Hard X-ray Component Research Articles

Nonthermal Hard X‐Ray Emission in Galaxy Clusters Observed with theBeppoSAXPDS

We study the X-ray emission in a Sample of galaxy clusters using the BeppoSAX PDS instrument in the 20 - 80 keV energy band. We estimate the non-thermal hard X-ray cluster emission (HXR) by modeling the thermal contribution from the cluster gas and the non-thermal contamination from the unobscured AGN in the clusters. We also evaluate the systematic uncertainties due to the background fluctuations. Assuming negligible contamination from the obscured AGN, the resulting non-thermal component is detected at a 2 sigma level in approx. 50% of the non-significantly AGN-contaminated clusters: A2142, A2199, A2256, A3376, Coma, Ophiuchus and Virgo. The data are consistent with a scenario whereby relaxed clusters have no hard X-ray component of non-thermal origin, whereas merger clusters do. The co-added spectrum of the above clusters indicates a power-law spectrum for the HXR with a photon index of 2.8+0.3-0.4 in the 12 - 115 keV band, and we find indication that it has extended distribution. These indications argue against significant contamination from obscured AGN, which have harder spectra and centrally concentrated distribution. These results are supportive of the assumption of the merger shock acceleration of electrons in clusters, which has been proposed as a possible origin of the non-thermal hard X-ray emission models. Assuming that the Cosmic Microwave Background photons experience Inverse Compton scattering from the merger-accelerated relativistic electrons, and thus produce the observed HXR, the measured hard X-ray slope corresponds to a differential momentum spectra of the relativistic electrons with a slope of mu = 3.8 - 5.0. In presence of cluster magnetic fields this relativistic electron population produces synchrotron emission with a spectral index of 1.4 - 2.1, consistent with radio halo observations of merger clusters. Thus both hard X-ray and radio observations of merger clusters are consistent with the Inverse Compton model. The observed slope of HXR is also consistent with that predicted by the non-thermal bremsstrahlung, which chus cannot be ruled by the fit to the current data, even though this model requires an extreme, untenable cluster energetics. Assuming centrally concentrated distribution of HXR, the data requires a harder slope for the HXR spectrum, which is consistent with secondary electron models, but this model yields a worse fit to the PDS data and thus seems to be disfavored over the primary electron Inverse Compton model.

Hard X-ray emission from neutron star X-ray binaries

Abstract In this paper we review our current knowledge of the hard X–ray emission properties of accreting X–ray Binary Pulsars and old accreting neutron stars in Low Mass X–ray Binaries in light of 7 years of BeppoSAX and RXTE observations. The paper is divided in two parts. In the first part we review the more recent findings on the phase-dependent broad band continua and cyclotron resonance scattering features observed in many systems of the X-ray Binary Pulsar class. In the second part we review the hard X-ray emission of LMXRB focussing on the hard X-ray components extending up to energies of a few hundred keV that have been clearly detected in sources of both the atoll and Z classes. The presence and characteristics of these hard emission components are then discussed in relation to source properties and spectral state. We, also, briefly mention models that have been proposed for the hard X-ray emission of neutron star X-ray binaries.

BeppoSAX observations of soft X-ray intermediate polars

We present broad-band (0.1-90 keV) spectral and temporal properties of the three Intermediate Polars, RE 0751+144 (PQ Gem), RX J0558.0+5353 (V405 Aur) and RX J1712.6 2414 (V2400 Oph) based on simultaneous soft and hard X-ray observations with the BeppoSAX satellite. The analysis of their spectra over the wide energy range of BeppoSAX instruments allows us to identify the soft and hard X-ray components and to determine simultaneously their temperatures. The black-body temperatures of the irradiated poles of the white dwarf atmosphere are found to be 60-100 eV, much higher than those found in their synchronous analogues, the Polars. The temperature of the optically thin post-shock plasma is well con- strained in RX J1712.6 2414 and in RE 0751+144 (13 and 17 keV) and less precisely determined in RX J0558.0+5353. In the first two systems evidence of subsolar abundances is found, similarly to that estimated in other magnetic Cataclysmic Variables. A Compton reflection component is present in RX J0558.0+5353 and in RE 0751+144 and it is favoured in RX J1712.6 2414. Its origin is likely at the irradiated white dwarf surface. Although these systems share common properties (a soft X-ray compo- nent and optical polarized radiation), their X-ray power spectra and light curves at dierent energies suggest accretion geome- tries that cannot be reconciled with a single and simple configuration.

