Local Hot Bubble Research Articles

A New View of the LHB and ¼ keV X-ray Halo

AbstractThe X-ray sky at ¼ keV is completely dominated by diffuse emission. It has become clear that it originates as at least three separate components: local emission within the nearest ~ 100 pc from the Sun, halo emission from beyond most of the neutral material of the Galactic disk, and the superposition of unresolved extragalactic sources. The only way to determine the temperatures and relative emission measures of the hot plasma responsible for the Galactic components is to use the X-ray intensity variations due to column density variations in the intervening Hi to separate the components. “Shadowing” studies have been pursued for individual objects using ROSAT data, from both pointed observations and the all-sky survey with considerable success.This paper presents the results of an all-sky analysis of the ¼ keV background from the ROSAT survey. A Local Hot Bubble is found consistent with, although somewhat smaller than, previous models. It has a temperature of 106.1 K and an emission measure which varies by a factor of ~ 3.3 over large angles. The halo emission has a temperature near 106.0 K with an emission measure which varies from near zero to more than five times that of the local emission.

Intensity Variations of the Soft X-ray Background: the Boundary Structure of the Local Hot Bubble at Low Galactic Latitudes

Abstract42 ROSAT PSPC pointed observations in the Galactic plane (l ~ 4° – 26°) are mosaicked in order to study the spatial structure of the X-ray emitting gas in the Local Hot Bubble (LHB). Degree scale X-ray intensity variations are detected at the ±10% level in the ¼ keV band, which imply a likely influence from a clumpy boundary shell of the LHB in the observed ¼ keV band X-ray background. The possible origins of such a clumpy boundary structure of the LHB are discussed.

On the Zero-Level of the Soft X-ray Background

AbstractThe soft X-ray background (SXRB, 0.1 – 2.0 keV) is generally believed to be comprised of a local component, the Local (Hot) Bubble (LHB), which is only marginally absorbed, and of distant emission (galactic halo, extragalactic background). It is vital to know the foreground emission (spectrum, intensity) if one wants to disentangle and determine the 3-dimensional structure of the X-ray emitting regions. During the ROSAT PSPC All-Sky Survey non-cosmic enhancements of the measured X-ray count rate have been observed. These lasted typically for up to 8 hours (i.e. a couple of orbits). Using PSPC data it has been shown that these so-called “long-term enhancements” (LXE) are related to geomagnetic storms and variations of the solar wind. Moreover, during an observation of the comet Hyakutake indications for a correlation of the X-ray background with the X-ray brightness of the comet have been found. This can be interpreted in terms of an X-ray emitting region around the Earth. Although the variable component of these enhancements could be subtracted from the sky maps an uncertainty remained concerning an undetected and relatively constant contribution which might be present in soft X-ray observations. As a consequence this could explain or contribute to the high PSPC count rate observed from the dark side of the Moon.

Radio, Millimeter and Infrared Observations of the Local Hot Bubble and Its Environment

AbstractWe present a definition of the local void of neutral gas from observations in the radio frequency window. We question the concept of the Local Bubble in sense of a more or less spherical volume which is surrounded by a shell of denser gas. The concept of the Local Bubble is challenged by the discovery of numerous neutral, dense clouds inside the local void. The search for a “shell” around the suspected Bubble has resulted only in inconclusive findings so far. The sample of high latitude molecular clouds illustrates the situation particularly well. The statistical properties of their spatial distribution, e.g. the mean distance, seem to fit very nicely to the spatial extent of the Local Bubble. But a more detailed investigation shows that the concept of a bubble – in particular an expanding bubble – is not supported. We suggest that the local void is nothing more than a typical place in an interarm region of our Galaxy.Finally, a discussion of the high latitude boundary of the local void does not give strong evidence for the concept of a bubble, that has once been in rapid expansion and is still showing signs of interaction with its environment. However, indications for interactions of IVCs or HVCs with their surroundings are found. These hint at the presence of a gaseous disk which is much more vertically extended than previously believed, or at a Galactic wind which may be blowing from the Galactic neighbourhood of the Sun.

Small-Scale ¼ keV Fluctuations Due to the Local Hot Bubble

AbstractTo determine fluctuations in the ¼ keV emission due to the LHB, without contamination due to sources within the galactic disk, the Galactic halo or the extragalactic background, we have formed a mosaic of PSPC observations towards the Hyades, a direction where distant emission is effectively blocked by a Galactic column of many optical depths. After removing the point sources from this image, we find the ¼ keV emission to be smooth on scales from 6′ to a few degrees.

The Interstellar Gas in the Line of Sight to ϵ Canis Majoris

We analyse Hubble Space Telescope GHRS observations of the interstellar medium in the direction to ϵ CMa, the strongest EUV source in the sky located 200 pc away in a region deficient in neutral gas. We show that the neutral gas density is the lowest yet measured in a galactic sight-line. The line of sight contains three main components among which the Local Cloud, and we derive their column densities, their velocity their temperature and their turbulence velocity. We discuss the ionization of the Local Cloud and we show that we detect the conductive interface between diffuse local cloud and the hot local bubble.

