Constraints on a hadronic model for unidentified off-plane galactic gamma-ray sources

Recently the H.E.S.S. collaboration announced the detection of an unidentified gamma-ray source with an off-set from the galactic plane of 3.5°: HESS J1507-622. If the distance of the object is larger than about one kpc it would be physically located outside the galactic disk. The density profile of the ISM perpendicular to the galactic plane, which acts as target material for hadronic gamma-ray production, drops quite fast with increasing distance. This fact places distance dependent constraints on the energetics and properties of off-plane gamma-ray sources like HESS J1507-622 if a hadronic origin of the gamma-ray emission is assumed. For the case of this source it is found that there seems to be no simple way to link this object to the remnant of a stellar explosions.     

Cold gas contents of galaxies and its relation to fueling: Gas dynamics and related phenomena in bar-driven evolution of disk galaxies

The distribution and the dynamics of the cold gas in the circumnuclear regions (r ≲ [1 − a few] kpc) of disk galaxies have been observed at high resolutions of a few arcsec in λ2.6 mm CO (J=1→0) line emission. Distinct CO features are commonly found in the observed disk galaxies and they are different from galaxy to galaxy. They are explained by means of orbit-resonance theories and dynamical evolution. The evolutionary sequence in time is constructed based on the studies of dynamics in theoretical works and numerical simulations in the literature. Not only the behavior of the cold gas but also the starburst, outbreaks of the active galactic nucleus, and the evolution of global structures in disk galaxies are discussed in the bar-driven evolution scenario.     

NMA survey of CO and HCN emission from nearby active galaxies

The Nobeyama Millimeter Array has been used to map CO(1-0) and HCN(1-0) emission in nearby Seyfert galaxies. A wide variety of molecular gas distributions are found, and there appears to be no “typical” gas distribution either in type-1 Seyferts or type-2s. All the gas distributions and kinematics in the observed Seyferts can be understood as a response to a non-axisymmetric potential in the central regions, suggesting that a small scale (a few 100 pc — a few kpc) distortion of the underlying potential is necessary for Seyfert activity, although it is not a sufficient condition. Circumnuclear star formation in the host of the observed Seyferts can occur via gravitational instabilities of the molecular gas, as in the case of star forming regions in non-Seyfert galaxies. Our results may support the idea that the host galaxies of Seyferts (both type-1s and 2s) and non-Seyferts are the same in terms of the fuel and trigger of star formation. Near the center of Seyfert nuclei (r < a few 100 pc), we find that the molecular gas tends to be gravitationally stable. We find that the R_HCN/CO value ranges over an order of magnitude, from 0.086 to 0.6 among Seyfert galaxies. It seems that the presence of kpc scale jet/outflows is related to the extremely high R_HCN/CO values.     

Neutral clouds in a pervasive X-ray gas between the sun and northern HVCs

We present evidence that the soft X-ray distribution observed with the ROSAT PSPC instrument is not adequately explained by the standard Local Hot Bubble model (/1/). We discuss the X-ray absorbing cloud LVC 88 + 36 − 2 embedded in the hot plasma of the Local Hot Bubble, the X-ray shadow of the Draco nebula and other clouds inside and outside the galactic disk, and the X-ray emission associated with halo type objects like the HVC's M I and M II. They populate the distance range from about 60 pc to more than a few kpc and imply the presence of X-ray emitting plasma between the sun and and the outer galactic halo. These observations are consistent with a pervasive X-ray emitting plasma in which neutral clouds are embedded. However, the volume filling factor of this plasma is not known. A model which adequately describes the observed features has been developed and published by Hirth et al. (/28/).

For the first time in the literature we present results of a correlation analysis of X-ray shadows and H I or IR images of a molecular cloud. This is a new technique for the determination of the total column density of hydrogen nuclei for molecular clouds.     

No direct correlation between galactic cosmic rays and earth surface temperature

We have searched for a correlation between galactic cosmic rays (GCRs) and global earth surface air temperature (GST) datasets going back to the year 1900. The linear correlation coefficient between GCRs and GST varies erratically, exhibiting both positive and negative values over time scales varying from about 5–20 years. Since the finding of no persistent correlation is not supported by predictive theory but is what one should expect for two random, un-correlated time series, we infer that GCRs do not influence global surface air temperature.     