The XMM-Newton view of the X-ray halo and jet of NGC 6251

We present an XMM-Newton observation of the radio jet and diffuse halo of the nearby radio galaxy NGC 6251. The EPIC spectrum of the galaxy's halo is best-fitted by a thermal model with temperature keV and sub-solar abundances. Interestingly, an additional hard X-ray component is required to fit the EPIC spectra of the halo above 3 keV, and is independently confirmed by an archival Chandra observation. However, its physical origin is not clear. Contribution from a population of undetected Low Mass X-ray Binaries seems unlikely. Instead, the hard X-ray component could be due to inverse Compton scattering of the CMB photons (IC/CMB) off relativistic electrons scattered throughout the halo of the galaxy, or non-thermal bremsstrahlung emission. The IC/CMB interpretation, together with limits on the diffuse radio emission, implies a very weak magnetic field, Gauss, while a non-thermal bremsstrahlung origin implies the presence of a large number of very energetic electrons. We also detect X-ray emission from the outer (~3.5′) jet, confirming previous ROSAT findings. Both the EPIC and ACIS spectra of the jet are best-fitted by a power law with photon index , fixed Galactic column density, and 1 keV flux nJy. A thermal model is formally ruled out by the data. Assuming an origin of the X-rays from the jet via IC/CMB, as suggested by energetic arguments, and assuming equipartition implies a large Doppler factor (). Alternatively, weaker beaming is possible for magnetic fields several orders of magnitude lower than the equipartition field.

X-ray and gamma-ray emission from millisecond pulsars

We present a self-consistent model to describe X-ray and γ-ray emission from millisecond pulsars (MSPs). The X-rays of MSPs are produced by the backflow of primary charged particles from the outer gap and most likely consist of three components, two thermal components and one power law component if there is a strong multipole magnetic field on the stellar surface. The backflow of ultra-relativistic particles emits photons with energies about several tens of GeV via curvature radiation. These photons cause an electromagnetic cascade about 2–3 stellar radii above the polar cap. The synchrotron radiation of these cascade e pairs produces hard X-rays with a power law index ~. Near 105 cm above the stellar surface, the primary charged particles encounter the strong surface magnetic field, which alters the local radius of curvature greatly, and they quickly loose more than half of their remaining energies to curvature radiation. These curvature photons heat up the polar cap area with a radius ~ 105 cm, which produce the softer thermal X-ray component. Finally, the primary charged particles deposit their remaining energies in a much smaller polar cap area, which corresponds to the footprints of outer gap and produce the medium hard X-ray component. γ-rays are produced in the outer gap through synchro-curvature radiation. We have applied this model to the MSPs which emit pulsed X-rays and likely γ-rays such as PSR J0437-4715, PSR J2124-3358, PSR J0218+4232 and PSR B1821-24. Our results give an agreement between predicted spectrum and the observed spectrum of MSP emission.

