HESS J1616−508: likely to be powered by PSR J1617−5055

HESS J1616−508 is one of the brightest emitters in the TeV sky. Recent observations with the IBIS/ISGRI telescope onboard the INTEGRAL spacecraft have revealed that a young, nearby and energetic pulsar, PSR J1617−5055, is a powerful emitter of soft γ-rays in the 20–100 keV domain. In this paper, we present an analysis of all available data from the INTEGRAL , Swift , BeppoSAX and XMM–Newton telescopes with a view to assessing the most likely counterpart to the High Energy Stereoscopic System (HESS) source. We find that the energy source that fuels the X/γ-ray emissions is derived from the pulsar, both on the basis of the positional morphology, the timing evidence and the energetics of the system. Likewise the 1.2 per cent of the pulsar's spin-down energy loss needed to power the 0.1–10 TeV emission is also fully consistent with other HESS sources known to be associated with pulsars. The relative sizes of the X/γ-ray and very high energy sources are consistent with the expected lifetimes against synchrotron and Compton losses for a single source of parent electrons emitted from the pulsar. We find that no other known object in the vicinity could be reasonably considered as a plausible counterpart to the HESS source. We conclude that there is good evidence to assume that the HESS J1616−508 source is driven by PSR J1617−5055 in which a combination of synchrotron and inverse-Compton processes combine to create the observed morphology of a broad-band emitter from keV to TeV energies.     

Injected spectrum for TeV-γ-ray emission from the galactic center

The detection of very high energy γ-ray emission from the Galactic center has been reported by four independent groups. One of these γ-ray sources, the 10TeV γ-ray radiation reported by HESS, has been suggested as having a hadronic origin when relativistic protons are injected into and interact with the dense ambient gas. Assuming that such relativistic protons required by the hadronic model come from the tidal disruption of a star by the massive black hole of Sgr A*, we explore the spectrum of the relativistic protons. In the calculations, we investigate cases where different types of stars are tidally disrupted by the black hole of Sgr A*, and we consider that different diffusion mechanisms are used for the propagation of protons. The initial energy distribution of the injected spectrum of protons is assumed to follow a power-law with an exponential cut-off, and we derive the different indices of the injected spectra for the tidal disruption of different types of stars. For the best fit to the spectrum of photons detected by HESS, the spectral index of the injected relativistic protons is about 2.05 when a red giant is tidally disrupted by the black hole of Sgr A* and the diffusion mechanism is the Effective Confinement of Protons.     

Hard X-ray observations of PSR J1833−1034 and its associated pulsar wind nebula

PSR J1833−1034 and its associated pulsar wind nebula (PWN) have been investigated in depth through X-ray observations ranging from 0.1 to 200 keV. The low-energy X-ray data from Chandra reveal a complex morphology that is characterized by a bright central plerion, no thermal shell and an extended diffuse halo. The spectral emission from the central plerion softens with radial distance from the pulsar, with the spectral index ranging from  Γ= 1.61  in the central region to  Γ= 2.36  at the edge of the PWN. At higher energy, INTEGRAL detected the source in the 17–200 keV range. The data analysis clearly shows that the main contribution to the spectral emission in the hard X-ray energy range is originated from the PWN, while the pulsar is dominant above 200 keV. Recent High Energy Stereoscopic System (HESS) observations in the high-energy gamma-ray domain show that PSR J1833−1034 is a bright TeV emitter, with a flux corresponding to ∼2 per cent of the Crab in 1–10 TeV range. In addition, the spectral shape in the TeV energy region matches well with that in the hard X-rays observed by INTEGRAL . Based on these findings, we conclude that the emission from the pulsar and its associated PWN can be described in a scenario where hard X-rays are produced through synchrotron light of electrons with Lorentz factor  γ∼ 10⁹  in a magnetic field of ∼10 μG. In this hypothesis, the TeV emission is due to inverse-Compton interaction of the cooled electrons off the cosmic microwave background photons. Search for PSR J1833−1034 X-ray pulsed emission, via RXTE and Swift X-ray observations, resulted in an upper limit that is about 50 per cent.     

