X-ray Emission Properties Research Articles

Spatially ResolvedChandraImaging Spectroscopy of the Classical/Weak‐Lined T Tauri System V710 Tau

We present spatially resolved X-ray observations of the binary T Tauri star system V710 Tau. Using Chandra's Advanced CCD Imaging Spectrometer (ACIS), we imaged this 3.2'' separation binary system, consisting of a classical T Tauri star, V710 Tau N, and a weak-lined T Tauri star, V710 Tau S. The Chandra ACIS-S3 images—obtained in two 9 ks exposures separated by about 3 months (2004 December and 2005 April)—cleanly resolve the V710 Tau binary, demonstrating that both stars emit X-rays, and thereby enabling the first spectral/temporal study of the individual components of this mixed (classical and weak-lined) T Tauri star binary system. The northern component, V710 Tau N, appears to have been in a flaring state during the first (2004 December) exposure. During this flare event, the X-ray flux of the classical T Tauri star hardened significantly. Single-component plasma models with plasma temperatures in the range kTX ~ 0.7–1.1 keV are adequate to fit the observed X-ray spectra of V710 Tau S in 2004 December and both stars in 2005 April. The 2004 December flare-state observation of V710 Tau N requires a higher temperature plasma component (kTX ~ 2.5–3.0 keV) in addition to the soft component (kTX ~ 0.5 keV), and is better fit by a model that includes a slightly enhanced Ne/Fe abundance ratio. These results are generally consistent with statistical contrasts between the X-ray emission properties of classical (rapidly accreting) versus weak-lined (weakly accreting or nonaccreting) T Tauri stars.

Hydrostatic equilibrium of a porous intracluster medium: implications for mass fraction and X-ray luminosity

The presence of dilute hot cavities in the intracluster medium (ICM) at the cores of clusters of galaxies changes the relation between gas temperature and its X-ray emission properties. Using the hydrostatic equations of a porous medium, we solve for the ICM density for a given temperature as a function of the filling factor of dilute bubbles. We find that at a given temperature, the core X-ray luminosity increases with the filling factor. If the frequency of active galactic nuclei (AGN) in clusters were higher in the past, then the filling factor could correspondingly be significant, with implications for the cluster scaling relations at high redshifts. This is especially important for the core properties, including the temperature–luminosity (LX–T) relation and estimates of the core gas mass. The results imply an epoch-dependent sensitivity of the LX–T relation in the core to the porosity of the ICM. Detection of such an effect would give new insights into AGN feedback.

X-ray emission properties from Circinus X-1

The X-ray activity of Circinus X-1, a periodically flaring source, was studied using observational data from the all-sky monitor onboard the Rossi X-ray Timing Explorer orbital observatory in the 2–12 keV energy band. The cross-correlation analysis of the 1 day averaged data set obtained in 2001 has confirmed the fundamental 16.6 day orbital period. The variability pattern of source emission in September–November 2005 has a complicated structure that may be explained by X-ray emission pressure on the accretion disc and periodically ejected matter. The spectral analysis of X-ray emission has revealed the source behaviour in high- and low-activity states that may be caused by differential absorption of emission by the dynamically changed environment in a close binary system.

The Sloan Digital Sky Survey Quasar Catalog. IV. Fifth Data Release

We present the fourth edition of the Sloan Digital Sky Survey (SDSS) Quasar Catalog. The catalog contains 77,429 objects; this is an increase of over 30,000 entries since the previous edition. The catalog consists of the objects in the SDSS Fifth Data Release that have luminosities larger than Mi = -22.0 (in a cosmology with H0 = 70 km s-1 Mpc-1, ΩM = 0.3, and ΩΛ = 0.7), have at least one emission line with FWHM larger than 1000 km s-1 or have interesting/complex absorption features, are fainter than i ≈ 15.0, and have highly reliable redshifts. The area covered by the catalog is ≈5740 deg2. The quasar redshifts range from 0.08 to 5.41, with a median value of 1.48; the catalog includes 891 quasars at redshifts greater than 4, of which 36 are at redshifts greater than 5. Approximately half of the catalog quasars have i < 19; nearly all have i < 21. For each object the catalog presents positions accurate to better than 0.2'' rms per coordinate, five-band (ugriz) CCD-based photometry with typical accuracy of 0.03 mag, and information on the morphology and selection method. The catalog also contains basic radio, near-infrared, and X-ray emission properties of the quasars, when available, from other large-area surveys. The calibrated digital spectra cover the wavelength region 3800-9200 A at a spectral resolution of 2000; the spectra can be retrieved from the public database using the information provided in the catalog. The average SDSS colors of quasars as a function of redshift, derived from the catalog entries, are presented in tabular form. Approximately 96% of the objects in the catalog were discovered by the SDSS.

