Galactic Ridge Research Articles

Diffuse radio recombination line emission on the Galactic plane between ℓ=36° and44°

Radio recombination lines (RRLs) can be used to determine the emission measure unambiguously along the Galactic plane. We use the deep (2100 s per beam) H I Parkes Zone of Avoidance (ZOA) survey which includes three RRLs (H166α, H167α and H168α) within its bandwidth. The region � = 36 ◦ to 44 ◦ , b =− 4 ◦ to +4 ◦ is chosen to include emission from the Local, Sagittarius and Scutum arms. An 8 ◦ × 8 ◦ data cube centred at (� , b) = (40 ◦ ,0 ◦ )i s constructed of RRL spectra with velocity and spatial resolution of 27 km s −1 and 15.5 arcmin, respectively. Well-known H II regions are identified as well as the diffuse RRL emission on the Galactic plane. A Galactic latitude section of the integrated RRL emission across the Galactic plane delineates the brightness temperature (T b) distribution which has a half-power width in latitude of � 1. ◦ 5. A value of the electron temperature T e � 8000 K is derived from a comparison with the Wilkinson Microwave Anisotropy Probe (WMAP) free–free maximum entropy method. The T b distribution from the present RRL data is combined with the WMAP 5-yr data to derive the latitude distribution of the residual ‘anomalous’ emission on the Galactic ridge. In this paper, we demonstrate that diffuse ionized emission on the Galactic ridge can be recovered using RRLs from the ZOA survey. This method is therefore able to complement the Hα data at low Galactic latitudes, to enable an all-sky free–free template to be derived.

INTEGRALhard X-ray spectra of the cosmic X-ray background and Galactic ridge emission

We derive the spectra of the cosmic X-ray background (CXB) and of the Galactic ridge X-ray emission (GRXE) in the ~20-200 keV range from the data of the IBIS instrument aboard the INTEGRAL satellite obtained during the four dedicated Earth-occultation observations of early 2006. We analyse the modulation of the IBIS/ISGRI detector counts induced by the passage of the Earth through the field of view of the instrument. Unlike previous studies, we do not fix the spectral shape of the various contributions, but model instead their spatial distribution and derive for each of them the expected modulation of the detector counts. The spectra of the diffuse emission components are obtained by fitting the normalizations of the model lightcurves to the observed modulation in different energy bins. The obtained CXB spectrum is consistent with the historic HEAO-1 results and falls slightly below the spectrum derived with Swift/BAT. A 10% higher normalization of the CXB cannot be completely excluded, but it would imply an unrealistically high albedo of the Earth. The derived spectrum of the GRXE confirms the presence of a minimum around 80 keV with improved statistics and yields an estimate of ~0.6 M_Sun for the average mass of white dwarfs in the Galaxy. The analysis also provides updated normalizations for the spectra of the Earth's albedo and the cosmic-ray induced atmospheric emission.

Broad view on hard X-ray background emission of the Galaxy

AbstractThe nature of the Galactic Ridge X-Ray Emission (GRXE) has been under scientific debate since its discovery more than 30 years ago. It is observed as extended emission along the Galactic disk. The question was: is GRXE truly diffuse or is it composed from a large number of unresolved point sources? Using near-infrared Galaxy maps measured with the DIRBE experiment and data from the INTEGRAL observatory, we show that the galactic background in the energy range 20-60 keV originates from the stellar population of the Galaxy, which is in contrast to the diffuse nature believed before (Krivonos et al., 2007). Here we show preliminary results of studying the transition region from hard X-rays to gamma diffuse background of the Galaxy, revealing the broad band picture of Galactic Background emission.

Warm ionized ISM in the bulge of Andromeda galaxy

AbstractWe demonstrate that unresolved X-ray emission from the bulge of M31 is composed of at least three different components: (i) Broad-band emission from a large number of faint sources – mainly accreting white dwarfs and active binaries, associated with the old stellar population, similar to the Galactic Ridge X-ray emission of the Milky Way. (ii) Soft emission from ionized gas with temperature of about ~ 300 eV and mass of ~ 4 × 106 M⊙. The gas distribution is significantly elongated along the minor axis of the galaxy suggesting that it may be outflowing in the direction perpendicular to the galactic disk. The shadows cast on the gas by spiral arms and the 10-kpc star-forming ring confirm large off-plane extent of the gas. (iii) Hard unresolved emission from spiral arms, most likely associated with protostars and young stellar objects located in the star-forming regions.

