The observed 511 keV line from the Galactic Bulge is a real challenge for theoretical astrophysics: despite a lot of suggested mechanisms, there is still no convincing explanation and the origin of the annihilated positrons remains unknown. Here we discuss the possibility that a population of slowly evaporating primordial black holes with the mass around $10^{16}-10^{17}$ g ejects (among other particles) low--energy positrons into the Galaxy. In addition to positrons, we have also calculated the spectrum and number density of photons and neutrinos produced by such black holes and found that the photons are potentially observable in the near future, while the neutrino flux is too weak and below the terrestrial and extra--terrestrial backgrounds. Depending on their mass distribution, such black holes could make a small fraction or the whole cosmological dark matter.
We analyze the processes relevant for star formation in a model with dark matter in the form of sterile neutrinos. Sterile neutrino decays produce an X-ray background radiation that has a two-fold effect on the collapsing clouds of hydrogen. First, the X-rays ionize the gas and cause an increase in the fraction of molecular hydrogen, which makes it easier for the gas to cool and to form stars. Second, the same X-rays deposit a certain amount of heat, which could, in principle, thwart the cooling of gas. We find that, in all the cases we have examined, the overall effect of sterile dark matter …
The 511 keV photons from the galactic center can be explained by positrons produced through Q-ball decay. In the scheme of gauge-mediated supersymmetry breaking, large Q balls with lepton charge are necessarily long-lived. In particular, the lifetime can be as long as (or even longer than) the age of the universe. If kinematically allowed, such large Q balls decay into positrons, which eventually annihilate with electrons into 511 keV photons. Our scenario is realized within the minimal supersymmetric standard model in the inflationary universe, which is very plausible.
We present a possible explanation of the recently observed 511 keV $γ$-ray anomaly with a new ``millicharged'' fermion. The new fermion is light (${\cal O}({\rm MeV})$) but has never been observed by any collider experiments mainly because of its tiny electromagnetic charge $εe$. We show that constraints from its relic density in the Universe and collider experiments allow a parameter range such that the 511 keV cosmic $γ$-ray emission from the galactic bulge may be due to positron production from this millicharged fermion.
Sterile right-handed neutrinos can be naturally embedded in a low scale gauged $U(1)_{B-L}$ extension of the standard model. We show that, within a low reheating scenario, such a neutrino is an interesting candidate for dark matter. We emphasize that if the neutrino mass is of order of MeV, then it accounts for the measured dark matter relic density and also accommodates the observed flux of 511 keV photons from the galactic bulge.
We follow the evolution of the baryonic top-hat overdensity in a single-zone approximation. Our goal is to juxtapose the evolution of the gas temperature in the primordial clouds in the lambda cold dark matter model and the warm dark matter model with keV sterile neutrinos and to check the effects of their decays, into one X-ray photon and one active neutrino, on the structure formation. We find that, in all the cases we have examined, the overall effect of sterile dark matter is to facilitate the cooling of gas and to reduce the minimal mass of the halo able to collapse. Hence, we conclude th…
We propose a possible explanation for the recently observed anomalous 511 keV line with a new "millicharged" fermion. This new fermion is light [${\cal O}({\rm MeV})$]. Nevertheless, it has never been observed by any collider experiments by virtue of its tiny electromagnetic charge $εe$. In particular, we constrain parameters of this millicharged particle if the 511 keV cosmic $γ$-ray emission from the galactic bulge is due to positron production from this new particle.
We show that the $e^++e^-$ decay of a light scalar boson of mass 1-10 MeV may account for the fluxes of 511 keV gamma ray observed by SPI/INTEGRAL. We argue that candidates of such a light scalar boson is one of the string moduli or a scalar partner of the axion in a supersymmetric theory.