Search for TeV Gamma-Rays from 3EG J1234-1318 with the Cangaroo-II Telescope

We observed 3EG J1234-1318 with the CANGAROO-II telescope in March, 2002. This source is a steady unidentified EGRET gamma-ray source at high Galactic latitude which has been associated with a dynamically merging clusters of galaxies, and is therefore a candidate TeV source. TeV gamma-ray emission is also expected as a result of large-scale cosmological structure formation. We here report the resulting upper limit on the TeV gamma-ray emission from 3EG J1234-1318. For the CANGAROO Collaboration.     

High energy processes in pulsar wind nebulae

Young pulsars produce relativistic winds which interact with matter ejected during the supernova explosion and the surrounding interstellar gas. Particles are accelerated to very high energies somewhere in the pulsar winds or at the shocks produced in collisions of the winds with the surrounding medium. As a result of the interactions of relativistic leptons with the magnetic field and low energy radiation (of synchrotron origin, thermal, or microwave background), non-thermal radiation is produced from the lowest possible energies up to ～100 TeV. The high energy (TeV) γ-ray emission has been originally observed from the Crab Nebula and recently from several other objects. Recent observations by the HESS Cherenkov telescopes allow to study for the first time the morphology of the sources of high energy emission, showing unexpected spectral features. They might be also interpreted as due to acceleration of hadrons. However, theory of particle acceleration in the PWNe and models for production of radiation are still at their early stage of development since it becomes clear that realistic modeling of these objects should include their time evolution and three-dimensional geometry. In this paper we concentrate on the attempts to create a model for the high energy processes inside the PWNe which includes existence of not only relativistic leptons but also of hadrons inside the nebula. Such model should also take into account evolution of the nebula in time. Possible high energy expectations based on such a model are discussed in the context of new observations.     

The INTEGRAL – HESS/MAGIC connection: A new class of cosmic high energy accelerators from keV to TeV

The recent completion and operation of the High Energy Stereoscopic System [1], an array of ground based imaging Cherenkov telescopes, has provided a survey with unprecedented sensitivity of the inner part of the Galaxy and revealed a new population of very high energy gamma-rays sources emitting at E > 100 GeV. Most of them were reported to have no known radio or X-ray counterpart and hypothesised to be representative of a new class of dark nucleonic cosmic sources. In fact, very high energy gamma-rays with energies E > 10¹¹ eV are the best proof of non-thermal processes in the universe and provide a direct in-site view of matter-radiation interaction at energies by far greater than producible in ground accelerators. At lower energy INTEGRAL has regularly observed the entire galactic plane during the first 1000 day in orbit providing a survey in the 20–100 keV range resulted in a soft gamma-ray sky populated with more than 200 sources, most of them being galactic binaries, either Black Hole Candidates (BHC) or Neutron Stars (NS) [5]. Very recently, the INTEGRAL new source IGR J18135-1751 has been identified as the soft gamma-ray counterpart of HESS J1813-178 [18] and AXJ1838.0-0655 as the X/gamma-ray counterpart of HESS J1837-069 [14].Detection of non-thermal radio, X and gamma-ray emission from these TeV sources is very important to discriminate between various emitting scenarios and, in turn, to fully understand their nature.The implications of these new findings in the high energy Galactic population will be addressed.On behalf of the IBIS Survey Team     

Prediction of the diffuse neutrino flux from cosmic ray interactions near supernova remnants

In this paper, we present high-energy neutrino spectra from 21 Galactic supernova remnants (SNRs), derived from gamma-ray measurements in the GeV–TeV range. We find that only the strongest sources, i.e. G40.5-0.5 in the north and Vela Junior in the south could be detected as single point sources by IceCube or KM3NeT, respectively. For the first time, it is also possible to derive a diffuse signal by applying the observed correlation between gamma-ray emission and radio signal. Radio data from 234 supernova remnants listed in Green’s catalog are used to show that the total diffuse neutrino flux is approximately a factor of 2.5 higher compared to the sources that are resolved so far. We show that the signal at above 10 TeV energies can actually become comparable to the diffuse neutrino flux component from interactions in the interstellar medium. Recently, the IceCube collaboration announced the detection of a first diffuse signal of astrophysical high-energy neutrinos. Directional information cannot unambiguously reveal the nature of the sources at this point due to low statistics. A number of events come from close to the Galactic center and one of the main questions is whether at least a part of the signal can be of Galactic nature. In this paper, we show that the diffuse flux from well-resolved SNRs is at least a factor of 20 below the observed flux.     

