Dense Interstellar Medium Research Articles

The interaction of planetary nebulae and their asymptotic giant branch progenitors with the interstellar medium

Abstract Interaction with the interstellar medium (ISM) cannot be ignored in understanding planetary nebula (PN) evolution and shaping. In an effort to understand the range of shapes observed in the outer envelopes of PNe, we have run a comprehensive set of three-dimensional hydrodynamic simulations, from the beginning of the asymptotic giant branch (AGB) superwind phase until the end of the post-AGB/PN phase. A ‘triple-wind’ model is used, including a slow AGB wind, fast post-AGB wind and third wind reflecting the linear movement through the ISM. A wide range of stellar velocities, mass-loss rates and ISM densities have been considered. We find that ISM interaction strongly affects outer PN structures, with the dominant shaping occurring during the AGB phase. The simulations predict four stages of PN–ISM interaction whereby (i) the PN is initially unaffected, (ii) then limb-brightened in the direction of motion, (iii) then distorted with the star moving away from the geometric centre, and (iv) finally so distorted that the object is no longer recognizable as a PN and may not be classed as such. Parsec-size shells around PNe are predicted to be common. The structure and brightness of ancient PNe are largely determined by the ISM interaction, caused by rebrightening during the second stage; this effect may address the current discrepancies in Galactic PN abundance. The majority of PNe will have tail structures. Evidence for strong interaction is found for all known PNe in globular clusters.

Supernova remnant evolution in uniform and non-uniform media

Aims. In this work numerical simulations showing the time evolution of supernova remnants (SNRs) in uniform and non-uniform interstellar medium (ISM) are presented. Methods. We use a hydrodynamic model including a kinematic calculation of the interstellar magnetic field. Important parameters influencing SNR evolution include the ejecta mass and energy of the remnant, as well as the ISM density and adiabatic index. Results. By varying these parameters we constructed an analytical expression giving the return time of the SNR reverse shock to the origin, in terms of these parameters. We also found that the reverse shock spends half of its time moving outward and the other half returning to the origin. Also computed is SNR evolution in non-uniform media where the blast wave moves from one medium into either a less or more dense medium. As the SNR moves into a medium of higher density a reflection wave is created at the interface between the two media which is driven back toward the center. This drives mass via a nonspherical flow away from the discontinuity. As this wave moves inward it also drags some of the ISM field lines (if the field is parallel with the interface) with it and heats the inside of the SNR resulting in larger temperatures in this region. When a SNR explodes in a medium with a high density and the blast wave propagates into a medium with a lower density, a cavity is being blown away changing the geometry of the high density region. Also, once the forward shock moves into the medium of less density a second reverse shock will start to evolve in this region.

Evolution of Dust in Primordial Supernova Remnants: Can Dust Grains Formed in the Ejecta Survive and Be Injected into the Early Interstellar Medium?

We investigate the evolution of dust that formed at Population III supernova (SN) explosions and its processing through the collisions with the reverse shocks resulting from the interaction of the SN ejecta with the ambient medium. In particular, we investigate the transport of the shocked dust within the SN remnant (SNR), and its effect on the chemical composition, the size distribution, and the total mass of dust surviving in SNR. We find that the evolution of the reverse shock, and hence its effect on the processing of the dust depends on the thickness of the envelope retained by the progenitor star. Furthermore, the transport and survival of the dust grains depend on their initial radius, a_{ini}, and composition: For Type II SNRs expanding into the interstellar medium (ISM) with a density of n_{H,0}=1 cm^{-3}, small grains with a_{ini} < ~ 0.05 micron are completely destroyed by sputtering in the postshock flow, while grains with a_{ini}= 0.05--0.2 micron are trapped into the dense shell behind the forward shock. Very large grains of a_{ini} > ~ 0.2 micron are ejected into the ISM without decreasing their sizes significantly. We find that the total mass fraction of dust that is destroyed by the reverse shock ranges from 0.2 to 1.0, depending on the energy of the explosion and the density of the ambient ISM. The results of our calculations have significant impact on the abundance pattern of subsequent generation of stars that form in the dense shell of primordial SNRs.

