Point-like X-ray Source Research Articles

X‐Ray Emission from Multiphase Shock in the Large Magellanic Cloud Supernova Remnant N49

The supernova remnant (SNR) N49 in the Large Magellanic Cloud has been observed with the Advanced CCD Imaging Spectrometer (ACIS) on board the Chandra X-Ray Observatory. The superb angular resolution of the Chandra/ACIS images resolves a point source, the likely X-ray counterpart of soft gamma-ray repeater SGR 0526-66, and the diffuse filaments and knots across the SNR. These filamentary features represent the blast wave sweeping through the ambient interstellar medium and nearby dense molecular clouds. We detect metal-rich ejecta beyond the main blast wave shock boundary in the southwest of the SNR, which appear to be explosion fragments, or "bullets," ejected from the progenitor star. The detection of strong H-like Si line emission in the eastern side of the SNR requires multiphase shocks in order to describe the observed X-ray spectrum, whereas such a multiphase plasma is not evident in the western side. This complex spectral structure of N49 suggests that the postshock regions toward the east of the SNR might have been reheated by the reverse shock off the dense molecular clouds while the blast wave shock front has decelerated as it propagates into the dense clouds. The X-ray spectrum of the detected pointlike source is continuum-dominated and can be described with a power law of Γ ~ 3. This provides a confirmation that this pointlike X-ray source is the counterpart of SGR 0526-66 in the quiescent state.

ASCA observations of the Crab-like pulsar/nebula system PSR B0540-69

The ASCA satellite has observed the 50-ms pulsar PSR B0540–69 and its associated supernova remnant SNR 0540–693 in the Large Magellanic Cloud 15 times in total, at least once a year, between 1993 June and 1999 November, including six observations with the pulsar as the main target. In all the observations, a point-like X-ray source was clearly detected at the position of PSR B0540–69 and the X-ray pulsations were detected at large significance. Folded light curves are identical throughout the observations, showing one broad pulse with a small notch at the top. Pulse frequency history during the 6.4 yr of the ASCA observations is well characterized by a second-order polynomial with a breaking index of 2.10±0.05, assuming no glitches occurred during the period. The frequency history smoothly follows the Ginga and ROSAT results and connects to the RXTE results without a measurable discontinuity. The X-ray spectrum of the whole source is well represented by a single power-law model of photon index, α=2.00±0.02, and a small photoelectric absorption, NH=(4.3±0.2)×1021 cm-2. The X-ray spectrum can also be represented by a pulsed power-law spectrum with photon index of 1.80±0.05 with 24 per cent in amplitude at 1 keV, embedded in a constant power-law component with photon index of 2.17±0.02 ascribed to a nebular component. Spectral properties of the X-ray emission from the pulsar and the nebula remain unchanged over the six years and are quite similar to those of the Crab pulsar and nebula.

ITAL]Chandra[/ITAL] Observation of Supernova Remnant G54.1+0.3: A Close Cousin of the Crab Nebula

We present a Chandra Advanced CCD Imaging Spectrometer observation of supernova remnant G54.1+0.3. This supernova remnant is resolved into several distinct X-ray-emitting components: a central bright pointlike source (CXOU J193030.13+185214.1), a surrounding ring, bipolar elongations, plus low surface brightness diffuse emission. The spectra of these components are all well described by power-law models; the spectral index steepens with increasing distance from the pointlike source. We find no evidence for any thermal plasma emission that would correspond to shocked interstellar medium or ejecta. The observed morphological and spectral characteristics suggest that G54.1+0.3 is the closest cousin of the Crab Nebula-a pulsar wind nebula driven by a combination of equatorial and polar outflows from the putative pulsar represented by the pointlike X-ray source.

