Recent progress in pushing the sensitivity of the Imaging Atmospheric Cherenkov Technique into the 10 mCrab regime has enabled first sensitive observations of the innermost few 100 pc of the Milky Way in Very High Energy (VHE; >100 GeV) gamma rays. These observations are a valuable tool to understand the acceleration and propagation of energetic particles near the Galactic Centre. Remarkably, besides two compact gamma-ray sources, faint diffuse gamma-ray emission has been discovered with high significance. The current VHE gamma-ray view of the Galactic Centre region is reviewed, and possible counterparts of the gamma-ray sources and the origin of the diffuse emission are discussed. The future prospects for VHE Galactic Centre observations are discussed based on order-of-magnitude estimates for a CTA type array of telescopes.
The intense Compton cooling of ultra-relativistic electrons in the Klein-Nishina regime in radiation dominated environments, such as that found in the Galactic Centre, may result in radically different electron spectra than those produced by Synchrotron cooling. We explore these effects and their impact on the X-ray and gamma-ray spectra produced in electron accelerators in this region in comparison to elsewhere in our galaxy. We discuss the broad-band emission expected from the newly discovered pulsar wind nebula G 359.95-0.04 and the possible relationship of this X-ray source to the central …
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 years. 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) …
Diffuse VHE gamma radiation from the Galactic Centre ridge observed by the H.E.S.S. telescope has been convincingly linked with the propagation of recently accelerated cosmic rays that interact with molecular hydrogen clouds during their diffusion. Through a series of time-dependent simulations of that diffusion for different propagation parameters we have obtained the most probable values of the diffusion coefficient for the Galactic Centre region. Assuming that the diffusion coefficient is of the form kappa(E) = kappa_0*(E/E_0)^delta, then for different optimal combinations of kappa_0 and de…
We report the discovery of two highly dispersed pulsars in the direction of the Galactic Centre made during a survey at 3.1 GHz with the Parkes radio telescope. Both PSRs J1745-2912 and J1746-2856 have an angular separation from the Galactic Centre of less than 0.3 degrees and dispersion measures in excess of 1100 cm-3pc, placing them in the top 10 pulsars when ranked on this value. The frequency dependence of the scatter-broadening in PSR J1746-2856 is much shallower than expected from simple theory. We believe it likely that the pulsars are located between 150 and 500 pc from the Galactic Ce…
We report on an analysis of data from the southern hemisphere SUGAR cosmic ray detector. We confirm the existence of an excess of $10^{18}$eV cosmic rays from a direction close to the Galactic Centre, first reported by the AGASA group. We find that the signal is consistent with that from a point source, and we find no evidence for an excess of cosmic rays coming from the direction of the Galactic Centre itself.
We report the status of a search for pulsars in the Galactic Centre, using a completely revised and improved high-sensitivity double-horn system at 4.85-GHz. We also present calculations about the success rate of periodicity searches for such a survey, showing that in contrast to conclusions in recent literature pulsars can be indeed detected at the chosen search frequency.
Recent Chandra observations have revealed a large population of faint X-ray point sources in the Galactic Centre. The observed population consists of about 2000 faint sources in the luminosity range ~10^31-10^33 erg/s. The majority of these sources (70%) are described by hard spectra, while the rest are rather soft. The nature of these sources still remains unknown. Belczynski & Taam (2004) demonstrated that X-ray binaries with neutron star or black hole accretors may account for most of the soft sources, but are not numerous enough to account for the observed number and X-ray properties of th…
Recent results from the HESS gamma ray telescope have shown the presence of both a diffuse, extended, flux of gamma rays above ~0.4 TeV and discrete sources in and near the Galactic Centre. Here, we put forward a possible explanation in terms of the diffusion of cosmic ray protons from a succession of supernova remnants (SNR) in the SgrA* region of the Galaxy plus a contribution from SNR in the rest of the Galactic Centre Region, to be called the Galactic Centre Ridge (GCR). Protons are favoured over electrons because the mG magnetic fields in the Region will attenuate energetic electrons seve…
The influence of rotation and magnetic fields on the physical properties of isothermal gas clouds is discussed. The presence of rotation and/or magnetic fields results in an increase of the critical cloud mass with respect to gravitational instability for clouds of a given temperature and external pressure. Rotating clouds have higher densities. Consequently, they are more stable against tidal shear than non-rotating clouds. They can approach the Galactic Centre up to a radius of ~2 pc without being disrupted by the tidal shear due to the gravitational potential. For smaller radii the clouds e…
We present early results from the first IR imaging of the weak X-ray sources discovered in a recent Chandra survey towards the Galactic Centre. From our VLT observations we will identify likely counterparts to a sample of the hardest sources in order to place constraints on the nature of this previously unknown population.