Discrete sources as the origin of the Galactic X-ray ridge emission

An unresolved X-ray glow (at energies above a few kiloelectronvolts) was discovered about 25 years ago and found to be coincident with the Galactic disk-the Galactic ridge X-ray emission. This emission has a spectrum characteristic of a approximately 10(8) K optically thin thermal plasma, with a prominent iron emission line at 6.7 keV. The gravitational well of the Galactic disk, however, is far too shallow to confine such a hot interstellar medium; instead, it would flow away at a velocity of a few thousand kilometres per second, exceeding the speed of sound in the gas. To replenish the energy losses requires a source of 10(43) erg s(-1), exceeding by orders of magnitude all plausible energy sources in the Milky Way. An alternative is that the hot plasma is bound to a multitude of faint sources, which is supported by the recently observed similarities in the X-ray and near-infrared surface brightness distributions (the latter traces the Galactic stellar distribution). Here we report that at energies of approximately 6-7 keV, more than 80 per cent of the seemingly diffuse X-ray emission is resolved into discrete sources, probably accreting white dwarfs and coronally active stars.

Iron Emission Lines on the Galactic Ridge Observed with Suzaku

In order to elucidate the origin of galactic ridge X-ray emission, we analyzed Suzaku data taken at various regions along the galactic plane, and studied their Fe-K emission line features. Suzaku resolved the Fe line complex into three narrow lines at $\sim$ 6.4 keV, $\sim$ 6.7 keV and $\sim$ 6.97 keV, which are K-lines from neutral (or low-ionized), He-like, and H-like iron respectively. The 6.7 keV line is clearly seen in all of the observed regions, and its longitudinal distribution is consistent with that determined from previous observations. The 6.4 keV emission line was also found in various galactic plane regions ($b\sim0^{\circ}$). Differences in the flux ratios of the 6.4 keV$/$6.7 keV and 6.97 keV$/$6.7 keV lines between the galactic plane and the galactic center regions were studied and its implication was discussed.

Decaying dark matter can explain the e± excesses

PAMELA and ATIC recently reported excesses in e± cosmic rays. Since the interpretation in terms of DM annihilations was found to be not easily compatible with constraints from photon observations, we consider the DM decay hypothesis and find that it can explain the e± excesses compatibly with all constraints, and can be tested by dedicated HESS observations of the Galactic Ridge. ATIC data indicate a DM mass of about 2 TeV: this mass naturally implies the observed DM abundance relative to ordinary matter if DM is a quasi-stable composite particle with a baryon-like matter asymmetry. Technicolor naturally yields these type of candidates.

Unresolved emission and ionized gas in the bulge of M31

We study the origin of unresolved X-ray emission from the bulge of M31 based on archival Chandra and XMM-Newton observations. We demonstrate that three different components are present. (i) Broad-band emission from a large number of faint sources - mainly accreting white dwarfs and active binaries, associated with the old stellar population, similar to the Galactic ridge X-ray emission of the Milky Way. The X-ray to K-band luminosity ratios are compatible with those for the Milky Way and for M32; in the 2-10 keV band, the ratio is (3.6 ± 0.2) x 10 27 erg s -1 L -1 ⊙ . (ii) Soft emission from ionized gas with a temperature of about ∼300 eV and a mass of ∼2 x 10 6 M ⊙ . The gas distribution is significantly extended along the minor axis of the galaxy, suggesting that it may be outflowing in the direction perpendicular to the galactic disc. The mass and energy supply from evolved stars and Type Ia supemovae is sufficient to sustain the outflow. We also detect a shadow cast on the gas emission by spiral arms and the 10-kpc star-forming ring, confirming significant extent of the gas in the 'vertical' direction. (iii) Hard extended emission from spiral arms, most likely associated with young stellar objects and young stars located in the star-forming regions. The L x /SFR (star formation rate) ratio equals ∼9 x 10 38 (erg s -1 )(Mz yr -1 ) -1 , which is about ∼1/3 of the high-mass X-ray binary contribution, determined earlier from Chandra observations of other nearby galaxies.