Spectral measurements of Cyg X-3: A thermal source embedded in hot plasma within a cold shell

The attempts at unified model fitting to explain the spectral variations in Cyg X-3 suggest equally probable fits with a combination of an absorbed blackbody and a separately absorbed power law with an exponential cut-off or a composite of absorbed free-free emission with a power law hard X-ray component apart from the iron emission line. These seemingly ordinary but ad hoc mixtures of simple X-ray emission mechanisms have a profound implication about the geometry of the X-ray source. While the first set suggests a black-hole nature of the compact object, the second combination is consistent with a neutron star binary picture. The spectral variability at hard X-ray energies above 30 keV can provide crucial input for the unified picture. In this paper, we present spectral observations of Cyg X-3, made in our on-going survey of galactic and extragalactic X-ray sources in the 20–200 keV energy region, using Large Area Scintillation counter Experiment. The data show a clear power-law photon spectrum of the form dN/dE ∼ E−2.8 in the 20 to 130 keV energy range. A comparison with earlier data suggests that the total number of X-ray photons in the entire 2–500 keV energy band is conserved at all time for a given luminosity level irrespective of the state. We propose that this behaviour can be explained by a simple geometry in which a thermal X-ray source is embedded in a hot plasma formed by winds from the accretion disk within a cold shell. The high/soft and low/hard X-ray states of the source are simply the manifestation of the extent of the surrounding scattering medium in which the seed photons are Comptonized and hot plasma can be maintained by either the X-ray driven winds or the magneto-centrifugal winds.

The Variabilities of the Soft and Hard X-ray Components of NGC 7314 and NGC 7582 and the Distribution of Absorbing Matter in Type II AGNs

ASCA observations of the two Type II AGNs, NGC 7314 and NGC 7582, show clear variations in the broad X-ray band (0.4 –10 keV) on short timescales ∼10 4 s. Spectral analysis indicates that they both have an absorbed hard X-ray component and an unabsorbed soft ``excess component. To clarify the origin of the latter, we made a cross-correlation analysis of the two components. The results show that, for NGC 7314, the soft X-ray variability is proportional to that of the hard X-ray component. This indicates that the active nucleus of NGC 7314 must be partially covered and so the soft emission is a ``leaking of the variable hard component. For NGC 7582, there is no detectable variability in the soft component, although there is a definite one in the hard component. This indicates that the variable nucleus of NGC 7582 must be fully blocked by absorbing matter, and the soft emission is most likely the scattered component predicted by the AGN unified model.

ChandraX‐Ray Observations of the Spiral Galaxy M81

A Chandra X-Ray Observatory ACIS-S imaging observation is used to study the population of X-ray sources in the nearby Sab galaxy M81 (NGC 3031). A total of 177 sources are detected with 124 located within the D25 isophote to a limiting X-ray luminosity of ~3 × 1036 ergs s-1. Source positions, count rates, luminosities in the 0.3-8.0 keV band, limiting optical magnitudes, and potential counterpart identifications are tabulated. Spectral and timing analysis of the 36 brightest sources are reported including the low-luminosity active galactic nucleus, SN 1993J, and the Einstein-discovered ultraluminous X-ray source X6. The nucleus accounts for ~86%, or 5 × 1040 ergs s-1, of the total X-ray emission from M81. Its spectrum is well fitted by an absorbed power law with photon index 1.98 ± 0.08, consistent with previous observations (average index 1.9). SN 1993J has softened and faded since its discovery. At an age of 2594 days, SN 1993J displayed a complex thermal spectrum from a reverse shock rich in Fe L and highly ionized Mg, Si, and S but lacking O. A hard X-ray component, emitted by a forward shock, is also present. X6 is spatially coincident with a stellar object with optical brightness and colors consistent with an O9-B1 main-sequence star. It is also coincident with a weak radio source with a flux density of ~95 μJy at λ = 3.6 cm. The continuum-dominated X-ray spectrum of X6 is most closely reproduced by a blackbody disk model suggesting the X-ray source is an ~18 M☉ object accreting at nearly its Eddington limit.