Superdiscs in radio galaxies: jet–wind interactions

Taking a clue from their sharp-edged (strip-like) morphology observed in several cases, a new mechanism is proposed for the formation of the emission gaps seen between the radio lobes of many powerful extragalactic double radio sources. Canonical understanding of the radio gaps invokes either blocking of the back-flowing lobe plasma by the denser interstellar medium (ISM) of the host galaxy, or "squeezing" of the radio bridge in the middle through buoyancy force exerted by either the ISM or the surrounding intra-cluster medium (ICM). These pictures encounter difficulties in explaining situations where the sharp-edged radio gaps associated with non-cluster radio galaxies have widths running into several tens (even hundreds) of kiloparsecs. More particularly, the required dense high-pressure ISM/ICM is likely to be lacking at least in the case of high-redshift radio galaxies. We propose here that radio emission gaps in at least such cases could arise from a dynamical interaction between the powerful thermal wind outflowing from the active galactic nucleus and the back-flowing synchrotron plasma in the two radio lobes, which occurs once the rapidly advancing jets have crossed out of the wind zone into the intergalactic medium. A simple analytical scheme is presented to explore the plausibility of the side-ways confinement of the thermal wind by the radio lobe pair, which would "freeze" pancake shaped conduits in the space, along which the hot, metal enriched wind from the AGN can escape (roughly orthogonal to the radio axis). Some other possible consequences of this scenario are pointed out.

Molecular oxygen in the ρ Ophiuchi cloud

Molecular oxygen, O2 has been expected historically to be an abundant component of the chemical species in molecular clouds and, as such, an important coolant of the dense interstellar medium. However, a number of attempts from both ground and from space have failed to detect O2 emission. The work described here uses heterodyne spectroscopy from space to search for molecular oxygen in the interstellar medium. The Odin satellite carries a 1.1 m sub-millimeter dish and a dedicated 119 GHz receiver for the ground state line of O2. Starting in 2002, the star forming molecular cloud core rho Oph A was observed with Odin for 34 days during several observing runs. We detect a spectral line at v(LSR) = 3.5 km/s with dv(FWHM) = 1.5 km/s, parameters which are also common to other species associated with rho Ohp A. This feature is identified as the O2 (N_J = 1_1 - 1_0) transition at 118 750.343 MHz. The abundance of molecular oxygen, relative to H2,, is 5E-8 averaged over the Odin beam. This abundance is consistently lower than previously reported upper limits.

GRB 050410 and GRB 050412: are they really dark gamma-ray bursts?

Aims. We present a detailed analysis of the prompt and afterglow emission of GRB 050410 and GRB 050412 detected by Swift for which no optical counterpart was observed. Methods. We analysed data from the prompt emission detected by the Swift BAT and from the early phase of the afterglow obtained by the Swift narrow field instrument XRT. Results. The $15{-}150$ keV energy distribution of the GRB 050410 prompt emission shows a peak energy at 53$_{-21}^{+40}$ keV. The XRT light curve of this GRB decays as a power law with a slope of $\alpha=$ 1.06 ± 0.04. The spectrum is well reproduced by an absorbed power law with a spectral index $\Gamma_{\rm x}=2.4$ ± 0.4 and a low energy absorption N H = 4$^{+3}_{-2}$ $\times$ 10 21 cm -2 which is higher than the Galactic value. The $15{-}150$ keV prompt emission in GRB 050412 is modelled with a hard ( Γ = 0.7 ± 0.2) power law. The XRT light curve follows a broken power law with the first slope $\alpha_1$ = 0.7 ± 0.4, the break time T break = 254$_{-41}^{+79}$ s and the second slope $\alpha_2$ = 2.8$_{-0.8}^{+0.5}$. The spectrum is fitted by a power law with spectral index $\Gamma_{\rm x}=1.3$ ± 0.2 which is absorbed at low energies by the Galactic column. Conclusions. The GRB 050410 afterglow light curve reveals the expected characteristics of the third component of the canonical Swift light curve. Conversely, a complex phenomenology was detected in the GRB 050412 because of the presence of the very early break. The light curve in this case can be interpreted as being the last peak of the prompt emission. The two bursts present tight upper limits for the optical emission, however, neither of them can be clearly classified as dark. For GRB 050410, the suppression of the optical afterglow could be attributed to a low density interstellar medium surrounding the burst. For GRB 050412, the evaluation of the darkness is more difficult due to the ambiguity in the extrapolation of the X-ray afterglow light curve.