We consider possible mechanisms via which electroweak scale WIMPs χ^0 could provide the source of the INTEGRAL/SPI 511 keV photon flux from the galactic centre. We consider scenarios where the WIMP spectrum contains near-degeneracies, with MeV-scale splitting, and focus on three possible production mechanisms for galactic positrons: (i) collisional excitation of the WIMP to a nearby charged state, χ^0 + χ^0 -> χ^+ + χ^-, with the subsequent decay producing positrons; (ii) capture of the WIMP by nuclei in the galactic interstellar medium, χ^0 + N -> e^+ + (χ^- N); and (iii) the decay of a nearb…
We propose a mechanism to generate large-scale magnetic fields with correlation lengths of 100 kpc. Domain walls with QCD scale internal structure form and coalesce obtaining Hubble scale correlations and align nucleon spins. Due to strong CP violation, nucleons in these walls have anomalous electric and magnetic dipole moments and thus the walls are ferromagnetic. This induces electromagnetic fields with Hubble size correlations. The same CP violation also induces a maximal helicity (Chern-Simons) correlated through the Hubble volume which supports an inverse cascade allowing the initial corr…
We point out that the lower bound on the primordial magnetic field required to seed the galactic dynamo is significantly relaxed in an open universe or in a universe with a positive cosmological constant. It is shown that, for reasonable cosmological parameters, primordial seed fields of strength 10^{-30} Gauss or less at the time of galaxy formation could explain observed galactic magnetic fields. As a consequence, mechanisms of primordial magnetic seed-field generation that have previously been ruled out could well be viable. We also comment on the implications of the observation of micro-Ga…
In order to elucidate origin of the 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 ~6.4 keV,~6.7 keV and ~6.97 keV, which are K-lines from neutral (or low-ionized), He-like, and H-like iron ions, respectively. The 6.7 keV line is clearly seen in all 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 pla…
It was recently shown that the vanishing of the Coulomb field related to the electric charge of a black hole by virtue of a tiny but non-vanishing photon mass can produce an electric asymmetry in the universe. This process can have place both at a cosmological early stage, through primordial black holes, and in contemporary universe, by means of supermassive black holes at galactic centers. Possible consequences of the latter case about the role of electrogenesis in the generation of magnetic fields coherent on galactic scale have been investigated. In this paper we will discuss the electrogen…
The deuterium abundances inferred from observations of the interstellar medium within 1-2 kpc of the Sun range over a factor of three and the corresponding oxygen abundances show an even larger dispersion. While the lower D (and O) abundances likely result from depletion onto dust, the higher D abundances are consistent with the BBN-predicted primordial D abundance and chemical evolution models of the Galaxy with infall of primordial or nearly primordial material. The large ranges in deuterium and oxygen abundances suggest that the effects of depletion and/or infall have not been homogenized i…
We systematically study the shortest time duration gamma ray bursts and find unique features that are best interpreted as sources of a galactic origin. There is a significant angular asymmetry and the V/Vmax distribution provides evidence for a homogenous or Euclidean source distribution. We eview the arguments that primordial black hole evaporation can give such GRBs. The rate of events is consistent with a PBH origin if we assume on enhanced local density, as are the other distributions. We suggest further tests of this hypothesis.
The pre-Galactic abundance of lithium offers a unique window into non-thermal cosmological processes. The primordial Li abundance is guaranteed to be present and probes big bang nucleosynthesis (BBN), while an additional Li component is likely to have been produced by cosmic rays accelerated in large scale structure formation. Pre-Galactic Li currently can only be observed in low metallicity Galactic halo stars, but abundance measurements are plagued with systematic uncertainties due to modeling of stellar atmospheres and convection. We propose a new site for measuring pre-Galactic Li: low-met…
The origin of both the diffuse high-latitude MeV gamma-ray emission and the 511 keV line flux from the Galactic bulge are uncertain. Previous studies have invoked dark matter physics to independently explain these observations, though as yet none has been able to explain both of these emissions within the well-motivated framework of Weakly-Interacting Massive Particles (WIMPs). Here we use an unstable WIMP dark matter model to show that it is in fact possible to simultaneously reconcile both of these observations, and in the process show a remarkable coincidence: decaying dark matter with MeV …
Globular clusters in galaxies have a mutual feedback with the environment, which is tuned by their dynamical evolution. This feedback may be the explanation of various features of both the globular cluster system and the host galaxy. Relevant examples of these are the radial distribution of globulars and the violent initial activity of galaxies as AGN.