XMM‐NewtonDetection of Hard X‐Ray Emission in the Nitrogen‐Type Wolf‐Rayet Star WR 110

We have used the excellent sensitivity of XMM-Newton to obtain the first high-quality X-ray spectrum of a Wolf-Rayet (W-R) star that is not known to be a member of a binary system. Our target, the nitrogen-type star WR 110 (HD 165688), was also observed and detected with the Very Large Array at four different frequencies. The radio flux density increases with frequency according to a power law Sν ∝ ν+0.64±0.10, in very good agreement with the behavior expected for free-free wind emission. The radio data give an ionized mass-loss rate = 4.9 × 10-5 M☉ yr-1 for an assumed spherical constant-velocity wind. The undispersed CCD X-ray spectra reveal strong emission lines from He-like ions of Mg, Si, and S. The emission measure distribution shows a dominant contribution from cool plasma with a characteristic temperature kTcool ≈ 0.5 keV (≈6 MK). Little or no excess absorption of this cool component above the value expected from the visual extinction is present. We conclude that the bulk of the cool plasma detected by XMM-Newton lies at hundreds of stellar radii or more if the wind is approximately spherical and homogeneous, but it could lie closer to the star if the wind is clumped. If the cool plasma is due to instability-driven wind shocks, then typical shock velocities are vs ≈ 340-550 km s-1 and the average filling factor of X-ray-emitting gas in the wind is no larger than f ~ 10-6. A surprising result is the unambiguous detection of a hard X-ray component that is clearly seen in the hard-band images and the spectra. This hard component accounts for about half of the observed flux and can be acceptably fitted by a hot, optically thin thermal plasma or a power-law model. If the emission is thermal, then a temperature kThot ≥ 3 keV is derived. Such high temperatures are not predicted by current instability-driven wind shock models, and a different mechanism is thus required to explain the hard X-rays. We examine several possible mechanisms and show that the hard emission could be accounted for by the W-R wind shocking onto a close stellar companion that has so far escaped detection. However, until persuasive evidence for binarity is found, we are left with the intriguing possibility that the hard X-ray emission is produced entirely by the Wolf-Rayet star.

ASCAView of the Supernova Remnant γ Cygni (G78.2+2.1): Bremsstrahlung X‐Ray Spectrum from Loss‐flattened Electron Distribution

With the ASCA X-ray satellite, we perform spatial and spectral studies of the shell-type supernova remnant (SNR) γ Cygni that is associated with the brightest EGRET unidentified source 3EG J2020+4017. At energies below 3 keV, the bulk of the X-ray flux from the remnant is well described by the emission of thermal plasma with characteristic temperature kTe 0.5-0.9 keV. In addition to this thermal emission, we found an extremely hard X-ray component from several clumps localized in the northern part of the remnant. This component, which dominates the X-ray emission from the remnant above 4 keV, is described by a power law with a photon index of Γ 0.8-1.5. Both the absolute flux and the spectral shape of the nonthermal X-rays cannot be explained by the synchrotron or inverse-Compton mechanisms. We argue that the unusually hard X-ray spectrum can be interpreted naturally in terms of nonthermal bremsstrahlung from Coulomb-loss-flattened electron distribution in dense environs with the gas density about 10-100 cm-3. For a given spectrum of the electron population, the ratio of the bremsstrahlung X- and γ-ray fluxes depends on the position of the in the electron spectrum. Formally, the entire high-energy γ-ray flux detected by EGRET from γ Cygni could originate in the hard X-ray regions. However, it is more likely that the bulk of γ-rays detected by EGRET comes from the radio-bright and X-ray-dim cloud at the southeast, where the very dense gas and strong magnetic field would illuminate the cloud in the radio and γ-ray bands but suppress the bremsstrahlung X-ray emission due to the shift of the Coulomb break in the electron spectrum toward higher energies.