Arp 220 – IC 4553/4: understanding the system and diagnosing the ISM

AbstractArp 220 is a nearby system in final stages of galaxy merger with powerful ongoing star-formation at and surrounding the two nuclei. Arp 220 was detected in HI absorption and OH Megamaser emission and later recognized as the nearest ultra-luminous infrared galaxy also showing powerful molecular and X-ray emissions. In this paper we review the available radio and mm-wave observational data of Arp 220 in order to obtain an integrated picture of the dense interstellar medium that forms the location of the powerful star-formation at the two nuclei.

Three-dimensional hydrodynamic simulations of asymmetric pulsar wind bow shocks

We present three-dimensional, nonrelativistic, hydrodynamic simulations of bow shocks in pulsar wind nebulae. The simulations are performed for a range of initial and boundary conditions to quantify the degree of asymmetry produced by latitudinal variations in the momentum flux of the pulsar wind, radiative cooling in the postshock flow, and density gradients in the interstellar medium (ISM). We find that the bow shock is stable even when travelling through a strong ISM gradient. We demonstrate how the shape of the bow shock changes when the pulsar encounters density variations in the ISM. We show that a density wall can account for the peculiar bow shock shapes of the nebulae around PSR J2124-3358 and PSR B0740-28. A wall produces kinks in the shock, whereas a smooth ISM density gradient tilts the shock. We conclude that the anisotropy of the wind momentum flux alone cannot explain the observed bow shock morphologies but it is instead necessary to take into account external effects. We show that the analytic (single layer, thin shell) solution is a good approximation when the momentum flux is anisotropic, fails for a steep ISM density gradient, and ap- proaches the numerical solution for efficient cooling. We provide analytic expressions for the latitudinal dependence of a vacuum-dipole wind and the associated shock shape, and compare the results to a split-monopole wind. We find that we are unable to distinguish between these two wind models purely from the bow shock morphology.

Numerical Modeling of the Radio Nebula from the 2004 December 27 Giant Flare of SGR 1806−20

We use the relativistic hydrodynamics code Cosmos++ to model the evolution of the radio nebula triggered by the Dec. 27, 2004 giant flare event of soft gamma repeater 1806-20. We primarily focus on the rebrightening and centroid motion occurring subsequent to day 20 following the flare event. We model this period as a mildly relativistic (gamma ~ 1.07 - 1.67) jetted outflow expanding into the interstellar medium (ISM). We demonstrate that a jet with total energy ~ 10^46 ergs confined to a half opening angle ~ 20 degrees fits the key observables of this event, e.g. the flux lightcurve, emission map centroid position, and aspect ratio. In particular, we find excellent agreement with observations if the rebrightening is due to the jet, moving at 0.5c and inclined ~ 0 - 40 degrees toward the observer, colliding with a density discontinuity in the ISM at a radius of several 10^16 cm. We also find that a jet with a higher velocity, >~ 0.7c, and larger inclination, >~ 70 degrees, moving into a uniform ISM can fit the observations in general, but tends to miss the details of rebrightening. The latter, uniform ISM model predicts an ISM density more than 100 times lower than that of the former model, and thus suggests an independent test which might discriminate between the two. One of the strongest constraints of both models is that the data seems to require a non-uniform jet in order to be well fit.