We show that, during false vacuum inflation, a primordial magnetic field can be created, sufficiently strong to seed the galactic dynamo and generate the observed galactic magnetic fields. Considering the inflaton dominated regime, our field is produced by the Higgs-field gradients, resulting from a grand unified phase transition. The evolution of the field is followed from its creation through to the epoch of structure formation, subject to the relevant constraints. We find that it is possible to create a magnetic field of sufficient magnitude, provided the phase transition occurs during the …
Observations of the central radian of the Galaxy by the Reuven Ramaty High Energy Solar Spectroscopy Imager (RHESSI) have yielded a high-resolution measurement of the 1809 keV line from 26Al, detected at 11 sigma significance in nine months of data. The RHESSI result for the width of the cosmic line is 2.03 (+0.78, -1.21) keV FWHM. The best fit line width of 5.4 keV FWHM reported by Naya et al. (1996) using the Gamma-Ray Imaging Spectrometer (GRIS) balloon instrument is rejected with high confidence.
We study the primordial magnetic field generated by stochastic currents produced by scalar charged particles created at the beginning of the radiation dominated epoch. We find that for the mass range 10^(-6)GeV \leq m \leq 10^2 GeV, a field of sufficient intensity to seed different mechanisms of galactic magnetic field generation, while still consistent with observational and theoretical constraints, is created coherently over a galactic scale.
The galactic evolutionary theory is now sufficiently mature to provide information about the galaxy formation phase. From evolutionary models we establish a link between the present features of late type galaxies and the protogalactic density fluctuations. This link is used to estimate the fluctuation power spectrum amplitude at galactic scales. The agreement with the power spectrum derived from galaxy distribution and extrapolated to galactic scales, is satisfactory. Meanwhile, the comparison with respect to the standard cold dark matter power spectrum normalized to COBE satellite meassuremen…
We consider the observational constraints from the detection of antiprotons in the Galaxy on the amount of Primordial Black Holes (PBH) produced from primordial power spectra with a bumpy mass variance. Though essentially equivalent at the present time to the constraints from the diffuse $γ$-ray background, they allow a widely independent approach and they should improve sensibly in the nearby future. We discuss the resulting constraints on inflationary parameters using a Broken Scale Invariance (BSI) model as a concrete example.
Galactic destruction of primordial deuterium is inevitably linked through star formation to the chemical evolution of the Galaxy. The relatively high present gas content and low metallicity suggest only modest D-destruction. In concert with deuterium abundances derived from solar system and/or interstellar observations this suggests a primordial deuterium abundance in possible conflict with data from some high-redshift, low-metallicity QSO absorbers. We have explored a variety of chemical evolution models including infall of processed material and early, supernovae-driven winds with the aim of…
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) …
In this paper, we propose a mechanism to generate large-scale magnetic fields with correlations lengths of 100 kpc. Domain walls with QCD scale internal structure form and coalesce obtaining Hubble size correlations and align nucleon spins. Due to strong CP violation, nucleons in these walls have anomalous electric and magnetic dipole moments and thus the walls are ferromagnetic. This induces electromagnetic fields with Hubble size correlations. The same CP violation also induces maximal helicity (Chern-Simons) correlated through the Hubble volume which supports an "inverse cascade" allowing t…
We provide first constraints on the morphology of the 511 keV line emission from the galactic centre region on basis of data taken with the spectrometer SPI on the INTEGRAL gamma-ray observatory. The data suggest an azimuthally symmetric galactic bulge component with FWHM of ~9 deg with a 2 sigma uncertainty range covering 6-18 deg. The 511 keV line flux in the bulge component amounts to (9.9+4.7-2.1) 10e-4 ph cm-2 s-1. No evidence for a galactic disk component has been found so far; upper 2 sigma flux limits in the range (1.4-3.4) 10e-3 ph cm-2 s-1 have been obtained that depend on the assume…