Models of Very-High-Energy Gamma-Ray Emission from the Jets of Microquasars: Orbital Modulation

The recent detection of very-high-energy (GeV – TeV) γ-ray emission from the Galactic black-hole candidate and microquasar LS 5039 has sparked renewed interest in jet models for the high-energy emission in those objects. In this work, we have focused on models in which the high-energy emission results from synchrotron and Compton emission by relativistic electrons in the jet (leptonic jet models). Particular attention has been paid to a possible orbital modulation of the high-energy emission due to azimuthal asymmetries caused by the presence of the companion star. Both orbital-phase dependentγγ absorption and Compton scattering of optical/UV photons from the companion star may lead to an orbital modulation of the gamma-ray emission. We make specific predictions which should be testable with refined data from HESS and the upcoming GLAST mission.     

TeV Gamma Rays From the Galactic Center Direct and Indirect Links to the Massive Black Hole in Sgr A

The recent detection of TeV gamma-radiation from the direction of the Galactic Center within several arc-minutes around Sgr A^* is the first model-independent evidence of existence of high-energy particle accelerator(s) in the central 10 pc region of our Galaxy. This is an extraordinary site that harbours many remarkable objects with the compact radio source Sgr A^* – a hypothetical super-massive black hole (SMBH) – in the dynamical center of the Galaxy. Here we explore the possible direct and indirect links of the reported TeV emission to the SMBH. We show that at least three γ-ray production scenarios that take place close to the event horizon of the SMBH can explain the reported TeV fluxes. An alternative (or additional) channel of TeV radiation is related to the run-away protons accelerated in Sgr A^*. Quasi-continuous injection of relativistic protons into the surrounding dense gas environment initiates detectable high-energy gamma-ray emission. The absolute flux and the energy spectrum of this radiation component strongly depend on the history of particle injection and the character of diffusion of protons during the last 10⁵ yr. For a reasonable combination of a few model parameters, one can explain the detected gamma-ray flux solely by this diffuse component.     

Time-Dependent Synchrotron and Compton Spectra from Microquasar Jets

Jet models for the high-energy emission of Galactic X-ray binary sources have regained significant interest with detailed spectral and timing studies of the X-ray emission from microquasars, the recent detection by the HESS collaboration of very-high-energy γ-rays from the microquasar LS 5039, and the earlier suggestion of jet models for ultraluminous X-ray sources observed in many nearby galaxies. Here we study the synchrotron and Compton signatures of time-dependent electron injection and acceleration, and adiabatic and radiative cooling in the jets of Galactic microquasars.     

Transient X-ray sources in the field of the unidentified gamma-ray source TeV J2032+4130 in Cygnus

We present an analysis of Chandra ACIS observations of the field of TeV J2032+4130, the first unidentified TeV source, detected serendipitously by HEGRA. This deep (48.7 ks) observation of the field follows up on an earlier 5 ks Chandra director’s discretionary observation. Of the numerous point-like X-ray sources in the field, the brightest are shown to be a mixture of early and late-type stars. We find that several of the X-ray sources are transients, exhibiting rapid increases in count rates by factors 3–10, and similar in nature to the one, hard absorbed transient source located in the earlier Chandra observation of the field. None of these transient sources are likely to correspond to the TeV source. Instead, we identify a region of diffuse X-ray emission within the error circle of the TeV source and consider its plausible association.     

A search for young Galactic supernova remnants

A sample of eight small-diameter radio sources has been selected from the Molonglo Galactic Plane Survey (MGPS) as candidates for young Galactic supernova remnants. The sources have been identified in the IRAS and Midcourse Space Experiment infrared data bases and imaged in the H107α radio recombination line (RRL) using the Australia Telescope Compact Array (ATCA). Seven of the sources display high ratios of infrared-to-radio-continuum flux density and/or detectable RRLs and are almost certainly H ii regions. One source (G282.8−1.2) is identified as a possible new young Galactic supernova remnant, based on its relatively weak infrared emission, steep radio spectrum, and possible X-ray emission. The adopted method for distinguishing thermal and non-thermal Galactic radio sources seems promising and could be fruitfully applied to more than 100 small-diameter sources listed in the MGPS.     

