Bright X-ray Emission Research Articles

The Cygnus Loop: A Soft‐shelled Supernova Remnant

We present ROSAT Position Sensitive Proportional Counter (PSPC) pointed observations of the Cygnus Loop supernova remnant in two broad soft X-ray bands, characteristic of energies 0.25 and 1.25 keV. The spatial differences between the bands are obvious in an X-ray softness map, which is the ratio of the 0.25 and 1.25 keV emission. The most striking feature of the softness map is a spatially thin (<5') shell that we detect around the entire rim of the supernova remnant. It is not an observational artifact, for the angular extent is greater than the instrumental resolution. Several known interactions with large interstellar clouds are prominent in the softest emission, and the brightest X-ray emission associated with these clouds appears to the interior of these soft features. We conclude that over most of the blast wave, genuinely lower temperatures behind slower shocks soften the X-ray spectrum, and this occurs where the cavity walls of the Cygnus Loop decelerate the blast wave.

The Supergiant Shell LMC 2. I. The Kinematics and Physical Structure

LMC 2 has the brightest, most coherent filamentary structure of all known supergiant shells in the Large Magellanic Cloud. The optical emission-line images show active star formation regions along the western edge and long filaments to the east. ROSAT PSPC and HRI images show bright X-ray emission from within the shell boundary, indicating the presence of hot gas. Counterintuitively, neither high-resolution echelle spectra in the Hα line nor aperture synthesis H I 21 cm emission-line observations show LMC 2 to have the kinematics expected of an expanding shell. Rather, LMC 2 appears to consist of hot gas confined between H I sheets. The interior surfaces of these sheets are ionized by the UV flux of massive stars in the star formation regions along the periphery of LMC 2, while the heating is provided by outflows of hot gas from the star formation regions and by SNRs interior to LMC 2. We have compared LMC 2 to other supergiant shells in the LMC and in more distant galaxies. When the spatial resolution of our data are degraded, we find that LMC 2 resembles supergiant shells observed at a distance of 4 Mpc that have previously been interpreted as expanding shells. Therefore, great caution should be exercised in the analysis and interpretation of the kinematics of distant supergiant shells to prevent overestimates of their velocities and total kinetic energies.

The 6.4-keV fluorescent iron line from cluster cooling flows

The fate of the cooling gas in the central regions of rich clusters of galaxies is not well understood. In one plausible scenario clouds of atomic or molecular gas are formed. However the mass of the cold gas, inferred from measurements of low-energy X-ray absorption, is hardly consistent with the absence of powerful CO or 21-cm emission lines from the cooling flow region. Among the factors which may affect the detectability of the cold clouds are their optical depth, shape and covering fraction. Thus, alternative methods to determine the mass in cold clouds, which are less sensitive to these parameters, are important. For the inner region of the cooling flow (e.g. within a radius of ∼50–100 kpc) the Thomson optical depth of the hot gas in a massive cooling flow can be as large as ∼ 0.01. Assuming that the cooling time in the inner region is few times shorter than the lifetime of the cluster, the Thomson depth of the accumulated cold gas can be accordingly higher (if most of the gas remains in the form of clouds). The illumination of the cold clouds by the X-ray emission of the hot gas should lead to the appearance of a 6.4-keV iron fluorescent line, with an equivalent width proportional to τT. The equivalent width only weakly depends on the detailed properties of the clouds, e.g. on the column density of individual clouds, as long as the column density is less than a few 1023 cm−2. Another effect also associated exclusively with the cold gas is a flux in the Compton shoulder of bright X-ray emission lines. It also scales linearly with the Thomson optical depth of the cold gas. With the new generation of X-ray telescopes, combining large effective area and high spectral resolution, the mass of the cold gas in cooling flows (and its distribution) can be measured.

New Radio and Optical Study of the Supernova Remnant W44

Abstract : We present new optical images of the supernova remnant (SNR) W44 in the H-alpha and [S II] lines covering the entire source for the first time. We also report on improved radio image of W44 at 1442.5 MHz, obtained after the reprocessing of existing VLA data. A spectral index a of -0.4 was derived for the whole source, without indication of significant spectral variations between 0.3 and 1.4 GHz across the remnant. Accurate multiwavelength comparisons were made based on existing observations of W44 in the different spectral regimes. We find excellent correlation between optical and radio emission along the northwest border of the remnant, suggesting that the optical radiation is originating through radiative cooling of the shocked gas immediately behind the shock front. Some diffuse optical emission is also observed towards the interior of W44, with no clear radio counterpart. We confirm that over most of the source, the bright X-ray emission corresponds with regions of low radio brightness. At the northern border of the remnant, diffuse X-ray emission exactly overlaps the radio and optical radiation. From the study of the immediate environs of W44 we conclude that the remnant is interacting with molecular clouds along the eastern border. Such a scenario is compatible with the filamentary structure of W44, the excitation of OH masers and the apparent lack of optical emission along the eastern border. To the north, an extended atomic cloud surrounds the remnant, although the interaction is not evident in this case.