A Thermal Bremsstrahlung Model for the Quiescent X‐Ray Emission from Sagittarius A*

I consider the thermal bremsstrahlung emission from hot accretion flows (Bondi/ADAFs), taking into account the finite size of the observing telescope's beam (Rbeam) relative to the Bondi accretion radius (RA). For Rbeam ≫ RA, soft X-ray emission from the hot interstellar medium surrounding the black hole dominates the observed emission, while for Rbeam ≪ RA, hard X-ray emission from the accretion flow dominates. I apply these models to Chandra observations of the Galactic center, for which Rbeam ≈ RA. I argue that bremsstrahlung emission accounts for most of the "quiescent" (nonflaring) flux observed by Chandra from Sgr A*; this emission is spatially extended on scales ~RA ~ 1'' and has a relatively soft spectrum, as is observed. If accretion onto the central black hole proceeds via a Bondi or ADAF flow, a hard X-ray power law should be present in deeper observations with a flux ~ of the soft X-ray flux; nondetection of this hard X-ray component would argue against ADAF/Bondi models. I briefly discuss the application of these results to other low-luminosity active galactic nuclei.

High energy phenomena in eta carinae

We have investigated with BeppoSAX the long term behaviour of the harder X-ray component of the supposed supermassive binary system η Car along its 5.52 year cycle. We have found that in March 1998 during egress from the last December 1997 eclipse, this component was the same as outside eclipse, but for a large (×3.5) increase of NH h , that can be attributed to the presence or formation of opaque matter in front of the source near periastron. Unexpectedly, at that time the iron 6.7 keV emission line was 40% stronger. BeppoSAX has for the first time found ahard X-ray tail extending to at least 50 keV, that cannot be adequately fitted with an additional hotter thermal component. The 2–100 keV spectrum of η Car is instead well fitted with an absorbed powerlaw spectrum with photon index 2.53, suggesting non-thermal emission as an alternative model for the core source.

ChandraX‐Ray Observations of NGC 4151

We present Chandra X-ray observations of the nearby Seyfert 1.5 galaxy NGC 4151. The images show the extended soft X-ray emission on the several hundreds of parsec scale with better sensitivity than previously obtained. We show that the hard X-ray component (greater than 2 keV) is spatially unresolved. The spectrum of the unresolved nuclear source may be described by a heavily absorbed (NH 3 × 1022 cm-3), hard power law (Γ 0.3) plus soft emission from either a power-law (Γ 2.6) or a thermal (kT 0.6 keV) component. The flux of the high-energy component has decreased from that observed by ASCA in 1993, and the spectrum is much harder (Γ ~ 0.3 in 2000 vs. Γ 1.5 in 1993). The large difference between the soft and hard spectral shapes does not favor the partial covering or scattering model of the soft excess. Instead, it is likely that the hard and soft nuclear components represent intrinsically different X-ray sources. The stronger nuclear emission lines of those seen by the Chandra HETGS spectrum are detected. Spectra of the extended emission to almost 1 kpc northeast and southwest of the nucleus have also been obtained. The spectra of these regions may be described by either thermal bremsstrahlung (kT 0.4-0.7 keV) or power-law (Γ 2.5-3.2) continua plus three emission lines. There is an excellent correlation between the extended X-ray and [O III] λ5007 line emissions. We discuss the nature of the extended X-ray emission. Because there is no extended electron-scattered hard X-ray emission, an upper limit to the electron scattering column can be obtained. This upper limit is much too low for the soft X-rays to be electron-scattered nuclear radiation, unless the nucleus radiates soft X-rays much more strongly toward the extended regions than toward Earth, a situation we consider unlikely. We favor a picture in which the extended X-ray-emitting gas is heated in situ by the nuclear radiation. Some of the X-rays may originate from a hot phase that confines the warm ionized gas seen optically, although X-ray emission produced via photoionization by the nucleus is also likely. A faint, probably background, compact X-ray source lies 22 from the nucleus to the southwest, approximately along an extension of the extended southwest X-ray and [O III] emission.