Clump-driven galaxy evolution and quasar activity

The collapse of a protogalaxy composed of dark matter and primordial gas has been investigated by numerical simulations and analytical multi-zone modelling in an attempt to examine the early evolution of disk galaxies. The importance of ample interstellar matter existing in young galactic disks has been highlighted. Confrontation of the theoretical results with the available observational data has led to a new picture for disk galaxy evolution, in which the bulge is the secondary object formed from disk matter. Occurrence of quasar activity is also discussed in relation to the dynamical evolution of the host galaxy.     

Nuclear-to-disk rotation curves and mass-to-luminosity ratio in galaxies

High-resolution nuclear-to-outer rotation curves for Sb, SBb, Sc, and SBc galaxies generally show a steep nuclear rise and flat rotation from the disk to the halo. The high-velocity central rotation indicates massive cores within bulges. Since this characteristic is common to most galaxies, the high-velocity central rotation cannot be due to a particular orientation of non-circular motion. Using these rotation curves, we derive the distributions of surface-mass density, and compare them directly with observed surface-luminosity distributions. The mass-to-luminosity ratio (M/L) increases from the outer bulge to the disk, indicating that the outer disk is already dominated by dark-mass. It, then, increases more rapidly toward the outer optical edge, indicating the massive halo. In the central regions of some galaxies, the M/L increases steeply toward the nucleus, reaching a value an order of magnitude greater in the central 100 pc region than that in the disk, which may indicate a massive core of radius ∼ 100 parsecs and mass of ∼ 10⁹ M_⊙. The core may be an object linking a bulge and a black hole at the nucleus.     

Improving the angular resolution of EGRET and new limits on supersymmetric dark matter near the galactic center

Using the EGRET data and an improved point source analysis, including an energy-dependent point spread function and an unbinned maximum likelihood technique, we have been able to place considerably lower limits on the γ-ray flux from the galactic center region. We also test this method on known sources, the Crab and Vela pulsars. In both cases, we find that our method improves the angular precision of EGRET data over the 3EG catalog.This new limit on γ-rays from the galactic center can be used to test models of annihilating supersymmetric dark matter and galactic halo profiles. We find that the present EGRET data can limit many supersymmetric models if the density of the galactic dark matter halo is cuspy or spiked toward the galactic center. We also discuss the ability of GLAST to test these models.     

Modelling of low-energy galactic electrons in the heliosheath

The modulation of cosmic ray electrons in the heliosphere plays an important role in improving our understanding and assessment of the processes applicable to low-energy galactic electrons. A full three-dimensional numerical model based on Parker’s transport equation is used to study the modulation of 10 MeV galactic electrons, in particular inside the heliosheath. The emphasis is placed on the role that perpendicular diffusion plays in causing the extraordinary large increase in the observed intensities of these electrons in the heliosheath. The modelling is compared with observations of 6–14 MeV electrons from the Voyager 1 mission. Results are shown for the radial intensity profiles of these electrons, as well as the modulation effects of varying the extent of the heliosheath by changing the location of the termination shock and the heliopause and the value of the local interstellar spectrum. We confirm that the heliosheath acts as a modulation ‘barrier’ for low-energy galactic electrons. The significance of this result depends on how wide the inner heliosheath is; on how high the very local interstellar spectrum is at these low energies (E < 100 MeV) and on how small perpendicular diffusion is inside the inner heliosheath.     

Results from the Far Ultraviolet Space Telescope (FAUST)

We present early results from the Far Ultraviolet Space Telescope (FAUST), which flew in March 1992 with the ATLAS space shuttle mission. The telescope provides wide-field images in the far ultraviolet (1400–1800 Å). Studies underway using the data obtained on this mission include establishing the brightness and distribution of far ultraviolet stars in the halo of our Galaxy, establishing the far ultraviolet properties of nearby galaxies and nearby clusters of galaxies, analyzing the diffuse galactic light, and searching for the origin of the extragalactic ultraviolet light. We discuss the instrument performance, and early results from these observations.     