I examine the effectiveness of Kozai oscillations in the centres of galaxies and in particular the Galactic centre using standard techniques from celestial mechanics. In particular, I study the effects of a stellar bulge potential and general relativity on Kozai oscillations, which are induced by stellar discs. Lockmann et al (2008) recently suggested that Kozai oscillations induced by the two young massive stellar discs in the Galactic centre drives the orbits of the young stars to large eccentricity ($e\approx 1$). If some of these young eccentric stars are in binaries, they would be disrupt…
The dust enshrouded star IRS~3 in the central light year of our galaxy was partially resolved in a recent VLTI experiment. The presented observation is the first step in investigating both IRS~3 in particular and the stellar population of the Galactic Centre in general with the VLTI at highest angular resolution. We will outline which scientific issues can be addressed by a complete MIDI dataset on IRS~3 in the mid infrared.
In X-rays, the Galactic Centre (GC) Region appears as a complex of diffuse thermal and non-thermal X-ray emission intermixed with a population of luminous discrete sources. Here we present some new findings from the XMM-Newton GC survey, focusing particularly on Sgr A*, Sgr A East, the Radio Arc region, and a newly discovered X-ray non-thermal filament.
Data from the Pierre Auger Observatory are analyzed to search for anisotropies near the direction of the Galactic Centre at EeV energies. The exposure of the surface array in this part of the sky is already significantly larger than that of the fore-runner experiments. Our results do not support previous findings of localized excesses in the AGASA and SUGAR data. We set an upper bound on a point-like flux of cosmic rays arriving from the Galactic Centre which excludes several scenarios predicting sources of EeV neutrons from Sagittarius $A$. Also the events detected simultaneously by the surfa…
Recent analyses of the anisotropy of cosmic rays at $10^{18}$ eV (the AGASA and SUGAR data) show significant excesses from regions close to the Galactic Centre and Cygnus. Our aim is to check whether such anisotropies can be caused by single sources of charged particles. We investigate propagation of protons in two models of the Galactic regular magnetic field (with the irregular component included) assuming that the particles are injected by a short lived discrete source lying in the direction of the Galactic Centre. We show that apart from a prompt image of the source, the regular magnetic f…
We examine influence of the circum-nuclear disc (CND) upon the orbital evolution of young stars in the Galactic Centre. We show that gravity of the CND causes precession of the orbits which is highly sensitive upon the semi-major axis and inclination. We consider such a differential precession within the context of an ongoing discussion about the origin of the young stars and suggest a possibility that all of them have originated in a thin disc which was partially destroyed due to the influence of the CND during the period of ~6Myr.