Nucleonic gamma-ray production in pulsar wind nebulae

Observations of the inner radian of the Galactic disk at very high energy (VHE) gamma-rays have revealed at least 16 new sources. Besides shell type super-nova remnants, pulsar wind nebulae (PWN) appear to be a dominant source population in the catalogue of VHE gamma-ray sources. Except for the Crab nebula, the newly discovered PWN are resolved at VHE gamma-rays to be spatially extended (5–20 pc). Currently, at least 3 middle aged (t>10 kyrs) PWN (Vela X, G18.0-0.7, and G313.3+0.6 in the “Kookaburra” region) and 1 young PWN MSH 15-52 (t=1.55 kyrs) have been identified to be VHE emitting PWN (sometimes called “TeV Plerions”). Two more candidate “TeV Plerions” have been identified and have been reported at this conference (Carrigan, These proceedings, in preparation). In this contribution, the gamma-ray emission from Vela X is explained by a nucleonic component in the pulsar wind. The measured broad band spectral energy distribution is compared with the expected X-ray emission from primary and secondary electrons. The observed X-ray emission and TeV emission from the three middle aged PWN are compared with each other.     

Analysis of the thin layer of Galactic warm ionized gas in the range

We present an analysis of the thin layer of Galactic warm ionized gas at an angular resolution ∼10 arcmin. This is carried out using radio continuum data at 1.4, 2.7 and 5 GHz in the coordinate region     . For this purpose, we evaluate the zero level of the 2.7- and 5-GHz surveys using auxiliary data at 2.3 GHz and 408 MHz. The derived zero-level corrections are   T _zero(2.7 GHz) = 0.15 ± 0.06 K  and   T _zero(5 GHz) = 0.1 ± 0.05 K  . We separate the thermal (free–free) and non-thermal (synchrotron) component by means of a spectral analysis performed adopting an antenna temperature spectral index −2.1 for the free–free emission, a realistic spatial distribution of indices for the synchrotron radiation and by fitting, pixel-by-pixel, the Galactic spectral index. We find that at 5 GHz, for  | b | = 0°  , the fraction of thermal emission reaches a maximum value of 82 per cent, while at 1.4 GHz, the corresponding value is 68 per cent. In addition, for the thermal emission, the analysis indicates a dominant contribution of the diffuse component relative to the source component associated with discrete H  ii regions.     

The multi-band non-thermal emission from the supernova remnant RX J1713.7−3946

Non-thermal X-rays and very high energy (VHE) γ-rays have been detected from the supernova remnant (SNR) RX J1713.7−3946, and the recent observations with the Suzaku satellite clearly reveal a spectral cut-off in the X-ray spectrum, which directly relates to the cut-off of the energy spectrum of the parent electrons. However, whether the origin of the VHE γ-rays from the SNR is hadronic or leptonic is still in debate. We studied the multi-band non-thermal emission from RX J1713.7−3946 based on a semi-analytical approach towards the non-linear shock acceleration process by including the contribution of the accelerated electrons to the non-thermal radiation. The results show that the multi-band observations on RX J1713.7−3946 can be well explained in the model with appropriate parameters, and the TeV γ-rays have hadronic origin, i.e. they are produced via proton–proton interactions as the relativistic protons accelerated by the shock collide with the ambient matter.     

On the evolution of the cosmic supernova rates

The Galactic radio-emitting X-ray binary Cygnus X-3 is known to be a source of large-scale radio jets associated with periods of intense radio flaring. These jets have been found to have an expansion velocity of ∼0.3 c and are believed (on kinematic grounds) to lie close to the plane of the sky. We present new observations of Cygnus X-3 using the VLBA at 15 GHz. These observations, which included the detection of two small flares, show an additional kind of behaviour with apparent superluminal expansion along both major and minor axes. Evidence for superluminal activity has been found in a number of X-ray binary systems such as GRS 1915+105 and GRO J1655−40 with their superluminal radio jets. Apparently similar morphologies of the Galactic and extragalactic jet sources have led to the X-ray binaries being described as 'micro-quasars'. The superluminal expansion seen in our results appears to be different in nature from these other two sources, and a number of mechanisms are presented and discussed.     