The annihilation of new dark matter candidates with masses $m_X$ in the MeV range may account for the galactic positrons that are required to explain the 511 keV $γ$-ray flux from the galactic bulge. We study the impact of MeV-mass thermal relic particles on the primordial synthesis of $^2$H, $^4$He, and $^7$Li. If the new particles are in thermal equilibrium with neutrinos during the nucleosynthesis epoch they increase the helium mass fraction for $m_X\alt 10$ MeV and are thus disfavored. If they couple primarily to the electromagnetic plasma they can have the opposite effect of lowering both…
We report the first measurements of the 511 keV line emission from the Galactic Centre (GC) region performed with the spectrometer SPI on the space observatory INTEGRAL (International Gamma-Ray Astrophysics Laboratory). Taking into account the range of spatial distribution models which are consistent with the data, we derive a flux of $9.9^{+4.7}_{-2.1} \times 10^{-4}$ ph cm$^{-2}$ s$^{-1}$ and an intrinsic line width of $2.95^{+0.45}_{-0.51}$ keV (FWHM). The results are consistent with other high-spectroscopy measurements, though the width is found to be at the upper bound of previously repor…
We present a mechanism for generating primordial magnetic fields with large correlation lengths on the order of 100 kpc today. The mechanism is based on recently conjectured QCD domain walls or similar CP violating domain walls with QCD scale structure. Such domain walls align the electric and magnetic dipole moments of the nucleons shortly after the QCD phase transition, producing electromagnetic fields correlated along the domain walls. Through the Kibble mechanism, the domain walls attain Hubble-scale correlations which they transfer to the aligned electromagnetic fields. Due to the CP viol…
We investigate cosmic-ray antiprotons emitted from the galactic primordial black holes in the Randall-Sundrum type-2 braneworld. The recent results of the BESS antiproton observation implies the existence of exotic primary sub-GeV antiprotons, one of whose most probable origin is PBHs in Our Galaxy. We show that the magnitude of antiproton flux from PBHs in the RS braneworld is proportional to negative power of the AdS radius, and immediately find that a large extra-dimension can relax upper-limits on the abundance of the Galactic PBHs. If actually there are more PBHs than the known upper-limi…
The Galactic background radiation near the Scutum Arm was observed simultaneously with RXTE and OSSE in order to determine the spectral shape and the origin of the emission in the hard X-ray/soft gamma-ray band. The spectrum in the 3 keV to 1 MeV band is well modeled by 4 components: a high energy continuum dominating above 500 keV that can be characterized by a power law of photon index ~ 1.6 (an extrapolation from measurements above ~ 1 MeV); a positron annihilation line at 511 keV and positronium continuum; a variable hard X-ray/soft gamma-ray component that dominates between 10-200 keV (wi…
In this letter we present a map of the GC in the 1-0 line of SiO covering the same region than the ASCA observations of the Fe 6.4 keV line. The SiO emission is restricted to molecular clouds with radial velocity between 10 and 60 km/s in the Sgr A and Sgr B complexes. We find a correlation between the SiO morphology and the spatial distribution of the Fe 6.4 keV line, on the large scale and also within Sgr A and Sgr B. The SiO abundance increases by a factor of 20 in the regions with strong Fe 6.4 keV line. This indicates that the Fe 6.4 keV line mainly arises from molecular clouds with large…
We have observed a typical Galactic plane field at (l,b) = (28.46d, -0.20d) with Suzaku for 100 ksec to carry out a precise spectral study of the Galactic Ridge X-ray Emission (GRXE). The field is known to be devoid of X-ray point sources brighter than ~2 x 10^{-13} ergs s^{-1} cm^{-2} (2--10 keV), and already deeply observed with Chandra. Thanks to the low and stable background and high spectral resolution of Suzaku, we were able to resolve, for the first time, three narrow iron K-emission lines from low-ionized (6.41 keV), helium-like (6.67 keV), and hydrogenic ions (7.00 keV) in the GRXE sp…
We consider the phenomenology of the 511 keV emission in the Galactic bulge, as recently observed by INTEGRAL, and propose a model is which the positrons are produced by gamma-ray bursts (GRB) associated with mini starbursts in the central molecular zone (CMZ). We show that the positrons can easily diffuse across the bulge on timescales of about 10^7 yr, and that their injection rate by GRBs is compatible with the observed fluxes if the mean time between two GRBs in the bulge is about 8 10^4 yr x E_GRB_51. We also explain the low disk-to-bulge emission ratio by noting that positrons from GRBs …