Galactic cosmic ray modulation for sunspot cycle 23

We present a study of the galactic cosmic ray modulation for sunspot cycle 23. We use the monthly and the annual mean hourly, pressure corrected, data from neutron monitors of the global network (monthly rate is calculated as the average of the hourly pressure corrected values). We draw attention to an asymmetry in the galactic cosmic ray (GCR) recovery during odd and even cycles for the monthly mean hourly rate data. For over half a century of observations, we find that the recovery for the odd cycles is to a higher level than for the even cycles. Qualitatively the effect is ascribed to charged particle drifts in inhomogeneous interplanetary magnetic field. Even so it has not been possible to arrive at a quantitative, self-consistent, explanation in terms of drifts at higher and lower GCR rigidities. We also study the rigidity dependence of the amplitude of 11-year modulation over a wide range (1–200 GV) of GCR spectrum; it is a power law in rigidity with an exponent −1.22. We discuss the implication of these findings on quasi-linear diffusion theories of modulation. We reflect on GCR recovery pattern for 2006–2009.     

The radial distribution of gamma rays and cosmic rays in the outer Galaxy

The radial distribution of the high-energy (70 MeV-5 GeV) gamma-ray emissivity in the outer Milky Way is derived. The kinematics of HI are used to construct column-density maps in three galacto-centric distance ranges in the outer Galaxy. These maps are used in combination with COS-B gamma-ray data to determine gamma-ray emissivities in these distance ranges. A steep negative gradient of the emissivity for the 70 MeV-150 MeV energy range is found in the outer Galaxy. The emissivity for the 300 MeV-5 GeV range is found to be approximately constant (within 20%) and equal to the local value out to large (20 kpc) galacto-centric distances. These results imply a hardening of the gamma-ray spectrum with increrasing distance and for R > 16 kpc the spectrum is shown to be consistent with a π°-decay spectrum with the intensity expected from the local measurement of the cosmic-ray nuclei spectrum. The energy-dependent decrease is interpreted as a steep gradient in the cosmic-ray electron density and a near constancy of the nuclear component. The galactic origin of electrons with energies up to several hundreds of MeV is confirmed, while for cosmic-ray nuclei with energies of a few GeV either confinement in a large galactic halo or an extragalactic origin is suggested by the data.     

Probing the galactic halo with ROSAT

We discuss the current status of ROSAT shadowing observations designed to search for emission from million degree gas in the halo of the Milky Way galaxy. Preliminary results indicate that million degree halo gas is observed in the 1/4 keV band in some directions, most notably toward the Draco cloud at (ℓ, b) = (92°, +38°), but that the halo emission is patchy and highly anisotropic. Our current understanding of this halo emission is based on a small handful of observations which have been analyzed to date. Many more observations are currently being analyzed or are scheduled for observation within the next year, and we expect our understanding of this component of the galactic halo to improve dramatically in the near future.     

Two intense Forbush decreases of solar activity cycle 22

We study two intense Forbush decreases that occurred during two adjacent SOLTIP (Solar connection of Transient Interplanetary Processes) intervals; namely SOLTIP 1 (22–27 March 1991) and SOLTIP 2 (1–17 June 1991); galactic cosmic ray intensity at the depth of the second Forbush decrease was the lowest ever recorded since continuous monitoring by Climax neutron monitor began in 1951 (58% below the solar minimum value of 1954), indicating extreme conditions in the heliosphere that prevented galactic cosmic rays from reaching the Earth. These decreases were seen propagating in outer heliosphere by the deep space missions Voyagers 1, 2 and Pioneer 10, 11, with suitable time delays. We analyze hourly, pressure corrected, neutron monitor data from the global sites in both hemispheres, and muon telescopes located underground; they respond to 10–300 GV range of the galactic cosmic ray spectrum. This circumstance provides us an ideal opportunity to study the rigidity dependence of the amplitudes of the two Forbush decreases. In both cases the amplitude is found to be a power law in rigidity, with negative exponents.     