Power Spectra of Black Holes and Neutron Stars as a Probe of Hydrodynamic Structure of the Source: Diffusion Theory and Its Application to Cygnus X‐1 and Cygnus X‐2 X‐Ray Observations

We present a model of Fourier power density spectrum (PDS) formation in accretion-powered X-ray binary systems derived from diffusion theory. Timing properties of X-ray emission are considered a result of diffusive propagation of the driving perturbations in a bounded medium. We prove that the integrated power Px of the resulting PDS is only a small fraction of the integrated power Pdr of the driving oscillations, which is distributed over the disk. Furthermore, we demonstrate that Px is inversely proportional to the characteristic frequency of the driving oscillations νdr, which likely scales with the frequency of the local gravity waves in the disk (Keplerian frequency). Because νdr increases toward soft states, we conclude that Px declines toward soft states. This dependence Px ∝ ν explains the well-known observational phenomenon that the X-ray variability power decreases when the source evolves to softer states. The resulting PDS continuum is a sum of a low-frequency (LF) component, which presumably originates in an extended accretion disk, and a high-frequency (HF) component, which originates in the innermost part of the source (Compton cloud or corona). The LF PDS component has a power-law shape with an index of 1.0-1.5 at higher frequencies ("red" noise) and a flat spectrum below a characteristic (break) frequency ("white" noise). This white-red noise (WRN) continuum spectrum holds information about the bounded extended medium, the diffusion timescale, and the dependence law of viscosity versus radius. We apply our model of the PDS to RXTE and EXOSAT timing data from Cygnus X-1 and Cygnus X-2, which describes adequately the spectral transitions in these sources. The presented PDSs are shown in frequency range from 10-8 to 102 Hz, 10 orders of magnitude.

X-ray emission properties of the old pulsar PSR B2224+65

Using archival Chandra data we studied the X-ray emission properties of PSR B2224+65 and its environment. Albeit limited by photon statistics the spectral analysis suggests that the bulk of the emission from PSR B2224+65 is non-thermal. Fitting a power-law to the observed energy spectrum yields a photon index of $\Gamma=1.58^{+0.43}_{-0.33}$. The possible origin of the non-thermal pulsar emission is discussed in the context of the outer-gap model. We did not find any evidence for a compact nebula around PSR B2224+65 though the Chandra data reveal the existence of an extended feature which appears to be associated with PSR B2224+65. It extends from the pulsar position about 2 arcmin to the north-west. Its orientation deviates by $\sim118^{\circ}$ from the pulsar's proper motion direction. Investigating its energy spectrum shows that the emission of this extended feature is much harder than that of the pulsar itself and is non-thermal in nature.