X-ray telescope onboard Astro-E II Ground-based x-ray characterization

X-ray characterization measurements of the x-ray telescope (XRT) onboard the Astro-E satellite were carried out at the Institute of Space and Astronautical Science (Japan) x-ray beam facility by means of a raster scan with a narrow x-ray pencil beam. The on-axis half-power diameter (HPD) was evaluated to be 1.8?-2.2?, irrespective of the x-ray energy. The on-axis effective areas of the XRTs for x-ray imaging spectrometers (XISs) were approximately 440, 320, 240, and 170 cm(2) at energies of 1.49, 4.51, 8.04, and 9.44 keV, respectively. Those of the x-ray spectrometer (XRS) were larger by 5-10%. The replication method introduced for reflector production significantly improved the imaging capability of the Advanced Satellite for Cosmology and Astrophyics (ASCA) XRT, whose HPD is ~3.6?. The increase in the effective area by a factor of 1.5-2.5, depending upon the x-ray energy, compared with that of the ASCA, was brought about by mechanical scale up and longer focal lengths. The off-axis HPDs were almost the same as those obtained on the optical axis. The field of view is defined as the off-axis angle at which the effective area becomes half of the on-axis value. The diameter of the field of view was ~19? at 1.49 keV, decreasing with increasing x-ray energy, and became ~13? at 9.44 keV. The intensity of stray light and the distribution of this kind of light on the focal plane were measured at the large off-axis angles 30? and 60?. In the entire XIS field of view (25.4 mm x 25.4 mm), the intensity of the stray light caused by a pointlike x-ray source became at most 1% of the same pointlike source that was on the optical axis.

Modeling the X‐Ray–Optical Correlations in NGC 3516

We test the paradigm of optical-UV active galactic nucleus (AGN) variability, according to which the variations in this wavelength range are driven by a variable X-ray component, by detailed modeling of the correlated X-ray-optical (3590 and 5510 A) variability of the recent multiwavelength campaign of NGC 3516. To this end we produce model optical light curves by convolving the observed X-ray flux with the response function of an infinite, thin accretion disk, illuminated by a pointlike X-ray source located at a given height hX above the compact object (the lamppost model). We also produce the ionized X-ray reflection and Fe Kα line spectra that result from the reprocessing of the same X-rays on the disk surface. Then we compare the properties of the model light curves (amplitude, morphology, lags) as well as those of the Fe Kα line profiles to those observed. Our calculations improve on those of similar past treatments by including the effects of an X-ray-heated ionized layer in hydrostatic equilibrium on its surface, which greatly affects its X-ray timing and spectral properties. The results of our calculations do not provide a clear-cut picture that would either support or refute the reprocessing paradigm: despite the large (50%) amplitude excursions of the X-ray flux, the model optical light curves exhibit variability amplitudes of only 3%-4% and vary in synchrony in the two bands, in agreement with observations. However, the model light curves, when viewed in detail, do not have a direct correspondence to those observed. Futhermore, while the observed intraband synchrony generally points toward smaller values for the black hole mass (M = 107 M☉), the X-ray Fe Kα line, X-ray reflection, and optical-UV spectra seem to favor larger mass values (M = 108 M☉). Generally, no combination of the model parameters seems to produce agreement with the ensemble of the constraints imposed by optical/UV/X-ray spectral and timing observations; we are thus led to believe that the simplest version of the lamppost model geometry is inconsistent with the NGC 3516 observations.

A catalogue of soft X-ray sources in the galactic center region

We present a catalogue of 107 point-like X-ray sources derived from a systematic analysis of all the ROSAT PSPC observations of the galactic center region performed in 1992-1993. Besides SgrA*, the massive black hole at the galactic center, 41 X-ray sources have been positionally associated with already classified objects. Twenty are identified with foreground stars and five with known Low Mass X-ray Binaries. The majority of the sources in our catalogue still remains unidentified. They are hard and/or severely absorbed and probably represent a large population of X-ray binaries located in the galactic center region, accreting at low accretion rates, and still largely unknown.

New Optical Constraints on the Presence of a Compact Central Object in Cassiopeia A

Deep optical imaging at the location of the pointlike X-ray source at the center of the 300 yr old Cassiopeia A supernova remnant discovered in the Chandra first-light observation is presented. R-band images obtained with the Steward Observatory 2.3 m telescope and direct CCD on four nights in 1999 September are used to set a new brightness limit of R 26.3 mag (1 σ) on the presence of any optical object associated with the X-ray source. This new limit implies MR 10.3 mag and LX(0.2-10 keV)/Lopt 800. Accretion models are the most constrained by the optical results. Binary accretion models and scenarios involving very low accretion rates from a disk onto a neutron star or a black hole similar to models for quiescent transient low-mass X-ray binaries are effectively ruled out. Fallback accretion models may be ruled out in some circumstances, but the emitted optical and infrared flux is sensitive to the location of the transition radius, the geometry of the disk, and its inclination to the line of sight.