Measurements of the Size and Pressure of the Local Bubble Towards Eridanus

AbstractWe have measured the size and pressure of the Local Bubble in the direction of the constellation Eridanus using ROSAT PSPC observations of soft X-ray “shadows” cast by neutral clouds against the bright X-ray emission from the Orion-Eridanus superbubble. We discuss the technique and present our results for the size of the Local Bubble in the directions of our shadowing fields. Using these distances and the measured soft X-ray emission measures, we derive the Local Bubble’s thermal pressure. For four lines of sight in Eridanus, we obtain pressures of about 1.2 × 104 K cm−3, in good agreement with measurements in several other directions.

Metal Abundance of an X-Ray Emitting Gas in Two Groups of Galaxies: The NGC 5044 Group and HCG 51

We observed two groups of galaxies, the NGC 5044 group (WP 23) and Hickson's compact group HCG 51, with ASCA. We detected an extended bright soft X-ray emission, which indicates the existence of large amounts of a hot X-ray emitting gas in both targets. The temperature of the hot gas is ∼ 1 keV for both objects, which is equivalent to their galaxy velocity dispersion. The metal abundance of the gas is 0.3–0.5 solar value for both objects, which is similar to that of rich clusters of galaxies. The Si to Fe abundance ratio is smaller than that of rich clusters of galaxies, although the iron abundances determined by the Fe-L lines are somewhat uncertain. The X-ray luminosities in the 0.5–10 keV band are 1 × 1043 erg s−1 and 5 × 1042 erg s−1 for the NGC 5044 group and HCG 51, respectively, which are higher by an order of magnitude than those of X-ray bright elliptical galaxies. This makes HCG 51 one of the most X-ray luminous compact groups. Both objects have a total gravitating mass of Mtotal ∼ 2.0 × 1013M⊙, and a gas mass and a galaxy mass of Mgas ∼ Mgalaxy ∼ 1.5 × 1012M⊙. The ratios among these mass components are Mgas/Mgalaxy ∼ 1 and (Mgas + Mgalaxy)/ Mtotal ∼ 0.2 within ∼ 350 kpc for both objects. The latter is similar to previous ROSAT results for many groups of galaxies, while the former is larger than the average value of the ROSAT-observed groups. Moreover, the metal abundances of the present two groups are significantly higher than those of the NGC 2300 group and HCG 62, and are comparable to those of clusters. These suggest that the hot-gas mass and metal abundances can scatter widely among groups, compared to the small variance found among rich clusters.

A Far-Ultraviolet Spectrum of the Puppis A Supernova Remnant Using the Hopkins Ultraviolet Telescope

We report the first successful far-ultraviolet spectroscopic observation of a filament in the Puppis A supernova remnant. We observed a position on the eastern side of the remnant near the brightest X-ray region using the Hopkins Ultraviolet Telescope during the Astro-2 space shuttle mission in 1995 March. The spectrum covers the 820-1840 A spectral range with ~3 A resolution and shows numerous lines of C, N, O, Ne, He, and possibly Si. Comparison of this spectrum to solar abundance shock models indicates a shock velocity of approximately 160-180 km s-1. A slight overabundance of N is possible for this position, consistent with a shocked interstellar medium picture for the observed filament. This is at odds with the much larger overabundance of N indicated by optical data from other regions of the outer shell in Puppis A. Comparison with optical CCD images obtained at Las Campanas Observatory and with Einstein HRI data indicates that the region observed corresponds to a very recent encounter between the blast wave and an interstellar cloud. The interaction of the shock wave with this cloud is responsible for the bright X-ray emission in this region.

An X-ray and optical study of the interaction of the Cygnus Loop supernova remnant with an interstellar cloud

We have used the ROSAT high-resolution imager and optical emission line data to study the structure of a bright optical knot on the southeastern rim of the Cygnus Loop supernova remnant. This knot has been identified as an encounter between the blast wave and a small isolated cloud. The knot appears in projection just behind the blast wave, which is traced by Balmer line filaments which bound the X-ray emission. The knot is a prominent X-ray feature, consisting of a numerb of filaments which are correlated with the optical line emission. These data permit a detailed view of the blast wave interaction. By combining the optical and X-ray data it is possible to trace the blast wave as a continuous surface from its southern edge around the western side of the cloud that then continues on to the north. The southeastern knot is an indentation on the surface of the blast wave. Thus the southeastern knot is not a small cloud that has been overrun by the blast wave, but the tip of a larger cloud. Bright X-ray emission is associated with bright radiative filaments. The location of this emission, upstream of the radiative shocks, implies that the enhanced X-rays come from a reverse shock. The presence of a reverse shock is further evidence that the southeastern knot represents an early stage of a blast wave encountering a large cloud. A consistent picture of the Cygnus Loop as an explosion within a preexisting cavity is emerging. These observations agree with that picture.