On the nature of EXO 0423.4−0840

We present a Chandra observation of the candidate BL Lac object EXO 0423.4−0840. The X-ray emission from EXO 0423.4−0840 is clearly extended, and is associated with an optical early-type galaxy, MCG-01-12-005, at the centre of cluster ClG 0422-09. We do not detect a point source that can be associated with a BL Lac, but we found a small radio source in the centre of MCG-01-12-005. The cluster gas temperature mapped by the Chandra observation drops continuously from 80 kpc towards the centre, and is locally single phase. We measure a metallicity profile that declines outwards with a value  0.8 Z_⊙  in the centre, dropping to  0.35 Z_⊙  at larger radius, which we interpret as a superposition of cluster gas and a dense interstellar medium (ISM) in the central galaxy. Although the temperature profile suggests that conduction is not efficient, the ISM and intra-cluster medium seem not to have mixed. The entropy profile declines continuously towards the centre, in agreement with recent results on groups and clusters. The radio source appears to have had some effect in terms of gas heating, as seen in the small-scale (∼10 kpc) entropy core, and the asymmetric hard emission on the same scale.     

Re-identification of G35.6−0.4 as a supernova remnant

G35.6−0.4 is an extended radio source in the Galactic plane which has previously been identified as either a supernova remnant or an H  ii region. Observations from the Very Large Array Galactic Plane Survey at 1.4 GHz with a resolution of 1 arcmin allow the extent of G35.6−0.4 to be defined for the first time. Comparison with other radio survey observations show that this source has a non-thermal spectral index, with   S ∝ν^{−0.47±0.07}  . G35.6−0.4 does not have obvious associated infrared emission, so it is identified as a Galactic supernova remnant, not an H  ii region. It is  ≈15 × 11 arcmin²  in extent, showing partial limb brightening.     

Very High Energy Emission from the Binary System Cyg X-3

Cyg X-3 is actively studied in the entire range of the electromagnetic spectrum from the radio band to ultrahigh energies. Based on the detection of ultrahigh-energy gamma-ray emission, it has been suggested that Cyg X-3 could be one of the most powerful sources of charged cosmic-ray particles in the Galaxy. We present the results of long-term observations of the Cygnus Х-3 region at energies 800 GeV–100 TeV by the SHALON mirror Cherenkov telescope. In 1995 the SHALON observations revealed a new Galactic source of very high energy gamma-ray emission coincident in its coordinates with the microquasar Cyg X-3. To reliably identify the detected source with Cyg X-3, an analysis has been performed and an orbital period of 4.8 h has been found, which is a signature of Cyg X-3. A series of flares in Cyg X-3 at energies >800 GeV and their correlation with the activity in the X-ray and radio bands have been observed. The results obtained in a wide energy range for Cyg X-3, including those during the periods of relativistic jet events, are needed to find the connection and to understand the different components of an accreting binary system.     

On the origin of the 511-keV emission in the Galactic Centre

Diffuse 511-keV line emission, from the annihilation of cold positrons, has been observed in the direction of the Galactic Centre for more than 30 yr. The latest high-resolution maps of this emission produced by the SPI instrument on INTEGRAL suggest at least one component of the emission is spatially coincident with the distribution of ∼70 luminous, low-mass X-ray binaries detected in the soft gamma-ray band. The X-ray band, however, is generally a more sensitive probe of X-ray binary populations. Recent X-ray surveys of the Galactic Centre have discovered a much larger population (>4000) of faint, hard X-ray point sources. We investigate the possibility that the positrons observed in the direction of the Galactic Centre originate in pair-dominated jets generated by this population of fainter accretion-powered X-ray binaries. We also consider briefly whether such sources could account for unexplained diffuse emission associated with the Galactic Centre in the microwave (the Wilkinson Microwave Anisotropy Probe 'haze') and at other wavelengths. Finally, we point out several unresolved problems in associating Galactic Centre 511-keV emission with the brightest X-ray binaries.