It was suggested by several authors that hypothetical primordial black holes (PBHs) may contribute to the dark matter in our Galaxy. There are strong constraints based on the Hawking evaporation that practically exclude PBHs with masses m~1e15-1e16g and smaller as significant contributors to the Galactic dark matter. Similarly, PBHs with masses greater than about 1e26g are practically excluded by the gravitational lensing observation. The mass range between 10e16g<m<10e26g is unconstrained. In this paper, we examine possible observational signatures in the unexplored mass range, investigating …
We present X-ray observations of the ultraluminous infrared galaxy, UGC 5101, thought to contain a buried active galactic nucleus (AGN) based on observations in other wavebands. We detected an absorbed hard component at >3 keV, as well as soft emission in the energy range 0.5-2 keV. The soft X-ray component, possibly due to a modestly dust-obscured, extended starburst, has an absorption-corrected 0.5-2 keV X-ray luminosity of LX(0.5-2 keV) = 1.2 * 10^41 ergs s^-1. The 0.5-2 keV X-ray to infrared luminosity ratio is a factor of 5 lower than typical values for a normal starburst, suggesting that…
Observations of a strong and extended positron-electron annihilation line emission in the Galactic center (GC) region by the SPI/INTEGRAL are challenging to the existing models of positron sources in the Galaxy. In this paper, we study the possibility that pulsar winds from a millisecond pulsar population in the GC produce the 511 keV line. Our preliminary estimations predict that the e+/- annihilation rate in the GC is around 5 x 10E+42 sE-1, which is consistent with the present observational constraints. Therefore, the e+/- pairs from pulsar winds can contribute significantly to the positron…
The Arecibo HI Strip Survey probed the halos of ~300 cataloged galaxies and the environments of ~14 groups with sensitivity to neutral hydrogen masses > 10^7 M_sun. The survey detected no objects with properties resembling the High Velocity Clouds (HVCs) associated with the Milky Way or Local Group. If the HVCs were typically M_HI=10^{7.5} M_sun objects distributed throughout groups and galaxy halos at distances of 1 Mpc, the survey should have made ~70 HVC detections in groups and ~250 detections around galaxies. The null detection implies that HVCs are deployed at typical distances of <200 k…
We show that a tangle of light superconducting strings in the Milky Way could be the source of the observed 511 KeV emission from electron-positron annihilation in the Galactic bulge. The scenario predicts a flux that is in agreement with observations if the strings are at the ~ 1 TeV scale making the particle physics within reach of planned accelerator experiments. The emission is directly proportional to the galactic magnetic field and future observations should be able to differentiate the superconducting string scenario from other proposals.
The BeppoSAX satellite performed a survey of the Galactic Center Region in the 1-10 keV energy band with its Narrow Field Instruments. Several bright X-ray sources containing neutron stars and black holes have been observed and studied, including the possible counterpart of SgrA*. Here we report the results on the diffuse emission coming from the SgrA Complex. The emission from within 8 arcmin from SgrA* has a double-temperature thermal spectrum (kT1~0.6 keV and kT2~8 keV) and an energy-dependent morphology: the hard emission (5-10 keV) is elongated along the galactic plane, while the soft one…
There are several lines of evidence that the super-massive black hole at the Galactic center had higher activities in the past than directly observed at present. Here I show that these lines of evidence can quantitatively and consistently be explained if the mean accretion rate during the past ~10^7 yrs has been ~10^{3-4} times higher than the current rate, by the picture of radiatively inefficient accretion flow (RIAF) and associated outflow that has been successfully applied to Sgr A*. I argue that this increased rate and its duration are theoretically reasonable in the Galactic center envir…
A mosaic of 5 \ros~PSPC pointed observations in the Galactic plane ($l\sim25^{\circ}$) reveals X-ray shadows in the $0.5-2.0$ keV band cast by distant molecular clouds. The observed on-cloud and off-cloud X-ray fluxes indicate that $\sim15$% and $\sim37$% of the diffuse X-ray background in this direction in the \tq~keV and 1.5 keV bands, respectively, originates behind the molecular gas which is located at $\sim$3 kpc from the Sun. The implication of the derived background X-ray flux beyond the absorbing molecular cloud is consistent with, and lends further support to recent observations of a …