High-Energy X-Ray Timing Experiment Detections of Hard X-Ray Tails in Scorpius X-1

We report the detection of a nonthermal hard X-ray component from Sco X-1 based on the analysis of 20-220 keV spectra obtained with the High-Energy X-Ray Timing Experiment on board the Rossi X-Ray Timing Explorer satellite. We find that the addition of a power-law component to a thermal bremsstrahlung model is required to achieve a good fit in five of 16 observations analyzed. Using Proportional Counter Array data, we were able to track the movement of the source along the Z diagram, and we found that the presence of the hard X-ray tail is not confined to a specific Z position. However, we do observe an indication that the power-law index hardens with increasing , as indicated from the position on the Z diagram. We find that the derived nonthermal luminosities are ~10% of that derived for the brightest of the atoll sources.

HEXTE studies of Sco X-1 spectra: Detections of hard X-ray tails beyond 200 keV

Using the HEXTE experiment on-board the RXTE satellite, we performed a search for hard X-ray tails in Sco X-1 spectra. We found strong evidence for the presence of such a non-thermal component on several occasions. Using the PCA/ RXTE we were able to track the position of the source along the Z diagram, and we observed that the presence of the hard X-ray tail is not confined to a particular region. However, we found a correlation between the power law index of the non-thermal component and the position of the source in the Z diagram, suggesting that the hard X-ray spectrum (i.e., E > 50 keV) becomes flatter as the mass accretion rate increases. We were also able to study the temporal variation of the appearance/absence of the hard X-ray component. With our derived luminosities, we were also able to test the idea that X-ray luminosities can be used to distinguish between X-ray binary systems containing neutron stars and black holes.

ASCA spectroscopy of the hard X-ray emission from the colliding wind interaction in 2 Velorum

ABSTRA C T We discuss an ASCA observation of the eccentric WC8 1 O7:5 III binary g 2 Velorum near apastron. The X-ray spectrum is compared with two previous observations obtained when the system was near periastron. All three spectra display a hard-emission component that undergoes strong variability over the orbital cycle. The properties of the hard X-ray emission of g 2 Vel are constrained by taking into account the contribution from contaminating soft X-ray sources in the vicinity of g 2 Vel. We find that the observed variations are in qualitative agreement with the predictions of colliding wind models. We investigate for the first time the effect of uncertainties in the chemical composition of the X-ray emitting plasma on our understanding of the high-energy properties of the wind interaction region. Our results indicate that these uncertainties significantly affect the derived shock temperature and absorption column, but play a smaller role in determining the intrinsic X-ray luminosity of the colliding wind zone. We further find that the intrinsic luminosity from the hard X-ray component in g 2 Vel does not follow the 1=D distance relation expected from simple models

BeppoSAXObservation of NGC 7582: Constraints on the X‐Ray Absorber

This paper presents a BeppoSAX observation of NGC 7582 made during 1998 November and an optical spectrum taken in 1998 October. The new X-ray data reveal a previously unknown hard X-ray component in NGC 7582, peaking close to 20 keV. Rapid variability is observed with correlated changes in the 5-10 and 13-60 keV bands indicating that a single continuum component, produced by the active nucleus, provides the dominant flux across both bands. Changes in the nuclear X-ray flux appear unrelated to the gradual decline in optical flux noted since the high-state in 1998 July. The X-ray continuum is attenuated by a thick absorber of N_H ~ 1.6 x 10E24 cm^-2 covering ~60% of the nucleus, plus a screen with N_H ~ 1.4 x 10E23 cm^-2 covering the entire nucleus. Comparison of the BeppoSAX and ASCA spectra shows an increase in the full screen by N_H ~7 x 10E22 cm^-2 since 1994, confirming the absorption variability found by Xue et al. The increase in soft X-ray flux between 1994 and 1998 is consistent with the appearance of holes in the full screen producing some clear lines-of-sight to the broad-line-region.