32 GHz observations with the Effelsberg 100-m telescope revealed a highly polarized Galactic Centre filament: G0.087-0.087, running parallel to the well-known Arc structure. It has a similar flat spectrum, but it is an order of magnitude weaker. G0.087-0.087 is about 2.5' or 6.2 pc long and located at the western boundary of the expanding molecular cloud G0.11-0.11. This unusual cloud is also visible in X-rays and known to interact on its eastern periphery with the Arc. Acceleration of highly relativistic particles at the surface of G0.11-0.11 seems necessary to explain the properties of both …
Recent observations by the H.E.S.S. collaboration of the Galactic Centre region have revealed what appears to be gamma-ray emission from the decay of pions produced by interactions of recently accelerated cosmic rays with local molecular hydrogen clouds. Synthesizing a 3-D hydrogen cloud map from the available data and assuming a diffusion coefficient of the form kappa(E) = kappa_0(E/E0)^delta, we performed Monte Carlo simulations of cosmic ray diffusion for various propagation times and values of kappa_0 and delta. By fitting the model gamma-ray spectra to the observed one we were able to inf…
The origin of Galactic cosmic rays (with energies up to 10^15 eV) remains unclear, though it is widely believed that they originate in the shock waves of expanding supernova remnants. Currently the best way to investigate their acceleration and propagation is by observing the gamma-rays produced when cosmic rays interact with interstellar gas. Here we report observations of an extended region of very high energy (VHE, >100 GeV) gamma-ray emission correlated spatially with a complex of giant molecular clouds in the central 200 pc of the Milky Way. The hardness of the gamma-ray spectrum and the …
We carried out a systematic search for new star clusters in a field of 5$^{\circ} \times5^{\circ}$ centred close to the Galactic Nucleus using the infrared JHK$_s$ 2MASS Survey archive. In addition we searched for embedded clusters in the directions of HII regions and dark clouds for $|\ell|\le4^{\circ}$. As a result we present a list of 58 IR star clusters or candidates. We provide positions, sizes and reddening estimated from 100 $μ$m dust emission. Their angular distribution together with previously catalogued objects in the region and possible relation with star forming complexes are also …
BeppoSAX has detected a population of faint transient X-ray sources in the Galactic Centre. I show that a simple irradiated-disc picture gives a consistent fit to the properties of this population, and that it probably consists of low-mass X-ray binaries which have evolved beyond their minimum periods ~80 min. Since all post-minimum systems are transient, and neutron-star LMXBs are more common than black-hole LMXBs in the Galaxy, the majority of these systems should contain neutron stars, as observed. This picture predicts that the Galactic Centre transients should have orbital periods in the …
We present results of a multi-wavelength program to study the faint discrete X-ray source population discovered by Chandra in the Galactic Centre (GC). From IR imaging obtained with the VLT we identify candidate K-band counterparts to 75% of the X-ray sources in our sample. By combining follow-up VLT K-band spectroscopy of a subset of these candidate counterparts with the magnitude limits of our photometric survey, we suggest that only a small percentage of the sources are HMXBs, while the majority are likely to be canonical LMXBs and CVs at the distance of the GC. In addition, we present our …
The Infrared Space Observatory Long wavelength Spectrometerhas been used to map distribution of the emission from a sample of 22 atomic, molecular and ionised lines toward the Circumnuclear Disk at the Galactic Centre. The CND disc is clearly seen in the maps of molecular lines such as CO and OH, whilst the central region dominates in other atomic and ionised lines. The spectrum toward Sgr A star is best represented by the sum of a 58 K blackbody, superposed with 22 identifiable emission or absorption features, including four lines each attributed to CO and OH, two broad features that may be i…
We present results from the first near-IR imaging of the weak X-ray sources discovered in the Chandra/ACIS-I survey (Wang et al. 2002) towards the Galactic Centre (GC). These ~800 discrete sources, which contribute significantly to the GC X-ray emission, represent an important and previously unknown population within the Galaxy. From our VLT observations we will identify likely IR counterparts to a sample of the hardest sources, which are most likely X-ray binaries. With these data we can place constraints on the nature of the discrete weak X-ray source population of the GC.