The dark matter halo structure of “fossil” groups candidates

We investigated properties of four isolated giant elliptical galaxies with extended X-ray halo using ASCA data. The derived size of X-ray halo, X-ray luminosity, and gravitational mass of the dark halo are unusually large those of X-ray halo of a single galaxy, but are typical for X-ray halos of groups and poor clusters of galaxies. The measured temperatures and abundances of the X-ray halo gas in these galaxies are also similar to those of the groups and poor clusters. Based on these results we identified these galaxies as “isolated X-ray overluminous elliptical galaxy” (IOLEG). The radial profiles of dark halo in these objects were derived from X-ray data. It is found that some are similar to those of compact groups while others are the same as those of normal ellipticals. The dark halos of lOLEGs are thus indistinguishable from those of groups (and poor clusters), which appears to be consistent with a widely believed idea that lOLEGs are a product of dynamical evolution of a compact group. However, mass-to-light ratios of IOLEGs (M₂₀₀/L_B  100–1000) are far greater than those of Hickson compact groups M₂₀₀/L_B  40–60). Since it is hard to consider that total optical luminosity of a compact group decreases by an order of magnitude in the course of dynamical evolution, such difference in the observed mass-to-light ratio between IOLEGs and Hickson compact groups strongly suggests that most IOLEGs have not evolved from compact groups which are observed at present.     

Relation between galactic and solar cosmic radiation at aviation altitude

Cosmic radiation bombards us at high altitude with ionizing particles; the radiation has a galactic component, which is normally dominant, and a component of solar origin. Cosmic ray particles are the primary source of ionization in the atmosphere above 1 km; below 1 km radon is a dominant source of ionization in many areas.     

VLBA imaging of parsec-scale gas in Seyfert galaxies

Seyfert galaxies contain small-scale radio jets that indicate the presence of compact active galactic nuclei of the same type found in more powerful quasars and radio galaxies. Since most Seyferts are relatively nearby, their nuclear environments can be probed at much higher resolution than in those more powerful sources. In addition, the relative weakness of the active nucleus makes the effects of the circumnuclear gas more important. VLBA images of a number of Seyfert galaxies have been produced, often at multiple frequencies, in order to reveal the presence and nature of the gas in the inner 1–100 parsecs of the galaxies. Absorption effects at gigahertz frequencies, due to both synchrotron self-absorption and free-free absorption, are quite common; the free-free absorption indicates the presence of large quantities of ionized gas in the inner few parsecs of the galaxies. Changes in radio position angles on parsec scales also occur in several galaxies, implying that Seyfert nuclei may have multiple symmetry axes.     

Consequences of the application of the streamer fluid model to the study of the sprite inception mechanism

We present the results of a streamer-fluid model used to investigate the electrodynamical coupling between the troposphere and upper atmosphere due to the penetration of lightning electric fields into the mesosphere and the lower ionosphere, generating sprites. The model solves the continuity equation for electrons and ions coupled to Poisson equation. The dominant physical response of the atmosphere is the formation of a screening-ionization wave. The wave shields the atmosphere above it from the action of the lightning field and, together with the conductivity reduction below it due to attachment, the wave amplifies the total field below it, allowing for the penetration of intense electric fields in the mesosphere as it propagates downwards into regions of higher density that compress the wave. This is the key physical mechanism for sprite inception. We evaluated the effects of the thundercloud charge geometry, lightning current waveshape, atmospheric conductivity, via different electron density profiles, and the effect of ionization, attachment and electron mobility coefficients in the electrical breakdown process, related to halo production, and sprite streamer initiation. The results showed that electrons with higher mobility are more efficient in shielding the lightning electric field before breakdown, causing delay, and they contribute to the formation of the streamer seed after breakdown, anticipating the sprite streamer inception. Similarly, a higher effective ionization rate, produced by modifications in the attachment and ionization coefficients, anticipates sprite inception. The simulations with 6 different electron density profiles, and therefore conductivities, spanning 4 orders of magnitude, showed that the altitude of breakdown and sprite initiation, as well as their time delays from the lightning discharge are directly related to atmospheric conductivity: higher conductivities produce halo and sprite inception at lower altitudes with longer delays and may hinder sprite formation. We document that variations of 30 times in the lightning current leads to sprite initiation altitudes in the range 66.0–73.5 km, with delays between 1.550 and 34.500 ms, while variations of 4 orders of magnitude in the conductivity profile lead to initiation altitudes 61.0–70.6 km, with delays in the range 3.825–9.825 ms. Consequently, we suggest that lightning characteristics dominate over atmospheric parameters in determining sprites’ initiation altitude and delay. The simulation of a −CG, with a constant current of 30 kA, did not produce a sprite seed, confirming an asymmetry in the response of the atmosphere to positive and negative lightning. This is due to the free electron drift direction that is away from the screening ionization wave, preventing the formation of the streamer seed for the great majority of −CGs. The same does not apply to halos, which depend on the occurrence of breakdown and can be produced by discharges of both polarities.