An X-ray and optical study of the cluster A33

We report the first detailed X-ray and optical observations of the medium-distant cluster A33 obtained with the Beppo-SAX satellite and with the UH 2.2 m and Keck II telescopes at Mauna Kea. The information deduced from X-ray and optical imaging and spectroscopic data allowed us to identify the X-ray source 1SAXJ0027.2-1930 as the X-ray counterpart of the A33 cluster. The faint, erg s-1 cm-2, X-ray source 1SAXJ0027.2-1930, arcmin away from the optical position of the cluster as given in the Abell catalogue, is identified with the central region of A33. Based on six cluster galaxy redshifts, we determine the redshift of A33, ; this is lower than the value derived by Leir & Van Den Bergh (1977). The source X-ray luminosity, erg s-1 cm-2, and intracluster gas temperature, keV, make this cluster interesting for cosmological studies of the cluster relation at intermediate redshifts. Two other X-ray sources in the A33 field are identified. An AGN at , and an M-type star, whose emissions are blended to form an extended X-ray emission arcmin north of the A33 cluster. A third possibly point-like X-ray source detected arcmin north-west of A33 lies close to a spiral galaxy at and to an elliptical galaxy at the same redshift as the cluster.

The Compact Central Object in Cassiopeia A: A Neutron Star with Hot Polar Caps or a Black Hole?

The central pointlike X-ray source of the Cassiopeia A supernova remnant was discovered in the Chandra first light observation and found later in the archival ROSAT and Einstein images. The analysis of these data does not show statistically significant variability of the source. Because of the small number of photons detected, different spectral models can fit the observed spectrum. The power-law fit yields the photon index gamma=2.6-4.1, and luminosity L(0.1-5.0 keV&parr0;=&parl0;2-60&parr0;x1034 ergs s-1 for d=3.4 kpc. The power-law index is higher, and the luminosity lower, than those observed from very young pulsars. One can fit the spectrum equally well with a blackbody model with T=6-8 MK, R=0.2-0.5 km, and Lbol=&parl0;1.4-1.9&parr0;x1033 ergs s-1. The inferred radii are too small, and the temperatures too high, for the radiation to be interpreted as emitted from the whole surface of a uniformly heated neutron star. Fits with the neutron star atmosphere models increase the radius and reduce the temperature, but these parameters are still substantially different from those expected for a young neutron star. One cannot exclude, however, the possibility that the observed emission originates from hot spots on a cooler neutron star surface. An upper limit on the (gravitationally redshifted) surface temperature is Tinfinitys<1.9-2.3 MK, depending on the chemical composition of the surface and the star's radius. Among several possible interpretations, we favor a model of a strongly magnetized neutron star with magnetically confined hydrogen or helium polar caps (Tinfinitypc approximately 2.8 MK, Rpc approximately 1 km) on a cooler iron surface (Tinfinitys approximately 1.7 MK). Such temperatures are consistent with the standard models of neutron star cooling. Alternatively, the observed radiation may be interpreted as emitted by a compact object (more likely, a black hole) accreting from a residual disk or from a late-type dwarf in a close binary.

ASCA Observations of Two Ultra-Luminous Compact X-Ray Sources in the Edge-On Spiral Galaxy NGC 4565

The edge-on spiral galaxy NGC 4565 was observed for ∼ 35 ks with ASCA in the 0.5–10 keV energy band. The X-ray emission was dominated by two bright sources, which could be identified with two point-like X-ray sources seen in the ROSAT HRI image. The observed 0.5–10 keV fluxes of these sources, 1.7 × 10−12 erg s−1 cm−2 and 0.7 × 10−12 erg s−1 cm−2, imply bolometric luminosities of 1.2 × 1040 erg s−1 and 4.6 × 1039 erg s−1, respectively. They exhibit similar spectra, which can be explained by emission from optically thick accretion disks with inner-disk temperature of 1.4–1.6 keV. One of them, coincident in position with the nucleus, shows too low absorption to be the active nucleus seen through the galaxy disk. Their spectra and high luminosities suggest that they are both mass-accreting black-hole binaries. However, the black-hole mass required by the Eddington limit is rather high (≥ 50 M⊙), and the observed disk temperature is too high to be compatible with the high black-hole mass. Several attempts have been made to solve these problems.