We report the discovery in the Galactic Centre region of two hard X-ray sources, designated as XMM J174457-2850.3 and XMM J174544-2913.0, which exhibited flux variations in the 2--10 keV band in excess of a factor of 100 in observations spanning roughly a year. In both cases the observed hydrogen column density is consistent with a location near to the Galactic Centre, implying peak X-ray luminosities of ~5 x 10^34 erg/s. These objects may represent a new population of transient source with very different properties to the much more luminous Galactic Centre transients associated with neutron s…
The detection of non-baryonic dark matter through its gamma-ray annihilation in the centre of our galaxy has been studied. The gamma fluxes according to different models have been simulated and compared to those expected to be observed with the Alpha Magnetic Spectrometer (AMS), during a long-term mission on board of the International Space Station. Under the assumption that the dark matter halo is composed of the lightest, stable supersymmetric particle, the neutralino, the results of the simulations in the framework of mSUGRA models, show that with a cuspy dark matter halo or a clumpy halo, …
We study the process of mass segregation through 2-body relaxation in galactic nuclei with a central massive black hole (MBH). This study has bearing on a variety of astrophysical questions, from the distribution of X-ray binaries at the Galactic centre, to tidal disruptions of main-sequence and giant stars, to inspirals of compact objects into the MBH, an important category of events for the future space borne gravitational wave interferometer LISA. In relatively small galactic nuclei, typical hosts of MBHs with masses in the range 1e4-1e7 Msun, the relaxation induces the formation of a steep…
We present updated results for anisotropy searches in the direction of the Galactic Center (GC) at energies in the ranges: 0.1<E<1 EeV and 1<E<10 EeV. We use data from the Pierre Auger Observatory up to March, 2007. Present analyzes are therefore based on a substantially larger data set than our previous published results. A limit on the flux coming from a hypothetical point-like neutron source at the GC for 1<E<10 EeV was imposed, and searches for extended excesses were also performed.
Compact remnants -- stellar mass black holes and neutron stars formed in the inner few parsec of galactic centres are predicted to sink into the central parsec due to dynamical friction on low mass stars, forming a high concentration cusp (Morris 1993). Same physical region may also contain very high density molecular clouds and accretion discs that are needed to fuel SMBH activity. Here we estimate gas capture rates onto the cusp of stellar remnants, and the resulting X-ray luminosity, as a function of the accretion disc mass. At low disc masses, most compact objects are too dim to be observa…
IRS 13E is an infrared, mm and X-ray source in the Galactic Centre. We present the first Chandra X-ray spectrum for IRS 13E and show that it is consistent with a luminous and highly absorbed X-ray binary system. Since the X-ray luminosity is too large for a solitary star, our interpretation is that of an early-type long-period binary with strong colliding winds emission. This naturally explains the observed X-ray spectrum and count rate as well as its lack of significant short term variability. Due to the short lifetime of any nebula 0.2 pc from the putative central super-massive black-hole, w…
There has been recent speculation (Davies & King 2005) that the cores of intermediate-mass stars stripped of their envelopes by tidal interaction with the supermassive black hole in the Galactic centre could form a population observationally similar to the so-called Sgr A* cluster or `S' stars, which have close eccentric orbits around the hole. We model the evolution of such stars, and show that the more luminous end of the population may indeed appear similar to young B stars within the observational limits of the Galactic Centre region. Whether some or all of these cluster stars can be accou…
We show that collisions with stellar--mass black holes can partially explain the absence of bright giant stars in the Galactic Centre, first noted by Genzel et al, 1996. We show that the missing objects are low--mass giants and AGB stars in the range 1-3 M$_{\odot}$. Using detailed stellar evolution calculations, we find that to prevent these objects from evolving to become visible in the depleted K bands, we require that they suffer collisions on the red giant branch, and we calculate the fractional envelope mass losses required. Using a combination of Smoothed Particle Hydrodynamic calculati…
A comparison of the XMM-Newton and Chandra Galactic Centre (GC) Surveys has revealed two faint X-ray transients with contrasting properties. The X-ray spectrum of XMM J174544-2913.0 shows a strong iron line with an equivalent width of ~2 keV, whereas that of XMM J174457-2850.3 is characterised by a very hard continuum with photon index ~1.0. The X-ray flux of both sources varied by more than 2 orders of magnitude over a period of months with a peak X-ray luminosity of 5 x 10^34 erg/s. We discuss the nature of these peculiar sources.
