variability. The extended emission detected in the gamma-rays by Fermi/LAT is investigated using the data of the spectrometre SPI in the 40-1000 keV range. Results: The hard X-ray spectrum of Cen A shows a significant cut-off at energies EC = 434 {+106 atop -73} keV with an underlying power-law of photon index Γ = 1.73 ± 0.02. A more physical model of thermal Comptonisation (compPS) gives a plasma temperature of kTe = 206 ± 62 keV within the optically thin corona with Compton parameter y = 0.42 {+0.09 atop -0.06}. The reflection component is significant at the 1.9σ level with R = 0.12 {+0.09 atop -0.10}, and a reflection strength R > 0.3 can be excluded on a 3σ level. Time resolved spectral studies show that the flux, absorption, and spectral slope varied in the range f3-30 keV = 1.2-9.2 × 10-10 erg cm-2 s-1, NH = 7-16 × 1022 cm-2, and Γ = 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, a broken or curved power-law model can also 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 3σ upper limit of f40-1000 keV ≲ 1.1 × 10-3 ph cm-2 s-1. Conclusions: 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, since it is either dominated by thermal inverse Compton emission or by non-thermal emission from the base of the jet. Based on observations with INTEGRAL, an ESA project with instruments and science data centre funded by ESA member states (especially the PI countries: Denmark, France, Germany, Italy, Switzerland and Spain), the Czech Republic, and Poland and with participation of Russia and the US.

On the Fe K absorption – accretion state connection in the Galactic Centre 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 data set of 38 XMM–Newton observations, including 11 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 Kα/Kβ ratio of the equivalent widths of the Fe xxv and Fe xxvi lines suggests that the column densities and turbulent velocities of the absorbing ionized plasma are in excess of NH ≃ 10^(23) cm^(−2) and vturb ≳ 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 characterized by jet emission.

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 (especially the magnetic field strength at the shock) and can be as high as roughly a half.

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

Context. Energetic pulsars can be embedded in a nebula of relativistic leptons that is powered by the dissipation of the rotational energy of the pulsar. The object PSR J0855−4644 is an energetic and fast-spinning pulsar ( u E = 1.1 × 10 36 erg s −1 , P = 65 ms) discovered near the southeast 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 γ-rays, which could be due to its putative pulsar wind nebula (PWN). Aims. 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. Methods. 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 SNR rim was compared with the absorption derived from the atomic (HI) and molecular ( 12 CO) gas distribution along the corresponding lines of sight to estimate the distance to the pulsar and to the SNR. Results. 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 to 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.

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 favoured 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 light curve corresponding to both the neutral Fe Kα line and the X-ray continuum emissions. We reveal a 30 per cent flux drop in 2012, detected with more than 4σ 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.

Extended Hard X-Ray Emission from the Vela Pulsar Wind Nebula

The nebula powered by the Vela pulsar is one of the best examples of an evolved pulsar wind nebula, allowing access to the particle injection history and the interaction with the supernova ejecta. We report on the INTEGRAL discovery of extended emission above 18 keV from the Vela nebula. The northern side has no known counterparts and it appears larger and more significant than the southern one, which is in turn partially coincident with the cocoon, the soft X-ray, and TeV filament toward the center of the remnant. We also present the spectrum of the Vela nebula in the 18-400 keV energy range as measured by IBIS/ISGRI and SPI on board the INTEGRAL satellite. The apparent discrepancy between IBIS/ISGRI, SPI, and previous measurements is understood in terms of the point-spread function, supporting the hypothesis of a nebula more diffuse than previously thought. A break at ~25 keV is found in the spectrum within 6 from the pulsar after including the Suzaku XIS data. Interpreted as a cooling break, this points out that the inner nebula is composed of electrons injected in the last ~2000 years. Broadband modeling also implies a magnetic field higher than 10 μG in this region. Finally, we discuss the nature of the northern emission, which might be due to fresh particles injected after the passage of the reverse shock.

Supernova remnants in the very–high–energy gamma-ray domain: the role of the Cherenkov telescope array

Supernova remnants are often presented as the most probable sources of Galactic cosmic rays. This idea is supported by the accumulation of evidence that particle acceleration is happening at supernova remnant shocks. Observations in the TeV range have especially contributed to increase the understanding of the mechanisms, but many aspects of the particle acceleration at supernova remnant shocks are still debated. The Cherenkov telescope array is expected to lead to the detection of many new supernova remnants in the TeV and multi-TeV range. In addition to the individual study of each, the study of these objects as a population can help constrain the parameters describing the acceleration of particles and increase our understanding of the mechanisms involved.

On the search for Galactic supernova remnant PeVatrons with current TeV instruments

The supernova remnant hypothesis for the origin of Galactic cosmic rays has passed several tests, but the firm identification of a supernova remnant pevatron, considered to be a decisive step to prove the hypothesis, is still missing. While a lot of hope has been placed in next-generation instruments operating in the multi-TeV range, it is possible that current gamma-ray instruments, operating in the TeV range, could pinpoint these objects or, most likely, identify a number of promising targets for instruments of next generation. Starting from the assumption that supernova remnants are indeed the sources of Galactic cosmic rays, and therefore must be pevatrons for some fraction of their lifetime, we investigate the ability of current instruments to detect such objects, or to identify the most promising candidates.