Modeling chemical growth processes in Titan’s atmosphere: 1. Theoretical rates for reactions between benzene and the ethynyl (C 2H) and cyano (CN) radicals at low temperature and pressure

Density functional theory calculations at the B3LYP/6-31+G ∗∗ level were employed to characterize the critical points for adducts, isomers, products, and intervening transition states for the reactions between benzene and the ethynyl (C 2H) or cyano (CN) radicals. Both addition reactions were found to have no barriers in their entrance channels, making them efficient at the low temperature and pressure conditions that prevail in the haze-forming region of Titan’s atmosphere as well as in the dense interstellar medium (ISM). The dominant products are ethynylbenzene (C 6H 5C 2H) and cyanobenzene (C 6H 5CN). Hydrogen abstraction reactions were also characterized but found to be non-competitive. Trajectory calculations based on potentials fit to about 600 points calculated at the ROMP2/6-31+G ∗∗ level for each interaction surface were used to determine reaction rates. The rates incorporated any necessary corrections for back reactions as ascertained from a multiwell treatment used to determine outcome distributions over the range of temperatures and pressures pertinent to Titan and the ISM and are in good agreement with the limited available experimental data.

The young star cluster NGC 2362: low-mass population and initial mass function from a Chandra X-ray observation

Context. We study the stellar population of the very young cluster NGC 2362, using a deep Chandra ACIS-I X-ray observation. This cluster, only 5 Myr old, has already cleared most of its inter- and circumstellar dust, and with its small and uniform reddening offers a unique opportunity of studying its pre-main-sequence stellar population with minimal disturbance from a dense interstellar medium. Aims. Our main purposes are to select cluster members down to low masses and to study their properties as a population (spatial properties, initial mass function, and coronal properties). Methods. We compare existing deep optical photometry and H α data with new X-ray data. We use combined optical and X-ray criteria to select cluster members. Results. We detect 387 X-ray sources down to $\log L_{\rm X} = 29.0$ (erg/s), and identify most of them (308) with star-like objects. The majority (88%) of optically identified X-ray sources are found to be very good candidate low-mass pre-main-sequence stars, with minimal field-object contamination. This increases the known cluster census by a substantial amount at low masses, with respect to previous optical/IR studies. The fraction of stars with active accretion is found to be in the range 5–9%. We find a significantly wider spatial distribution for low-mass stars than for massive stars (mass segregation). We find only a small spread around the low-mass cluster sequence in the HR diagram, indicating that star formation lasted only about 1–2 Myr. We have derived the cluster initial mass function, which appears to flatten (on the low-mass side) at higher masses with respect to other very young clusters. The quiescent X-ray emission of low-mass cluster stars is found to be rather strictly correlated with the stellar bolometric luminosity: the small spread in this correlation puts an upper bound on the amplitude of X-ray variability on time scales longer than one day (e.g., activity cycles) in such young coronal sources. We find significant X-ray spectral differences between low-mass stars brighter and fainter than $\log L_{\rm X} \sim 30.3$ (erg/s), respectively, with X-ray brighter stars showing hotter components ($kT \sim 2$ keV), absent in fainter stars.