Observations by the H.E.S.S. system of imaging atmospheric Cherenkov telescopes provide the most sensitive measurements of the Galactic Centre region in the energy range 150 GeV - 30 TeV. The vicinity of the kinetic centre of our galaxy harbours numerous objects which could potentially accelerate particles to very high energies (VHE, > 100 GeV) and thus produce the Gamma-ray flux observed. Within statistical and systematic errors, the centroid of the point-like emission measured by H.E.S.S. was found to be in good agreement with the position of the supermassive black hole Sgr A* and the recent…
We report the results of time-resolved photometric and spectroscopic near-infrared observations of the Ofpe/WN9 star and LBV candidate GCIRS34W in the Galactic Centre star cluster. Diffraction limited resolution photometric observations obtained in H and K bands show a strong, non-periodic variability on time scales from months to years in both bands accompanied by variations of the stellar colour. Three K band spectra obtained in 1996, 2003 and 2004 with integral field spectrometers are identical within their accuracies and exclude significant spectroscopic variability. The most probable expl…
We study the efficiency at which a black hole or dense star cluster spirals-in to the Galactic centre. This process takes place on a dynamical friction time scale, which depends on the value of the Coulomb logarithm ln(L). We determine the accurate value of this parameter using the direct N-body method, a tree algorithm and a particle-mesh technique with up to 2 million plus one particles. The three different techniques are in excellent agreement. Our result for the Coulomb logarithm appears to be independent of the number of particles. We conclude that ln(L) = 6.6 +/- 0.6 for a massive point …
A high level of complex structure, or ``granularity'', has been observed in the distribution of infrared-obscuring material towards the Galactic Centre (GC), with a characteristic scale of 5arcsec - 15arcsec, corresponding to 0.2 - 0.6pc at a GC distance of 8.5kpc. This structure has been observed in ISAAC images which have a resolution of 0.6arcsec, significantly higher than that of previous studies of the GC. We have discovered granularity throughout the GC survey region, which covers an area of 1.6deg x 0.8deg in longitude and latitude respectively (300pc x 120pc at 8.5kpc) centred on Sgr A…
The High-Energy Stereoscopic System (HESS) has detected intense diffuse TeV emission correlated with the distribution of molecular gas along the galactic ridge at the centre of our Galaxy. Earlier HESS observations of this region had already revealed the presence of several point sources at these energies, one of them (HESS J1745-290) coincident with the supermassive black hole Sagittarius A*. It is still not entirely clear what the origin of the TeV emission is, nor even whether it is due to hadronic or leptonic interactions. It is reasonable to suppose, however, that at least for the diffuse…
We report the discovery by XMM-Newton and Chandra of a hard extended X-ray source (XMM J174540-2904.5) associated with a compact non-thermal radio filament (the Sgr A-E `wisp'=1LC 359.888-0.086= G359.88-0.07), which is located within ~4 arcmin of the Galactic Centre. The source position is also coincident with the peak of the molecular cloud, M -0.13-0.08 (the `20 km/s' cloud). The X-ray spectrum is non-thermal with an energy index of 1.0 (+1.1 -0.9) and column density of 38 (+7 -11) x 10^22 H/cm2. The observed 2--10 keV flux of 4 x 10^-13 erg/s/cm2 converts to an unabsorbed X-ray luminosity o…
OH(1720 MHz) masers permit direct measurements of the magnetic field strength at the Galactic Centre, and their angular broadening constrains models for the medium that scatters radio waves in the central 100 pc of the Galaxy. As the 1720 MHz masers are unaccompanied by the main-line masers at 1665 and 1667 MHz, they must be pumped by collisions in molecular gas that is cooling after being overrun by a non-dissociative (i.e. C-type) shock wave. In particular, this confirms that the ``SNR'' Sgr A East is driving a shock into the M-0.02-0.07 molecular cloud. The intensity of the v=1--0 S(1) line…