This paper reports the diffuse X-ray features around the Galactic center observed with Chandra. We confirm the ASCA and Ginga discoveries of the large-scale thin-thermal plasma with strong lines in the Galactic center region. In addition, many small clumps of emission lines from neutral (6.4 keV line) to He-like (6.7 keV line) irons are discovered. The 6.4 keV line clumps would be reflection nebulae, while those of the 6.7 keV line are likely SNRs. We also find emission lines of intermediate energy between 6.5-6.7 keV, which are attributable to young SNRs in non equilibrium ionization. Non-the…
A mosaic of 7 ROSAT PSPC pointed observations in the direction of (l,b ~ 10,0 deg) reveals deep X-ray shadows in the 0.5-2.0 keV band cast by dense molecular gas. The comparison between the observed on-cloud and off-cloud X-ray fluxes indicates that ~43% of the diffuse X-ray background in this direction in both the 3/4 keV and 1.5 keV bands originates behind the molecular gas, which is located at 2-4 kpc from the Sun. Given the short mean free path of X-rays in the 3/4 keV band in the Galactic plane (~1 kpc assuming an average space density of 1 cm^-3), this large percentage of the observed fl…
A long BeppoSAX observation of the Galactic Center region shows that the spectrum of the diffuse X-ray emission from the SgrA Complex can be described with the sum of two thermal plasma models with temperatures of ~0.6 keV and \~8-9 keV. The spatial distribution of the diffuse emission is energy dependent. While the hard X-ray emission has a nearly symmetric distribution elongated along the Galactic Plane, the soft X-rays (E < 5 keV) are remarkably well correlated with the triangular radio halo of SgrA East. The parameters derived for the soft component support the interpretation of the SgrA E…
We report on the first census of INTEGRAL/IBIS detections ($\gtrsim 4σ$ significance) above 100 keV based on the Core Program and public Open time observations up to April 2005. There are 49 sources detected in the 100-150 keV band of which 14 are also seen in the 150-300 keV range. The low energy sample is dominated by X-ray binary systems of both low and high mass, but also includes 10 active galaxies. Of the binary systems that are detected above 150 keV, more than 50% are associated with black hole candidates, often reported as microquasars. The present survey results are then used to cons…
We derive an X-ray luminosity function for active galactic nuclei (AGN) that accounts for the X-ray source counts in the 0.5-2.0 and 2-10 keV energy ranges, the redshift distribution of AGNs in the ROSAT Deep Survey (RDS), as well as the X-ray background (XRB) from 1-10 keV. We emphasize the role of X-ray absorption, which has a large effect on the faint end of the 2-10 keV source counts, as well as on the integrated X-ray background.
We compiled a sample of 95 AGNs serendipitously detected in the 3-20 keV band at Galactic latitude |b|>10 during the RXTE slew survey (XSS, Revnivtsev et al.), and utilize it to study the statistical properties of the local population of AGNs, including X-ray luminosity function and absorption distribution. We find that among low X-ray luminosity (Lx < 10^43.5 erg/s) AGNs, the ratio of absorbed (characterized by intrinsic absorption in the range 10^22 cm^-2 < NH < 10^24 cm^-2) and unabsorbed (NH < 10^22 cm^-2) objects is 2:1, while this ratio drops to less than 1:5 for higher luminosity AGNs. …
We present the results of a Suzaku study of a bright point-like source in the 6.7 keV intensity map of the Galactic center region. We detected an intense FeXXV 6.7 keV line with an equivalent width of ~1 keV as well as emission lines of highly ionized Ar and Ca from a spectrum obtained by the X-ray Imaging Spectrometer. The overall spectrum is described very well by a heavily absorbed (~2x10^{23}cm^{-2}) thin thermal plasma model with a temperature of 3.8+/-0.6 keV and a luminosity of ~3x10^{34} erg s^{-1} (2.0--8.0 keV) at 8 kpc. The absorption, temperature, luminosity, and the 6.7 keV line i…
We consider models in which the (transverse) photon mass is non-zero during inflation and drops to zero non-adiabatically at the end of inflation. Through this process, vacuum fluctuations of the photon field are converted into physical, long-wavelength modes with high amplitude. The resulting spectrum of the field strength is approximately B_l ~ 1/l, where l is the relevant coherence scale. With a reasonable model of field evolution we obtain, on comoving galactic scales, a magnetic field strong enough to seed a dynamo mechanism and generate the observed galactic magnetic fields.