INTEGRALSPI All‐Sky View in Soft Gamma Rays: A Study of Point‐Source and Galactic Diffuse Emission

We have processed the data accumulated with the INTEGRAL SPI instrument over 4 years (~51 Ms) to study the morphology of the Galactic emission in the 20 keV to 8 MeV energy range. To achieve this, we simultaneously derived an all-sky census of emitting sources and images of the Galactic ridge (GR) emission. In the central radian, the resolved point-source emission amounts to 88%, 91%, and 68% of the total in the 25-50, 50-100, and 100-300 keV domains, respectively. We compare the spatial distribution of the GR emission with the distributions obtained from CO and near-IR maps and quantify our results through latitude and longitude profiles. Below 50 keV, the SPI data are better traced by the latter, supporting a stellar origin for this emission. Furthermore, we find that the GR emission spectrum follows a power law with a photon index ~1.55 above 50 keV, while an additional component is required below that energy. This component shows a cutoff around 30 keV, reinforcing a stellar origin, as proposed by Krivonos et al. The component of the diffuse emission due to e± annihilations is extracted simultaneously, leading to the determination of the related parameters (positronium flux and fraction). Specific discussion is devoted to the annihilation-line distribution, since significant emission is detected over a region as large as ~80° × ~10°, potentially associated with the disk or halo surrounding the central regions of our Galaxy.

Diffuse TeV emission at the Galactic Centre

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 emission, the tight correlation of the intensity distribution with the molecular gas indicates a pionic-decay process involving relativistic protons. In this paper, we explore the possible source(s) of energetic hadrons at the galactic centre, and their propagation through a turbulent medium. We conclude that though Sagittarius A* itself may be the source of cosmic rays producing the emission in HESS J1745-290, it cannot be responsible for the diffuse emission farther out. A distribution of point sources, such as pulsar wind nebulae dispersed along the galactic plane, similarly do not produce a TeV emission profile consistent with the HESS map. We conclude that only a relativistic proton distribution accelerated throughout the inter-cloud medium can account for the TeV emission profile measured with HESS.

Inverse Compton Origin of the Hard X‐Ray and Soft Gamma‐Ray Emission from the Galactic Ridge

A recent re-determination of the non-thermal component of the hard X-ray to soft {gamma}-ray emission from the Galactic ridge, using the SPI instrument on the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) Observatory, is shown to be well reproduced as inverse-Compton emission from the interstellar medium. Both cosmic-ray primary electrons and secondary electrons and positrons contribute to the emission. The prediction uses the GALPROP model and includes a new calculation of the interstellar radiation field. This may solve a long-standing mystery of the origin of this emission, and potentially opens a new window on Galactic cosmic rays.

Suzaku Spectroscopy of Extended X-Ray Emission in M17

Abstract We present the results of a Suzaku spectroscopic study of soft extended X-ray emission in the HII region of M17. The spectrum of the extended emission was obtained with a high signal-to-noise ratio in a spatially resolved manner using the X-ray Imaging Spectrometer (XIS). We established that contamination by unresolved point sources, the Galactic Ridge X-ray emission, the cosmic X-ray background, and the local hot bubble emission is negligible in the background-subtracted XIS spectrum of the diffuse emission. Half a dozen emission lines were clearly resolved for the first time, including K$\alpha$ lines of highly ionized O, Ne, and Mg as well as L-series complex of Fe at 0.5-1.5keV. Based on diagnosis of these lines, we obtained the following results: (1) the extended emission is an optically thin thermal plasma, represented well by a single temperature of $\sim$3.0$\pm$0.4MK, (2) the abundances of elements with emission lines in the diffuse spectrum are 0.1-0.3 solar, while those of bright discrete sources are 0.3-1.5 solar, (3) the metal abundances relative to each other in the diffuse emission are consistent with solar, except for a Ne enhancement of a factor of $\sim$2, (4) both the plasma temperature and the chemical composition of the diffuse emission show no spatial variation across the studied spatial scale of $\sim$5pc.