Nature and evolution of the dominant carbonaceous matter in interplanetary dust particles: effects of irradiation and identification with a type of amorphous carbon

Aims. Interplanetary dust particle (IDP) matter probably evolved under irradiation in the interstellar medium (ISM) and the solar nebula. Currently IDPs are exposed to irradiation in the Solar System. Here the effects of UV and proton processing on IDP matter are studied experimentally. The structure and chemical composition of the bulk of carbon matter in IDPs is characterized. Methods. Several IDPs were further irradiated in the laboratory using ultraviolet (UV) photons and protons in order to study the effects of such processing. By means of infrared and Raman spectroscopy, IDPs were also compared to different materials that serve as analogs of carbon grains in the dense and diffuse ISM. Results. The carbonaceous fraction of IDPs is dehydrogenated by exposure to hard UV photons or 1 MeV protons. On the other hand, proton irradiation at lower energies (20 keV) leads to an efficient hydrogenation of the carbonaceous IDP matter. The dominant type of carbon in IDPs, observed with Raman and infrared spectroscopy, is found to be either a form of amorphous carbon (a-C) or hydrogenated amorphous carbon (a-C:H), depending on the IDP, consisting of aromatic units with an average domain size of 1.35 nm (5-6 rings in diameter), linked by aliphatic chains. Conclusions. The D- and 15 N-enrichments associated to an aliphatic component in some IDPs are probably the result of chemical reactions at cold temperatures. It is proposed that the amorphous carbon in IDPs was formed by energetic processing (UV photons and cosmic rays) of icy grains, maybe during the dense cloud stage, and more likely on the surface of the disk during the T Tauri phase of our Sun. This would explain the isotopic anomalies and morphology of IDPs. Partial annealing, 300-400 °C, is required to convert an organic residue from ice photoprocessing into the amorphous carbon with low heteroatom content found in IDPs. Such annealing might have occurred as the particles approached the Sun and/or during atmospheric entry heating.

Simulations of mixed-morphology supernova remnants with anisotropic thermal conduction

We explore the role of anisotropic thermal conduction on the evolution of supernova remnants (SNRs) through interstellar media with a range of densities via numerical simulations. We find that a remnant expanding in a dense environment can produce centre-bright hard X-ray emission within 20 kyr, and centre-bright soft X-ray emission within 60 kyr of the supernova event. In a more tenuous environment, the appearance of a centre-bright structure in hard X-rays is delayed until about 60 kyr. The soft X-ray emission from such a remnant may not become centre bright during its observable lifetime. This can explain the observations that show that mixed-morphology SNRs preferentially occur close to denser, molecular environments. Remnants expanding into denser environments tend to be smaller, making it easier for thermal conduction to make large changes in the temperatures of their hot gas bubbles. We show that the lower temperatures make it very favourable to use high-stage ions as diagnostics of the hot gas bubbles in SNRs. In particular, the distribution of O VIII transitions from shell bright at early epochs to centre bright at later epochs in the evolution of an SNR expanding in a dense interstellar medium when the physics of thermal conduction is included.

Soft gamma-ray background and light dark matter annihilation

The bulk of the extragalactic background between 10 keV and 10 GeV is likely to be explained by the emission of Seyfert galaxies, type Ia supernovae, and blazars. However, as revealed by the INTEGRAL satellite, the bulge of our galaxy is an intense source of a 511 keV gamma-ray line, indicating the production of a large number of positrons that annihilate. The origin of the latter is debated, and they could be produced, in particular, by the ($S$- or $P$-wave) annihilation of light dark matter particles into ${e}^{+}{e}^{\ensuremath{-}}$. In any case, the cumulated effect of similar sources at all redshifts could lead to a new background of hard x-ray and soft gamma-ray photons. On the basis of the hierarchical model of galaxy formation, we compute analytically the SNIa contribution to the background, and add it to Seyfert and blazars emission models. Confronting these expected contributions to observation, we find that any extra contribution to this unresolved background around 511 keV should be lower than about $4\text{ }\text{ }\mathrm{keV}\text{ }{\mathrm{cm}}^{\ensuremath{-}2}\text{ }{\mathrm{s}}^{\ensuremath{-}1}\text{ }{\mathrm{sr}}^{\ensuremath{-}1}$. We also estimate analytically the extragalactic background due to dark matter annihilation, increasing the accuracy of the earlier computations. Indeed, we take into account the large positron escape fraction from low mass dark matter halos, unable to confine a dense and magnetized interstellar medium. Our new background estimate turns out to be 1 order of magnitude lower so that the hypothesis of a light dark matter candidate remains compatible with the observed extragalactic background for a wider range of particle masses and cross sections.