Using stellar luminosity functions derived from ROSAT data, the contributions of Galactic stars to the diffuse X-ray background are calculated for ROSAT PSPC energy bands. The model follows that of Guillout et al. (1996), but uses ROSAT rather than {\it Einstein} data to determine the intrinsic luminosity distributions. The model adequately predicts the numbers of stellar sources observed in deep ROSAT surveys. The contribution of unresolved stellar sources to the ROSAT All-Sky Survey at the Galactic poles is 6.85, 4.76, and 4.91 $\times$ counts s$^{-1}$ arcmin$^{-2}$ in bands R12 (1/4 keV), R…
The imaging spectrometer SPI on board ESA's INTEGRAL observatory provides us with an unprecedented view of positron annihilation in our Galaxy. The first sky maps in the 511 keV annihilation line and in the positronium continuum from SPI showed a puzzling concentration of annihilation radiation in the Galactic bulge region. By now, more than twice as many INTEGRAL observations are available, offering new clues to the origin of Galactic positrons. We present the current status of our analyses of this augmented data set. We now detect significant emission from outside the Galactic bulge region. …
The shape of the gamma-ray line from radioactive 26Al, at 1808.7 keV energy in the frame of the decaying isotope, is determined by its kinematics when it decays, typically 10^6 y after its ejection into the interstellar medium from its nucleosynthesis source. Three measurements of the line width exist: HEAO-C's 1982 value of (0+3) keV FWHM, the GRIS 1996 value of (5.4 +/- 1.3) keV FWHM, and the recent RHESSI value of (2.0 +/- 0.8) keV FWHM, suggesting either "cold", "hot", or "warm" 26Al in the ISM. We model the line width as expected from Galactic rotation, expanding supernova ejecta, and/or …
We consider the cosmological model in which a part of the Universe Ω_h\sim 10^-5 is in the form of primordial black holes with mass \sim 10^5M_\odot. These primordial black holes would be centers for growing protogalaxies which experienced multiple mergers with ordinary galaxies. This process of galaxies formation is accompanied by the merging of central black holes in the galactic nuclei. It is shown that recently discovered correlations between the central black holes and bulges of galaxies are naturally reproduced in this scenario.
We consider the hypothesis that galactic magnetic fields are primordial. We also discuss the various objections to this hypothesis. We assume that there was a magnetic field present in the galactic plasma before the galaxy formed. After the galactic disk formed, the lines of force thread through it and remain connected to the external cosmic medium. They enter through one side of the disk, proceed horizontally a distance $ l $ in the disk, and then leave through the other side. We find that the lines of force are stretched rotation of the galactic disk, which amplifies the toroidal component o…
We observed the newly discovered Galactic jet source XTE J0421+560 (= CI Cam) with ASCA from 1998 April 3.3 to April 4.1 (UT), three days after the beginning of the outburst. The X-ray intensity in the 1-10 keV band gradually decreased with an e-folding time of about 30 hours; the decline was accompanied by spectral softening. Two flare-like intensity enhancements were detected below 1 keV. We could fit the average spectrum above 0.8 keV with a two-temperature model (5.7 and 1.1 keV) of thermal emission from an optically thin ionization-equilibrium plasma. The broad iron-K profile, however, re…
We review some of the expectations for Primordial Black Hole evaporation and we review the properties of short Gamma Ray Bursts. It is plausible that these GRB come from PBH evaporation. We then show that the angular distribution asymmetry recently observed by us could be due to a concentration of PBH in the Galactic Arms like the Orion arm.
We study the spatial distribution of Galactic metal-free stars by combining an extremely high-resolution (7.8 X 10^5 solar masses per particle) Cold Dark Matter N-body simulation of the Milky-Way with a semi-analytic model of metal enrichment. This approach allows us to resolve halos with virial temperatures down to the 10^4K atomic cooling limit, and it is sufficiently flexible to make a number of robust conclusions, despite the extremely uncertain properties of the first stars. Galactic metal-free stars are formed over a large redshift range, which peaks at z~10, but continues down to z~5, c…
We present results of a study of the Galactic ridge X-ray emission (GRXE) in hard X-rays performed with the IBIS telescope aboard INTEGRAL. The imaging capabilities of this coding aperture telescope make it possible to account for the flux from bright Galactic point sources whereas the wide field of view permits to collect large flux from the underlying GRXE. Extensive study of the IBIS/ISGRI detector background allowed us to construct a model that predicts the detector count rate with $\sim1-2$% accuracy in the energy band 17-60 keV. The derived longitude and latitude profiles of the ridge em…
We present X-ray spectra of four ultraluminous infrared galaxies (ULIRGs) with detectable broad near-infrared emission lines produced by active galactic nuclei (AGNs): Mrk 463, PKS 1345+12, IRAS 05189-2524, and IRAS 07598+6508. With the exception of IRAS 07598+6508, high quality X-ray spectra obtained with XMM or Chandra show modest (30-340 eV) equivalent widths of the 6.4 keV Fe K alpha emission line and clear signatures for absorption at a level of 4-33 * 10^22 cm^-2 for the main power-law components from the AGNs. These spectral properties are typical of Compton-thin AGNs, and so we estimat…