Spectral Study of the Galactic Ridge X-Ray Emission with Suzaku

Abstract In order to carry out a precise spectral study of the Galactic Ridge X-ray Emission using Suzaku, we have observed a typical Galactic plane field at $(l,b) =$ (28D46, $-$0D20), which is already deeply observed with Chandra, and known to be devoid of bright X-ray point sources. Thanks to the low background and high spectral resolution of Suzaku, we were able to resolve three narrow iron K-emission lines from low-ionized (6.41keV), helium-like (6.67keV), and hydrogenic ions (7.00keV). The cosmic-ray ion charge-exchange model or the non-equilibrium ionization plasma model are unlikely to explain these line features, since they require either broad emission lines or lines at intermediate ionization states. Collisional ionization equilibrium plasma is the likely origin for the 6.67keV and 7.00keV lines; however, the origin of the 6.41keV line, which is due to fluorescence from cold material, has not been elucidated. We could also precisely measure the absolute X-ray surface brightness in the direction of the Galactic plane. Excluding point sources brighter than $\sim$2 $\times 10^{-13}$ergs$^{-1}$cm$^{-2}$ (2-10keV), the total surface brightness on the Galactic plane is $\sim$6.1 $\times 10^{-11}$ergs$^{-1}$cm$^{-2}$deg$^{-2}$ (2-10keV), including the contribution of the cosmic X-ray background, which is estimated to be $\sim$1.3$\times 10^{-11}$ergs$^{-1}$cm$^{-2}$deg$^{-2}$.

Galactic X‐ray source populations

AbstractThe landscape of Galactic X‐ray sources made of accreting binaries, isolated objects and active stellar coronae has been significantly modified by the advent of the Chandra, XMM‐Newton and INTEGRAL satellites. New types of relatively low X‐ray luminosity X‐ray binaries have been unveiled in the Galactic disc, while deep observations of the central regions have revealed large numbers of X‐ray binaries of so far poorly constrained nature. Because of the high spatial resolution needed and faint X‐ray luminosities generally emitted, studying the dependency of the X‐ray source composition with parent stellar population, Galactic disc, bulge, nuclear bulge, etc., is only practicable in our Galaxy. The evolutionary links between low LX X‐ray binaries and classical X‐ray luminous accreting systems are still open in many cases. In addition, the important question of the nature of the compact sources contributing to the Galactic ridge hard X‐ray emission remains unresolved. We review the most important results gathered by XMM‐Newton over the last years in this domain and show how future observations could be instrumental in addressing several of these issues. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Chandra Detection of Diffuse Hot Gas in and around the M31 Bulge

We report the detection of diffuse hot gas in M31, using archival Chandra observations that allow us to map out a 30' × 30' field (covering a galactocentric radius up to 4.5 kpc) and to detect sources in the galaxy down to a 0.5-8 keV luminosity limit of ~1035 ergs s-1. We estimate the remaining stellar contribution from fainter X-ray sources (primarily cataclysmic variables and coronally active binaries), assuming that they spatially follow the stellar distribution. Indeed, the near-IR K-band light of the galaxy closely traces the 2-8 keV unresolved X-rays, indicating a collective stellar X-ray emissivity consistent with those determined for the Galactic ridge and M32, whereas the amount of the 0.5-2 keV unresolved emission is significantly greater than the expected stellar contribution, especially within a galactocentric radius of ~2 kpc. Morphologically, this soft X-ray excess appears substantially rounder than the bulge as seen in K band and is elongated approximately along the minor-axis at large radii. The excess thus most likely represents the emission of diffuse hot gas in and around the galactic bulge. Furthermore, the near side of the M31 disk casts an apparent shadow against the soft X-ray excess, indicating that the hot gas extends to at least 2.5 kpc from the galactic plane. We briefly discuss the implications of these results on the energy balance in the M31 bulge and on understanding the large-scale soft X-ray enhancement observed toward the inner region of our own Galaxy.

Resolving the Galactic ridge X-ray background

We use Chandra deep observations of the Galactic Center (GC) region to improve the constraints on the unresolved fraction of the Galactic ridge X-ray background in the 4–8 keV band. We emphasize the importance of correcting the measured source counts at low fluxes for bias associated with Poisson noise. We find that at distances of 2 � –4 � from Sgr A ∗ at least ∼40% of the total X-ray emission in the energy band 4–8 keV originates from point sources with luminosities L2−10 keV > 10 31 erg s −1 . From a comparison of the source number-flux function in the GC region with the known luminosity function of faint X-ray sources in the Solar vicinity, we infer that Chandra has already resolved a large fraction of the cumulative contribution of cataclysmic variables to the total X-ray flux from the GC region. This comparison further indicates that most of the yet unresolved X-ray flux from the GC region (∼60%) is likely produced by weak cataclysmic variables and coronally active stars with L2−10 keV < 10 31 erg s −1 . We conclude that the bulk of the high energy Galactic X-ray background in this direction is produced by discrete sources, and that this is likely to be the case for the rest of the Galactic ridge.