Pulse Power Compression by Cutting a Dense Z-Pinch with a Laser Beam

Abstract A thin cut made through a z-pinch by an intense laser beam can become a magnetically insulated diode crossed by an intense ion beam. For larger cuts, the gap is crossed by an intense relativistic electron beam, stopped by magnetic bremsstrahlung resulting in a pointlike intense x-ray source. In either case, the impedance of the pinch discharge is increased, with the power delivered rising in the same pro-portion. A magnetically insulated cut is advantageous for three reasons: First, with the ion current com-parable to the Alfvèn ion current, the pinch instabilities are reduced. Second, with the energy deposit-ed into fast ions, a non-Maxwellian velocity distribution is established increasing〈σ ν〉 value for nuclear fusion reactions taking place in the pinch discharge. Third, in a high density z-pinch plasma, the intense ion beam can launch a thermonuclear detonation wave propagating along the pinch discharge channel. For larger cuts the soft x-rays produced by magnetic bremsstrahlung can be used to drive a thermonuclear hohlraum target. Finally, the proposed pulse power compression scheme permits to use a cheap low power d.c. source charging a magnetic storage coil delivering the magnetically stored energy to the pinch discharge load by an exploding wire opening switch.

ASCA Observation of the Low-Luminosity Seyfert 1.5 Galaxy NGC 5033

We present the results of an ASCA observation of the low-luminosity Seyfert 1.5 galaxy NGC 5033. A point-like X-ray source with a luminosity of 2.3×1041 erg s−1 in the 2–10 keV band (at 18.7 Mpc; Tully 1988, AAA 045.002.054) was detected at the nucleus. The X-ray light curve shows variability on a timescale of ∼ 104 s with an amplitude of ∼ 20%. The X-ray continuum is represented by a weakly absorbed (NH ≈ 9×1020 cm−2) power-law with a photon index of 1.72±0.04, which is quite similar to Seyfert 1 galaxies with higher luminosities. An Fe Kα emission line is detected at 6.40+0.08−0.06 keV (redshift corrected) and the equivalent width is 290±100 eV. The line width is unresolved. The narrower line width and larger equivalent width compared to Seyfert 1s imply that fluorescent Fe Kα emission from matter further out from the center than the accretion disk significantly contributes to the observed Fe Kα line. We suggest that fluorescent Fe Kα emission from the putative torus contributes to the observed Fe Kα line.

Detection of an Iron K Emission Line from the LINER NGC 4579

We present the results of an ASCA observation of the LINER NGC 4579. A point-like X-ray source is detected at the nucleus with a 2-10 keV luminosity of 1.5x10^41 ergs/s assuming a distance of 16.8 Mpc. The X-ray spectrum is represented by a combination of a power-law with a photon index of ~1.7 and soft thermal component with kT~0.9 keV. An iron K emission line is detected at 6.73+/-0.13 keV (rest frame) with an equivalent width of 490 +180/-190 eV and is statistically significant at more than 99.9 % confidence. The line center energy is consistent with Helium-like iron and is significantly higher than 6.4 keV which is expected from fluorescence by cold (or a lower ionization state of) iron. The iron line profile shows no significant red tail in contrast to Seyfert 1 galaxies although the statistics are limited. The line center energy, equivalent width, and profile are consistent with an origin in an ionized accretion disk. However the large mass accretion rate necessary to ionize the accretion disk is not consistent with the observed luminosity and normal accretion models.

A candidate gamma-ray pulsar in the supernova remnant CTA 1

ABSTRA C T We present a detailed analysis of the high-energy gamma-ray source 2EG J0008+7307. The source has a steady flux and a hard spectrum, softening above 2 GeV. The properties of the gamma-ray source are suggestive of emission from a young pulsar in the spatially coincident CTA 1 supernova remnant, which has recently been found to have a non-thermal X-ray plerion. Our 95 per cent uncertainty contour around the> 1 GeV source position includes the point-like X-ray source at the centre of the plerion. We propose that this object is a young pulsar and is the most likely counterpart of 2EG J0008+7307.