First spectral observations of the diffuse background with the Extreme Ultraviolet Explorer

We present the first results from the analysis of the spectroscopic observations of diffuse extreme ultraviolet (EUV) emission taken with the Extreme Ultraviolet Explorer (EUVE) spectrometers in the wavelength range 160-740 A. Although not designed or optimized for diffuse observation, the EUVE spectrometers are the most sensitive diffuse EUV spectrometer in orbit. The spectral resolution for diffuse emission of the medium and long-wavelength spectrometers are 17 and 34 A FWHM, respectively. During the period from 1992 July 25 to 1992 August 19, the spectrometers surveyed a 2 x 20 deg field scanned from (l, b) = (24 deg, -28 deg) to (44 deg, -74 deg) with a total effective exposure time of 575,232 s. The only emission lines detected were those of He I and He II (584, 537, 304 A) with intensities consistent with local geocoronal and/or interplanetary scattering of solar radiation (584 A = 1.30 rayleighs; 537 A = 0.040 R; and 304 A = 0.029 R). Models of the soft X-ray background, which results from a 10(exp 6) K plasma (Local Bubble) surrounding the neutral gas near the Sun (Local Cloud), predict that most of the flux from the hot plasma appears as emission lines in the EUV. We have compared these spectral predictions with our observations to place limits on the emission measure versus temperature of the proposed hot plasma. Using the same plasma model, we derived emissions measures for our data and the C and B soft X-ray bands of the Wisconsin rocket survey. We find that our limits for the plasma emission measure are a factor of 5-10 below the C- and B-band emission measures over the temperature range from 10(exp 5.7) to 10(exp 6.4) K. We explore possible scenarios that could reconcile our results with the X-ray surveys and conclude that depletion or a nonequilibrium plasma state rather than absorption are the more likely explanations of the discrepancy. We also show that our spectrum is inconsistent with the spectrum from the approximately 10(exp 5) K gas at the conductive interface between the hot Local Bubble and the cooler Local Cloud given by Slavin (1989). In addition, we place new limits on the helium ionization parameter in the Sun's vicinity caused by the 10(exp 6) K plasma in the Local Bubble.

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.

Implications of ROSAT observations for the local hot bubble

The ROSAT all-sky survey and Guest Observer pointed observations have brought a wealth of high-quality data to the study of the soft X-ray diffuse background. Analysis of the spatial structure of the1/4 keV flux, including observations of shadows cast by discrete clouds in the interstellar medium, have allowed the separation of the observed flux into foreground and background components supporting a spatially varying local component consistent with previous ideas of the local hot bubble, a highly variable galactic halo component, and an extragalactic background.

The galactic diffuse component of soft X-rays and its source function

A new model for the source distribution of galactic soft X-ray (B and C band) emission is presented. From the mean dependence of count rates on galactic latitudeb (i.e., the brightness distribution), we derive the soft X-ray source functionQ as function of the optical depth τ by solving the equation of radiative transfer with the aid of a Laplace transform. Contrary to older ‘Heaviside’ step models,Q is found to increase strongly, but not abruptly, in the range 1.5<τ<2.5, indicating a noticeable emission of X-rays from beyond theHi scale height. Using standard X-ray absorption cross-sections for the interstellar medium, we find that the B band X-ray emission coefficient is non-zero within theHi disk and has a maximum at az-value slightly above the Hi scale height. In the C band, the emission coefficient slightly decreases with increasingz, at least up to theHi scale height. A non-zero source function near the galactic plane implies that the interstellar medium (ISM) within theHi scale height is not only an absorbing layer but is mixed with X-ray emitting regions. The so-called ‘local hot bubble’ is adopted as one of these regions. The maximum of the B band emission coefficient, together with the sharp increase ofQ, is strong evidence for the existence of a galactic soft X-ray ‘halo’, and, moreover, give rise to the assumption of a general intergalactic X-ray background. The ‘effective’ absorption cross-sections σ given in the literature, based on an (pure) exponential dependence in the negative correlation between count rates andHi column densities, were biased to be too small, in particular in the B band. In replacing the Heaviside step (in the ISM) by a smoothed transition region, these inconsistencies become spurious.

The hot interstellar medium

Abstract Several gas phases coexist in the interstellar medium. Besides the cold neutral, the warm neutral, and the warm ionized gases, the existence of hot gas (3.10 5 - 10 6 K) has been revealed through different measurements. Highly ionized species, like SiIV, CIV, NV, OVI have been detected through absorption lines in the spectra of stars by UV satellites (Copernicus, IUE). No strong constraint has been derived on the value of the filling factor of this hot gas in the galactic disk. These absorption lines, together with recently detected CIV emission lines, indicate that the hot gas probably extends as far as 3 kpc above the galactic plane. In this picture, the sun seems to occuppy a very peculiar position in a local hot bubble (T_~10 6 K) depicted by the soft X-ray measurements.

The Local Hot Bubble from X-Ray Spectroscopic Measurements

Soft X-ray surveys proved the existence of a local hot bubble surrounding the solar system. Recently two experiments measured the spectrum of this hot interstellar medium with solid state detectors during rocket flights between 300 and 1000 eV [4-7]. The good resolution of the experiment permitted to detect C V, C VI and O VII lines. In this paper we intend to compare results obtained during these two flights and discuss temperature and abundance determinations from the X-ray spectroscopic measurements already available. In particular the Solid State Spectrometer results are compared with those obtained by a Japanese Group using a Gas Scintillation Proportional Counter [3].

Non Equilibrium Ionization in the Local Hot Bubble

AbstractSoft X-ray surveys proved the existence of a local hot bubble surrounding the solar system (Mc Cammon et al 1983). Recently two experiments measured the spectrum of this hot interstellar medium with solid state detectors during rocket flights between 300 and 1000 eV (Rocchia et al 1984). The good resolution of the experiment permitted to measure CV, CVI and OVII line emissivities with good accuracy. We use these data to put some constraints on a model where the emission comes from a supernova remnant expanding in a finite pressure medium (Cox and Anderson 1982). We show the importance of non equilibrium ionization phenomena.