Stars within 0.1 pc of the supermassive black hole Sgr A* at the Galactic centre are expected to encounter a cluster of stellar-mass black holes (BHs) that have segregated to that region. Some of these stars will scatter off an orbiting BH and be kicked out of the Galactic centre with velocities up to ~2000 km/s. We calculate the resulting ejection rate of hypervelocity stars (HVSs) by this process under a variety of assumptions, and find it to be comparable to the tidal disruption rate of binary stars by Sgr A*, first discussed by Hills (1988). Under some conditions, this novel process is suf…
We present near-IR imaging of a sample of the faint, hard X-ray sources discovered in the 2001 Chandra ACIS-I survey towards the Galactic Centre (GC) (Wang et al. 2002). These ~800 discrete sources represent an important and previously undetected population within the Galaxy. From our VLT observations of 77 X-ray sources, we identify candidate K-band counterparts to 75% of the Chandra sources in our sample. The near-IR magnitudes and colours of the majority of candidate counterparts are consistent with highly reddened stars, indicating that most of the Chandra sources are likely to be accretin…
We have performed N-body simulations of the formation of hyper-velocity stars (HVS) in the centre of the Milky Way due to inspiralling intermediate-mass black holes (IMBHs). We considered IMBHs of different masses, all starting from circular orbits at an initial distance of 0.1 pc. We find that the IMBHs sink to the centre of the Galaxy due to dynamical friction, where they deplete the central cusp of stars. Some of these stars become HVS and are ejected with velocities sufficiently high to escape the Galaxy. Since the HVS carry with them information about their origin, in particular in the mo…
We have computed the galactic trajectories of twelve hypervelocity stars (HVSs) under the assumption that they originated in the Galactic Centre. We show that eight of these twelve stars are bound to the Galaxy. We consider the subsequent trajectories of the bound stars to compute their characteristic orbital period, which is 2 Gyr. All eight bound stars are moving away from the centre of the Galaxy, which implies that the stars' lifetimes are less than 2 Gyr. We thus infer that the observed HVSs are massive main sequence stars, rather than blue horizontal branch stars. The observations sugges…
Most galactic nuclei are now believed to harbour supermassive black holes. Studies of stellar motions in the central few light-years of our Milky Way Galaxy indicate the presence of a dark object with a mass of about 2.6 million solar masses. This object is spatially coincident with Sagittarius A* (Sgr A*), the unique compact radio source located at the dynamical centre of our Galaxy. By analogy with distant quasars and nearby active galactic nuclei (AGN), Sgr A* is thought to be powered by the gravitational potential energy released by matter as it accretes onto a supermassive black hole. How…
The contributions of different radiation mechanisms to the diffuse gamma-ray emission of the galactic disk are studied in a broad energy region from X-rays to very high energy gamma rays.
We study annihilation radiation of neutralinos in the Galactic centre, assuming the existence of a 'spike' in the dark matter density profile, due to adiabatic accretion onto the massive black hole lying at the Galactic centre. Under this assumption we find that it is possible to reproduce the observed SgrA* emission at radio and gamma-ray frequencies in a consistent scenario with a magnetic field close to the equipartition strength and with values of $γ$ (the density profile power law index) around 0.1.