Comparison between cool and hot plasma behaviors of surges

Ground-based coordinated observations with the Multichannel Subtractive Double Pass spectrograph (MSDP) allowed us to obtain chromospheric intensity and velocity field maps below coronal structures during the launch of the NIXT payload on 1991 July 11 (eclipse day). A large H-alpha ejection in AR 6713 (N38 W40) was detected during the NIXT flight. However, only a low level of X-ray emission was associated with this event. In contrast, bright X-ray emission associated with a subflare was observed in a nearby active region, but with only a weak associated ejection in H-alpha. A discussion of both of these events gives strong constraints on the triggering mechanisms of surges.

Multi-wave band SMM-VLA observations of an M2 flare and an associated coronal mass ejection

Results are presented of observations of an M2 flare and an associated coronal mass ejection CME by instruments on the SMM as well as by the VLA and other ground-based observatories on September 30, 1988. The multiwave band data show a gradual slowly changing event which lasted several hours. The microwave burst emission was found to originate in compact moderately circularly polarized sources located near the sites of bright H-alpha and soft X-ray emission. These data are combined with estimates of an electron temperature of 1.5 x 10 to the 7th K and an emission measure of about 2.0 x 10 to the 49th/cu cm obtained from Ca XIX and Fe XXV spectra to show that the microwave emission can be attributed to thermal gyrosynchrotron radiation in regions where the magnetic field strength is 425-650 G. The CME acceleration at low altitudes is measured on the basis of ground- and space-based coronagraphs.

Fast shells and X-ray emission in 30 Doradus: SNRs and superbubbles

We have mapped the kinematics of the ionized gas in 30 Dor, and find numerous examples of fast expanding shells. There is good correlation between the high-velocity material and the bright diffuse X-ray emission in 30 Dor. The fast shells are probably supernova remnants, stellar wind-blown bubbles, or combinations of both.

NEW OPTICAL DATA ON THE SUPERNOVA REMNANT 3C 400.2

ABSTRACT We present optical imaging and spectroscopic data for the faint galactic supernova remnant 3C 400.2. A deep CCD image of the southwestern region obtained with an Hα + [N II] interference filter shows a complex morphology with many chaotic filaments and considerable diffuse emission. A comparable exposure in the north-central region, centered on the brightest X-ray emission, shows faint, diffuse optical emission probably belonging to the remnant. Spectra of two of the optical filaments in the southwest indicate strong [N II], [S II], and [O I] emission lines relative to Ha, characteristic of shock-heated gas. The electron densities determined from the ratio of [S II] λ6717 and λ6731 at our two positions indicate a real variation is present. However, the range of pressures indicated for the optical filaments is still consistent with rough pressure equilibrium between the X-ray and optical gas.

Effect of Compact Objects Near Be Stars

Several Be stars are identified with bright X-ray sources. (Rappaport and Van den Heuvel, 1982). The bright X-ray emission and observed periodicities indicate the existence of compact objects (white dwarfs, neutron stars or black holes) near the Be stars. A prime example is the brightest X-ray source A0538-66 in LMC, which contains a neutron star with a rotation period of 59 ms. Apparao (1985) explained the X-ray emission, which occurs in periodic flares, by considering an inclined eccentric orbit for the neutron star around the assumed Be-star. The neutron star when it enters a gas ring (around the Be-star) accreting matter giving out X-rays.The X-ray emission from the compact objects, when the gas ring from the Be-star envelopes the objects, has interesting consequences. The X-ray emission produces an ionized region (compact object Stromgren sphere or COSS) in the gas surrounding the compact object (CO).