RXTE broad band X-ray spectrum of the burster 1E1724-3045

The X-ray burster 1E1724-3045 located in the globular cluster Terzan 2 is known as one of the persistent (though variable) hard X-ray sources as shown by the SIGMA observations of the Galactic Center region. 1E1724-3045 was observed with the PCA and HEXTE experiments onboard the Rossi X-ray Timing Explorer (RXTE) on November 1996 for about 100 ksec. The broad band spectral capability and sensitivity of RXTE enables to study simultaneously the X-ray (3–20 keV) and hard X-ray components (E>20 keV) of this source. During the observation, this “Atoll” source was in its “Island” state characterized by a hard Comptonized spectrum with an electron temperature kT e ∼ 29keV, an optical depth τ ∼ 2.9 (spherical geometry) and a temperature of the “seed” photons of kT W ∼ 1.2keV. Below 5 keV, there is a soft excess which we fit with a blackbody of kT BB∼ 0.67 keV. The Comptonization temperature is significantly lower those observed for black holes candidates in their low luminosity state (kT e ≳ 50keV). Finally, our observation allows us to associate the presence of an hard tail with a low luminosity X-ray state (1–20 keV luminosity of 1.0×10 37 ergs s −1 at 7.7 kpc).

Quasi‐periodic Oscillations and Spectral States in GRS 1915+105

We present results from the analysis of X-ray energy spectra and quasi-periodic oscillations (QPOs) from a set of observations that samples a broad range of time variability in GRS 1915+105. We first demonstrate that the frequency and integrated amplitude of a 0.5-10 Hz QPO is correlated with the apparent temperature of the accretion disk for the majority of observations. We then show that the behavior of GRS 1915+105 exhibits two distinct modes of accretion. In the first mode, the QPO is present between 0.5 and 10 Hz and variability in the source luminosity is dominated by the power-law component. In the second mode, the QPO is absent and the changes in the luminosity are dominated by thermal emission from the accretion disk. We find that the color radius and temperature of the inner accretion disk are empirically related by Rcol ∝ T + const. We discuss these results in terms of ongoing efforts to explain the origin of both the QPOs and the hard X-ray component in the spectrum of GRS 1915+105.

X‐Ray Observations of Gravitationally Lensed Quasars: Evidence for a Hidden Quasar Population

X-ray observations are presented of gravitationally lensed (GL) quasars with redshifts ranging between 1 and 4. The large magnification factors of GL systems allow us to investigate the properties of quasars with X-ray luminosities that are substantially lower than those of unlensed ones and also provide an independent means of estimating the contribution of faint quasars to the hard X-ray component of the cosmic X-ray background. Spectral indices have been estimated in the rest-frame energy bands 0.5-1 keV (soft), 1-4 keV (mid), and 4-20 keV (hard). Our spectral analysis indicates a flattening of the spectral index in the hard band for two radio-loud quasars in the GL quasar sample for which the data have moderate signal-to-noise ratio. These results are consistent with the reported spectral properties of nonlensed radio-loud quasars; however, there are no indications of spectral hardening toward fainter X-ray fluxes. We have identified a large fraction of broad absorption line (BAL) quasars among the GL quasar population. We find that approximately 35% of radio-quiet GL quasars contain BAL features, which is significantly larger than the 10% fraction of BAL quasars presently found in optically selected flux-limited quasar samples. We present a simple model that estimates the effects of attenuation and lens magnification on the luminosity function of quasars and that explains the observed fraction of GL BAL quasars. These observations suggest that a large fraction of BAL quasars are missed from flux-limited optical surveys. Modeling of several X-ray observations of the GL BAL quasar PG 1115+080 suggests that the observed large X-ray variability may be caused in part by a variable intrinsic absorber consistent with previously observed variability of the BAL troughs in the UV band. The observed large X-ray flux variations in PG 1115+080 offer the prospect of considerably reducing errors in determining the time delay with future X-ray monitoring of this system and hence constraining the Hubble constant H0.