ROSAT X-Ray Observations of the Radio Galaxy 4C46.09

4C46.09 is the radio source that shows up as a point-like x-ray source inside the supernova remnant HB9 (Leahy, 1987). Leahy, 1987 found a 0.2-4 keV Einstein IPC flux of approximately 1.5 × 10−12erg cm−2s−1 and a significantly higher hardness ratio than the rest of HB9. Too few counts were available for any spectral analysis. Seward et al, 1991, found 4C46.09 to be a large radio galaxy at redshift 0.195 and distance 1280 Mpc (Ho = 50 km s−1Mpc−1). 4C46.09 is of further interest due to the observation of a high energy component in the spectrum of HB9 observed by GINGA (Yamauchi and Koyama, 1993). Whether this was due to HB9 or to 4C46.09 could not be determined.

Pulsed intense electron beams generated in transient hollow cathode discharges: fundamentals and applications

For the commercial application of pulsed power, material processing with intense pulsed particle beams is a very interesting subject. Recently, high-voltage (1-70 kV), low-pressure (1-100 Pa) transient hollow-cathode discharges turned out to be sources for pulsed intense electron beam generation suitable for this application. The remarkable parameters of these electron beams-beam currents of 50-1000 A (10-30% of the maximum discharge current) with a high energy component (mean energy of about 0.25-0.75 of maximum applied voltage) of 20-70% of the maximum beam current, power density up to 10 W/cm/sup 2/, beam diameters of 0.1-3 mm, beam charge efficiency of 3-5%-captured the attention not only of the scientific community in the last decade. The electron beam is emitted during the early phases of the discharge, and only weak dependence of the high energetic peak of the beam current was found on the external capacity, which determine the development of the later high-current phases. However, the beam parameters depend on the breakdown voltage, gas pressure, and discharge geometry (including self-capacity). In this paper, the characteristics of the pulsed intense electron beams generated in two configurations-multigap pseudosparks and preionization-controlled open-ended hollow-cathode transient discharges (PCOHC)-are described. Such electron beams already were used successfully in a variety of pulsed power applications in material processing, deposition of superconducting (YBaCuO) and diamond-like thin films, microlithography, electron sources for accelerators, and intense point-like X-ray sources, and some preliminary experiments revealed new potential applications such as pumping of short-wavelength laser active media. These pulsed electron beams could be used further in any kind of pulsed power applications that require high-power density, small or high electron energy, and small-beam diameters.

A ROSAT PSPC Study of NGC 55.

The Position Sensitive Proportional Counter onboard ROSAT imaged the spiral galaxy NGC 55 in X-rays for 18.9 ksec. Twenty-two point-like X-ray sources are detected covering the PSPC field-of-view, of which 7 sources are candidate members of NGC 55. The spectra of the brightest sources that are nearest to NGC 55 are consistent either with a heavily absorbed source or a foreground star. The absorbed sources are very likely members of NGC 55. None of the brightest sources appear to be variable in a periodic fashion. Weak diffuse emission is detected in the plane of the galaxy, but only in the energy bands above ~0.5 keV. This emission is either the tail of a hot component of the ISM or the emission of unresolved point sources. We argue that the emission is a hot ISM component. A hot plume, visible in the 0.75 keV band, appears to align with an H I plume detected in a VLA total H I map. The cool H I gas appears to surround the hot gas, so the structure represents a chimney.