A sheet description of the emission from middle-aged supernova remnants

view Abstract Citations (106) References (40) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Sheet Description of the Emission from Middle-aged Supernova Remnants Hester, J. Jeff Abstract The hypothesis that sheetlike shock fronts propagating into large clouds immersed in a fairly uniform intercloud medium can provide a consistent picture accounting for essentially all the observational data for the Cygnus Loop SNR is considered. Filaments are interpreted as line-of-sight projections of portions of the sheet. Variations in the projection of the shock velocity along the line of sight lead naturally to 'turbulent' velocities consistent with observation. An analog model of a large, distorted, bumpy shock front is presented and shown to produce the observed morphology of the remnant's prominent optical filaments. Comparison with the observed morphology supports the idea that regions of optical emission lie on the local outer boundary of the shell and have not been engulfed by the blast wave. Bright X-ray emission arises when the blast wave encounters a preshock medium with an intermediate density higher than average for the adiabatic portion of the remnant, but still lower than that required for the shock to become radiative. Publication: The Astrophysical Journal Pub Date: March 1987 DOI: 10.1086/165049 Bibcode: 1987ApJ...314..187H Keywords: Nebulae; Sheets; Shock Fronts; Supernova Remnants; Astronomical Models; Interstellar Matter; X Ray Sources; Astrophysics; NEBULAE: INDIVIDUAL NAME: CYGNUS LOOP; NEBULAE: INDIVIDUAL ALPHANUMERIC: IC 443; NEBULAE: SUPERNOVA REMNANTS; SHOCK WAVES full text sources ADS | data products SIMBAD (2)

The Cygnus loop - A detailed comparison of X-ray and optical emission

A comparison at a resolution of 17 arcsec is presented between optical emission from forbidden O III and forbidden S II and the thermal X-ray emission for a field on the southeast edge of the Cygnus Loop SNR. The relationship between optical and X-ray emission in the field is described, generalizing to the Loop as a whole when possible. Several possible explanations for the presence of bright X-ray emission in the vicinity of optical emission are presented and evaluated based on the data. These mechanisms involve evaporation, gradual variations and gradients in the density of the preshock intercloud medium, and additional compression of material which has already been heated to X-ray temperatures by the adiabatic blast wave. The additional compression could result from rapid deceleration of the blast wave itself or from reshocking by reflected or bow shocks around dense clouds. Implications of the observations for models of SNR evolution and the interstellar medium are discussed.

AY Ceti - A flaring, spotted star with a hot companion

AY Ceti is a late-type single-line spectroscopic binary, a bright X-ray source (L/x/ equal to about 1.5 x 10 to the 31st ergs/s), and a spotted star, as evidenced by its prominent photometric wave. In this paper, observations made with the IUE satellite and the VLA radio interferometer are reported. The 1200-2000 A UV spectrum of AY Cet shows a hot stellar continuum and a very broad Ly-alpha absorption line from a previously unobserved white dwarf secondary. The UV spectrum can be matched to the energy distribution of a (T/eff/ = 18,000 K, log g = 8) model atmosphere. Superposed on this hot continuum are high-excitation emission lines typical of chromospheres and transition regions of active late-type stars, e.g., the spotted RS CVn binaries. It is concluded that the bright lines and soft X-ray emission of AY Cet arise from the cool primary star, rather than from mass transfer and accretion onto the secondary as has recently been proposed for the similar system 56 Peg. Two strong radio flares on AY Cet were observed. The second was rapidly variable and left-hand circularly polarized at levels up to pi(c) = 86 + or - 5 percent at 20 cm wavelength. The most likely emission mechanism is an electron-cyclotron maser.

Progress and prospects in the applications of laser produced plasmas

Reviews progress and prospects in just three major areas with particular reference to work by UK University groups using the SERC Central Laser Facility at the Rutherford Appleton Laboratory. (i) Laser driven implosions, which are used to create dense high pressure plasmas and which are the basis of ideas for inertially confined thermonuclear fusion. (ii) New applications of uniquely bright X-ray emission from laser produced plasmas for nanosecond time resolved EXAFS, X-ray diffraction etc. (iii) X-ray laser research based on laser produced plasmas as the amplifying media.

Multiple wavelength observations of a solar active region

The Solar Maximum Mission Satellite, the Sacramento Peak Vacuum Tower Telescope, the Very Large Array and the Westerbork Synthesis Radio Telescope have been used to observe active region AR 2490 on two consecutive days at soft X-ray, ultraviolet, optical and radio wavelengths (2, 6, and 20 cm), with comparable angular resolution (2″ to 15″) and field of view (4′ × 4′). The radio emissions at λ = 6 cm and 20 cm show a double structure in which one component is associated with bright Hα plage, C iv and soft X-ray emission, and the other component is associated only with sunspots. No radiation at λ = 2 cm is detected in this latter component. Coronal temperature and emission measure derived from X-ray lines indicate that the dominant radiation mechanism of the plage-associated component is due to thermal bremsstrahlung while the gyroresonance absorption coefficient must be invoked to account for the high brightness temperature (T b ≈2×106K) observed in the sunspot associated component. The high magnetic field strength needed (600 G at a level where T∼2×106K) is explained assuming a thin transition zone, in order to reach a high electron temperature close to the sunspot, where the magnetic fields are stronger. A higher temperature gradient above sunspots is also consistent with the absence of detectable C iv emission.