The radio source Sagittarius A* (Sgr A*), is thought to be a supermassive black hole located at the centre of our Galaxy, that is accreting gas from the surrounding region. Using the high inferred accretion rates, however, standard accretion models are unable to explain the low luminosity and observed spectrum from Sgr A*. A new accretion model has been proposed - an advection dominated accretion flow (ADAF), where most of the accretion energy is stored in the gas and lost into the black hole. The gas therefore has a two-temperature structure with the protons being much hotter than the electro…
Within the central parsec of the Galaxy, several tens of young stars orbiting a central supermassive black hole are observed. A subset of these stars forms a coherently rotating disc. Other observations reveal a massive molecular torus which lies at a radius ~1.5pc from the centre. In this paper we consider the gravitational influence of the molecular torus upon the stars of the stellar disc. We derive an analytical formula for the rate of precession of individual stellar orbits and we show that it is highly sensitive upon the orbital semi-major axis and inclination with respect to the plane o…
The photometry data base of the second phase of the OGLE microlensing experiment, OGLE-II, is a rich source of information about the kinematics and structure of the Galaxy. In this work we use the OGLE-II proper motion catalogue to identify candidate stars which have high proper motions. 521 stars with proper motion mu > 50 mas/yr in the OGLE-II proper motion catalogue (Sumi 2004) were cross-identified with stars in the MACHO high proper motion catalogue, and the DENIS and 2MASS infra-red photometry catalogues. Photometric distances were computed for stars with colours consistent with G/K and …
Radioastronomical studies have indicated that the magnetic field in the central few hundred parsecs of our Milky Way Galaxy has a dipolar geometry and a strength substantially larger than elsewhere in the Galaxy, with estimates ranging up to a milligauss. A strong, large-scale magnetic field can affect the Galactic orbits of molecular clouds by exerting a drag on them, it can inhibit star formation, and it can guide a wind of cosmic rays away from the central region, so a characterization of the magnetic field at the Galactic center is important for understanding much of the activity there. He…
The long-term evolution of stellar orbits bound to a massive centre is studied in order to understand the cores of star clusters in central regions of galaxies. Stellar trajectories undergo tiny perturbation, the origin of which is twofold: (i) gravitational field of a thin gaseous disc surrounding the galactic centre, and (ii) cumulative drag due to successive interactions of the stars with material of the disc. Both effects are closely related because they depend on the total mass of the disc, assumed to be a small fraction of the central mass. It is shown that, in contrast to previous works…
Adopting a recent calculation of the Galactic interstellar radiation field, we calculate the attenuation of the very high energy gamma rays from the Galactic sources. The infra-red radiation background near the Galactic Center is very intense due to the new calculation and our result shows that a cutoff of high energy gamma ray spectrum begins at about 20 TeV and reaches about 10% for 50 TeV gamma rays.
Excesses of Galactic gamma rays in the 1-100 GeV region observed by EGRET and by WMAP have been interpreted as signals of dark matter (LSP) annihilation. We argue that the excess of TeV gamma rays from the Galactic center observed by H.E.S.S. signals a heavier dark matter component with a very small relic density.
On the basis of ``sticky particle'' calculations, it is argued that the gas features observed within 10 pc of the Galactic Centre-- the circumnuclear disk (CND) and the ionized gas filaments-- as well as the newly formed stars in the inner one parsec can be understood in terms of tidal capture and disruption of gas clouds on low angular momentum orbits in a potential containing a point mass. The calculations demonstrate that a dissipative component forms a ``dispersion ring'', an asymmetric elliptical torus precessing counter to the direction of rotation, and that this shape can be maintained …
We have measured 21-cm absorption and emission spectra in the direction of a further 7 southern pulsars with the Parkes telescope to derive their kinematic distances and to study the interstellar medium. For the first time we have successfully obtained HI absorption measurements for PSRs J1602-5100, J1740-3015 and J1745-3040. We have also significantly improved the sensitivity and resolution on PSRs J1600-5044, J1752-2806, and J1825-0935, whose spectra have previously been measured, and have corrected an error in the published distance to PSR J1824-1945. Since the Frail & Weisberg summary of p…
An attempt is made to reconcile the large wind-loss rates of stars in the Galactic Centre (GC) with the predicted low accretion rate for Sgr A*, the putative blackhole at the heart of the Milky Way. It is found that, independent of the details of the accretion, the bound but unaccreted gas has been accumulating in the potential well of Sgr A* for <1000 yrs and thus is not in equilibrium. Otherwise, the gas flows of the region would be visible in both the IR and X-ray. It appears that the blackhole was more active in the recent past due to the passing of a supernova blast shock but is presently…
A massive black hole is present at the centre of our galaxy and inevitably accretes dark matter particles, creating a region of very high particle density. The annihilation rate is enhanced with a large number of e+e- pairs produced either directly or by successive decays of mesons. We evaluate the synchrotron emission (and self-absorption) associated with the propagation of these particles through the galactic magnetic field and give constraints on the values of mass and cross section of the dark matter particles.