Enrichment of deuterium in insoluble organic matter from primitive meteorites: A solar system origin?

Abstract Because of a systematic enrichment in deuterium, the insoluble organic matter (IOM) of the carbonaceous chondrites is considered to have formed in the interstellar medium. However, the D / H ratios in IOM remain much lower than those measured in the organic molecules commonly observed in the dense interstellar medium. In this study, the D / H ratio of different aromatic and aliphatic molecular fragments of IOM from the Orgueil meteorite was measured by GC-irMS (gas chromatography–isotopic ratio mass spectrometry). No correlation was observed between the D / H ratios and structural parameters characterizing the IOM, such as the H / C ratio. However, the δD of the benzylic, aliphatic and aromatic hydrogen into the IOM can be determined to be 1250‰, + 550‰ and + 150‰, respectively, relative to SMOW. This indicates that D-enrichment in IOM is correlated with the C–H bond dissociation energy. Such a correlation rules out IOM formation from observed interstellar molecules and suggests instead that the different components of IOM have acquired their D / H ratios by an exchange with a deuterium-rich reservoir after its synthesis. The same process can be invoked to account for the D / H composition of meteoritic water. Findings point to a common process for deuterium enrichment in the solar system.

SwiftPanchromatic Observations of the Bright Gamma‐Ray Burst GRB 050525a

The bright gamma-ray burst GRB050525a has been detected with the Swift observatory, providing unique multiwavelength coverage from the very earliest phases of the burst. The X-ray and optical/UV afterglow decay light curves both exhibit a steeper slope ~0.15 days after the burst, indicative of a jet break. This jet break time combined with the total gamma-ray energy of the burst constrains the opening angle of the jet to be 3.2 degrees. We derive an empirical `time-lag' redshift from the BAT data of z_hat = 0.69 +/- 0.02, in good agreement with the spectroscopic redshift of 0.61. Prior to the jet break, the X-ray data can be modelled by a simple power law with index alpha = -1.2. However after 300 s the X-ray flux brightens by about 30% compared to the power-law fit. The optical/UV data have a more complex decay, with evidence of a rapidly falling reverse shock component that dominates in the first minute or so, giving way to a flatter forward shock component at later times. The multiwavelength X-ray/UV/Optical spectrum of the afterglow shows evidence for migration of the electron cooling frequency through the optical range within 25000 s. The measured temporal decay and spectral indices in the X-ray and optical/UV regimes compare favourably with the standard fireball model for Gamma-ray bursts assuming expansion into a constant density interstellar medium.

J0041+3224: a new double-double radio galaxy

We report the discovery of a double-double radio galaxy (DDRG), J0041+3224, with the Giant Metrewave Radio Telescope (GMRT) and subsequent high-frequency observations with the Very Large Array (VLA). The inner and outer doubles are aligned within about 4 deg and are reasonably collinear with the parent optical galaxy. The outer double has a steeper radio spectrum compared with the inner one. Using an estimated redshift of 0.45, the projected linear sizes of the outer and inner doubles are 969 and 171 kpc respectively. The time scale of interruption of jet activity has been estimated to be about 20 Myr, similar to other known DDRGs. We have compiled a sample of known DDRGs, and have re-examined the inverse correlation between the ratio of the luminosities of the outer to the inner double and the size of the inner double, l_{in}. Unlike the other DDRGs with l_{in} larger than about 50 kpc, the inner double of J0041+3224 is marginally more luminous than the outer one. The two DDRGs with l_{in} less than about a few kpc have a more luminous inner double than the outer one, possibly due to a higher efficiency of conversion of beam energy as the jets propagate through the dense interstellar medium. We have examined the symmetry parameters and find that the inner doubles appear to be more asymmetric in both its armlength and flux density ratios compared with the outer doubles, although they appear marginally more collinear with the core than the outer double. We discuss briefly possible implications of these trends.