ROSAT Observations of the X-Ray Nebula around ARP 220

view Abstract Citations (88) References (50) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS ROSAT Observations of the X-Ray Nebula around ARP 220 Heckman, Timothy M. ; Dahlem, Michael ; Eales, Stephen A. ; Fabbiano, Giuseppina ; Weaver, Kimberly Abstract We report on the X-ray properties of Arp 22(#the most luminous object in the local universe and the nearest, brightest, and best-studied example of the class of "ultraluminous" infrared galaxies. New X-ray images and spectra obtained with ROSAT show that the X-ray emission in the 0.1-2.4 keV band is spatially extended with a size of ∼30 × 11 kpc, has a luminosity of between 4.3 × 1040 and 2.3 × 1041 ergs s-1 (depending on the amount of X-ray absorption), and can be fitted by a thermal spectrum with T ∼ 107 K. The total thermal energy associated with the hot gas is ∼2 x 1057 ergs. At most ∼20% of the observed X-ray emission can be associated with any single point source (e.g., a "buried" QSO). The X-ray nebula bears a strong morphological relationship to the expanding "double-bubble" system seen in optical emission-line images. We suggest that the X-ray and optical emission arise as the result of a bipolar "superwind" driven out from the nucleus by a starburst or a dust-shrouded QSO. A simple model of an expanding structure driven over a timescale of a few times 107 yr by a mechanical luminosity of ∼1043 ergs s-1 into a galactic halo with mean density ∼10-2 cm-3 satisfactorily accounts for all the observed global X-ray and optical properties of the nebula. Such a mechanical luminosity agrees with estimates for an ultraluminous starburst; however, a QSO-driven wind cannot be ruled out. We note that this energetic outflow in Arp 220 represents a local laboratory to study how newly formed galaxies and/or QSOs may have pumped energy and metals into the intergalactic medium at early epochs. Alternative models for the nebula are less attractive. Gas shock-heated as a result of a galactic merger will probably be too cool (T less than a few times 106 K) and would not be expected to have the regular, symmetric morphology we observe in Hα. Electron scattering of X-rays from a hidden QSO by warm (T < 106 K) halo gas underpredicts the total observed X-ray luminosity by more than an order of magnitude. However, it is possible that this mechanism may produce a bright central pair of pointlike X-ray sources that straddle the nucleus, which together account for ∼40% of the total X-ray emission. Our X-ray images also show a diffuse source located about 2' south-southwest of Arp 220. Optical images show that the source is probably associated with a group or poor cluster of galaxies at an estimated redshift of ∼0.1. This source contributes about half the emission seen in earlier Einstein X-ray observations of Arp 220. Publication: The Astrophysical Journal Pub Date: February 1996 DOI: 10.1086/176758 Bibcode: 1996ApJ...457..616H Keywords: GALAXIES: INDIVIDUAL NAME: ARP 220; X-RAYS: GALAXIES full text sources ADS | data products SIMBAD (2) NED (1) HEASARC (1)

Discovery of Be/x-ray stars in two supernova remnants in the Small Magellanic Cloud

We present ROSAT high resolution x-ray images of two previously cataloged supernova remnants in the Small Magellanic Cloud (SMC): 0101-72.4 and 0104-72.3. These remnants were known to show optical, x-ray, and radio emission based on earlier observations: the present data show the first evidence for arcsecond-scale x-ray structure. There is no diffuse x-ray emission associated with the optically emitting shell in 0101-72.4; we set a 3 sigma upper limit of 7 x 10(exp 34) erg s(exp -1) on the 0.02-2 keV luminosity from the region. The x-ray emission comes instead from a weak pointlike object near the limb of the remnant. Optical observations of this source reveal a m(sub v) = 14.8 blue star with H alpha and H beta in emission; we identify this as a Be star in the SMC. No evidence for variability down to time scales of about 1 s was found in the ROSAT data; however, a comparison of the ROSAT and Einstein fluxes indicates possible long term variability by a factor of approximately 2 over several years. The other SNR, 0104-72.3, also contains a pointlike x-ray source with a blue optical counterpart (m(sub v) = 16.7) and H alpha emission. We tentatively identify this as a Be star as well. In addition to the point source there is weak diffuse x-ray emission from 0104-72.3 (L(sub x) approximately 1.4 x 10(exp 35) erg s(exp -1)), but the remnant's appearance in the x-ray band is considerably different from that in either the radio or optical band. We argue for a physical association between the supernova remnants (SNRs) and Be/x-ray stars. A large space velocity (greater than or approximately 100 km s(exp -1)) for the Be/x-ray binaries is required if the explosions that produced the remnants also formed the neutron stars in the binaries. Alternatively, the associations could be the result of common membership in OB associations in the SMC.

Production of quasi-parallel X-ray beams using microchannel plate “X-ray lenses”

We describe the soft X-ray (0.28 and 1.74 keV) images produced by a spherically-slumped microchannel plate (MCP) optic with a nominal radius of curvature, R, of 1.4 m. With a pointlike X-ray source situated at the focus, an axial distance R2 from the MCP on the concave side, this structure produces, in the manner of a lens, a quasi-parallel output beam whose energy-dependent intensity profile corresponds closely to the predictions of (i) a simple 1D analytical model and (ii) a 2D Monte Carlo ray-trace code. With the source on the convex side of the slumped plate, we observe X-ray focusing which is again consistent with our models. Analysis of the X-ray image structure observed in focusing mode yields an upper limit to the rms surface roughness of the lead glass channel walls of 50 Å.