EBIT charge-exchange measurements and astrophysical applications

The past decade has seen a surge of interest in astrophysical charge exchange (CX). The impetus was the discovery of X-ray emission from comets in 1996, soon followed by the observation of CX emission in planetary atmospheres and throughout the heliosphere. Geocoronal and heliospheric CX are now recognized as contributing a considerable fraction of the soft X-ray background, and stellar-wind charge exchange is expected to occur in the astrospheres surrounding many stars. CX may also contribute to X-ray line emission in supernova remnants, the Galactic Center, and the Galactic Ridge. This article summarizes the key aspects of CX X-ray emission and its astrophysical relevance, and reviews related laboratory measurements and theoretical predictions with particular attention to spectroscopy experiments conducted on electron beam ion traps.PACS Nos.: 32.30.Rj, 34.70.+e, 39.10.+j, 95.30.Dr

On the contribution of point sources to the Galactic ridge X-ray emission

We analyzed deep Chandra observations of the Galactic plane region centered at l=28.55, b=-0.03 with the aim to obtain the best possible constraints on the contribution of weak point sources to the Galactic ridge X-ray emission (GRXE) in this region. We demonstrate that the vast majority of the detected sources are Galactic in origin and are probably cataclysmic variables and coronally active stars. We use the number-flux function of detected sources to constrain the luminosity function of Galactic X-ray sources in the range 1e30--1e32 erg/s and find good agreement with the luminosity function of sources in the Solar vicinity. The fraction of the total flux at energies 1--7 keV resolved into point sources at the current sensitivity level is ~25%. Excluding the expected contribution of extragalactic sources, ~19% of the GRXE is due to point Galactic sources with interstellar absorption corrected fluxes higher than 1.2e-15 erg/s/cm^2in the energy band 1-7 keV.

Source population synthesis and the Galactic diffuse gamma-ray emission

Population synthesis is used to study the contribution from unresolved sources to the Galactic ridge emission measured by EGRET. Synthesized source counts are compared with the 3rd EGRET catalogue at low and high latitudes. For pulsar-like populations, 5–10% of the emission >100 MeV comes from sources below the EGRET threshold. A steeper luminosity function can increase this to 20% without violating EGRET source statistics. Less luminous populations can produce much higher values without being detected. Since the unresolved source spectrum is different from the interstellar spectrum, it could provide an explanation of the observed MeV and GeV excesses above the predictions, and we give an explicit example of how this could work.

Obtaining Cosmic‐Ray Propagation Parameters from Diffuse Very High Energy Gamma‐Ray Emission from the Galactic Center Ridge

The recent discovery of diffuse, very high energy (VHE) γ-radiation from the Galactic center ridge by the HESS telescope allows for the first time the direct determination of the parameters of Galactic cosmic-ray propagation models. Whereas this discovery showed that the diffuse γ-radiation can be explained by the interaction of VHE cosmic-ray (CR) protons with the interstellar gas located in several giant molecular clouds, we show in this paper that the associated diffusion coefficient for the protons depends on the epoch of activity of the central source of protons: Assuming that the supernova remnant (SNR) Sgr A East was responsible for the particle acceleration, we infer a diffusion coefficient for the Galactic center region of κ = 1-5 kpc2 Myr-1 for a mean proton energy of ~3 TeV. More specifically, for impulsive injection in a 5-10 kyr SNR, we infer a value of κ = 1-2 kpc2 Myr-1, whereas for source activity timescales equal to the age of the SNR, the diffusion coefficient would increase to κ ~ 5 kpc2 Myr-1. These values are smaller than those inferred from local CR abundances. Finally, the above-mentioned values of κ for impulsive injection are equally valid if the required transient source of protons was due to an earlier epoch of stellar infall into the black hole Sgr A*.