High‐Ionization Emission in Metal‐deficient Blue Compact Dwarf Galaxies

Primordial stars are expected to be very massive and hot, producing copious amounts of hard ionizing radiation. The best place to study hard ionizing radiation in the local universe is in very metal-deficient Blue Compact Dwarf (BCD) galaxies. We have carried out a MMT spectroscopic search for [Ne V] 3426 (ionization potential of 7.1 Ryd), [Fe V] 4227 (ionization potential of 4 Ryd) and He II 4686 (ionization potential of 4 Ryd) emission in a sample of 18 BCDs. We have added data from previous work and from the Data Release 3 of the Sloan Digital Sky Survey. In total, we have assembled a BCD high-ionization sample with [Ne V] emission in 4 galaxies, [Fe V] emission in 15 galaxies and He II emission in 465 galaxies. With this large sample, we have reached the following conclusions. There is a general trend of higher [Ne V], [Fe V] and He II emission at lower metallicities. However metallicity is not the only factor which controls the hardness of the radiation. High-mass X-ray binaries and main-sequence stars are probably excluded as the main sources of the very hard ionizing radiation responsible for [Ne V] emission. The most likely source of [Ne V] emission is probably fast radiative shocks moving with velocities > 450 km/s through a dense interstellar medium with an electron number density of several hundreds per cm^-3 and associated with supernova explosions of the most massive stars. These have masses of ~ 50 - 100 Msun and are formed in very compact super-star clusters. The softer ionizing radiation required for He II emission is likely associated with less massive evolved stars and/or radiative shocks moving through a less dense interstellar medium.

A Radio Transient 0.1 Parsecs from Sagittarius A*

We report the discovery of a transient radio source 27 (0.1 pc projected distance) south of the Galactic center massive black hole, Sgr A*. The source flared with a peak of at least 80 mJy in 2004 March. The source was resolved by the Very Large Array into two components with a separation of ~07 and characteristic sizes of ~02. The two components of the source faded with a power-law index of 1.1 ± 0.1. We detect an upper limit to the proper motion of the eastern component of ~3 × 103 km s-1 relative to Sgr A*. We detect a proper motion of ~104 km s-1 for the western component relative to Sgr A*. The transient was also detected at X-ray wavelengths with the Chandra X-Ray Observatory and XMM-Newton and given the designation CXOGC J174540.0-290031. The X-ray source falls in between the two radio components. The maximum luminosity of the X-ray source is ~1036 ergs s-1, significantly sub-Eddington. The radio jet flux density predicted by the X-ray/radio correlation for X-ray binaries is orders of magnitude less than the measured flux density. We conclude that the radio transient is the result of a bipolar jet originating in a single impulsive event from the X-ray source and interacting with the dense interstellar medium of the Galactic center.

Detectable neutrino fluxes due to enhanced cosmic ray densities in the Galactic Centreregion

We examine in detail the detectability of a signal of diffuse high energy neutrinos producedin the Milky Way by the interaction of cosmic rays (CRs) with the interstellarmedium (ISM). We show that highly inhomogeneous galactic CR densities arisenaturally from large-scale drift effects and the inhomogeneous distribution ofCR sources. In particular, the CR density in the Galactic Centre region (wherethe ISM density is maximal) could be an order of magnitude larger than thelocal CR density. Hence, the expected diffuse flux of neutrinos from the GalacticCentre region becomes enhanced and could be detected at energies above∼104 GeVin a km3-size neutrino telescope within ten years of operation. The galactic anisotropy, which is themain signature of this flux, allows us to discriminate this signal from other possibleextraterrestrial diffuse fluxes of high energy neutrinos.