Iron Line and Diffuse Hard X‐Ray Emission from the Starburst Galaxy M82

We examine the properties of the diffuse hard X-ray emission in the classic starburst galaxy M82. We use new Chandra ACIS-S observations in combination with reprocessed archival Chandra ACIS-I and XMM-Newton observations. We find E~6.7 keV Fe He-alpha emission is present in the central |r| < 200 pc, |z| < 100 pc of M82 in all datasets at high statistical significance, in addition to a possibly non-thermal X-ray continuum and marginally significant E=6.4 keV Fe K-alpha line emission. No statistically significant Fe emission is found in the summed X-ray spectra of the point-like X-ray sources or the ULX in the two epochs of Chandra observation. The total nuclear region iron line fluxes in the 2004 April 21 XMM-Newton observation are consistent with those of the Chandra-derived diffuse component, but in the 2001 May 6 XMM-Newton observation they are significantly higher and also both E=6.4 and E=6.9 keV iron lines are detected. We attribute the excess iron line emission to the Ultra-Luminous X-ray source in its high state. In general the iron K-shell luminosity of M82 is dominated by the diffuse component. The total X-ray luminosity of the diffuse hard X-ray emission (corrected for emission by unresolved low luminosity compact objects) is L_X ~ 4.4 x 10^39 erg/s in the E=2-8 keV energy band, and the 6.7 keV iron line luminosity is L_X ~ (1.1 -- 1.7) x 10^38 erg/s. The 6.7 keV iron line luminosity is consistent with that expected from the previously unobserved metal-enriched merged supernova ejecta that is thought to drive the larger-scale galactic superwind. The iron line luminosity implies a thermal pressure within the starburst region of P/k ~ 2 x 10^7 K/cm^3, which is consistent with independent observational estimates of the starburst region pressure [Abstract abridged].

The onset of X-ray emission in young stellar objects

Aims. To study the properties of X-ray emission from young stellar objects (YSOs) through their evolution from Class I to Class III and determine whether Class 0 protostars emit X-rays.

X‐Ray Emission from Planetary Nebulae. I. Spherically Symmetric Numerical Simulations

(abridged) The interaction of a fast wind with a spherical Asymptotic Giant Branch (AGB) wind is thought to be the basic mechanism for shaping Pre-Planetary Nebulae (PPN) and later Planetary Nebulae (PN). Due to the large speed of the fast wind, one expects extended X-ray emission from these objects, but X-ray emission has only been detected in a small fraction of PNs and only in one PPN. Using numerical simulations we investigate the constraints that can be set on the physical properties of the fast wind (speed, mass-flux, opening angle) in order to produce the observed X-ray emission properties of PPNs and PNs. We combine numerical hydrodynamical simulations including radiative cooling using the code FLASH with calculations of the X-ray properties of the resulting expanding hot bubble using the atomic database ATOMDB. In this first study, we compute X-ray fluxes and spectra using one-dimensional models. Comparing our results with analytical solutions, we find some agreements and many disagreements. In particular, we test the effect of different time histories of the fast wind on the X-ray emission and find that it is determined by the final stage of the time history during which the fast wind velocity has its largest value. The disagreements which are both qualitative and quantitative in nature argue for the necessity of using numerical simulations for understanding the X-ray properties of PNs.

ChandraX‐Ray Observations of 19 Millisecond Pulsars in the Globular Cluster 47 Tucanae

We present spectral and long-timescale variability analyses of Chandra X-Ray Observatory ACIS-S observations of the 19 millisecond pulsars (MSPs) with precisely known positions in the globular cluster 47 Tucanae. The X-ray emission of the majority of these MSPs is well described by a thermal (blackbody or neutron star hydrogen atmosphere) spectrum with a temperature Teff ~ (1-3) × 106 K, emission radius Reff ~ 0.1-3 km, and luminosity LX ~ 1030-1031 ergs s-1. For several MSPs, there are indications that a second thermal component is required, similar to what is seen in some nearby field MSPs. The observed radiation most likely originates from the heated magnetic polar caps of the MSPs. The small apparent scatter in LX is consistent with thermal emission from the polar caps of a global dipole field, although the small emission areas may imply either a more complex small-scale magnetic field configuration near the neutron star surface or nonuniform polar cap heating. The radio eclipsing binary MSPs 47 Tuc J, O, and W show a significant nonthermal (power-law) component, with spectral photon index Γ ~ 1-1.5, which most likely originates in an intrabinary shock formed due to interaction between the relativistic pulsar wind and matter from the stellar companion. We reexamine the X-ray-spin-down luminosity relation (LX-Ė relation) and find that for the MSPs with thermal spectra LX ∝ Ėβ, where β ~ 0.2 ± 1.1. Due to the large uncertainties in both parameters, the result is consistent with both the linear LX-Ė relation and the flatter LX ∝ Ė0.5 predicted by polar cap heating models. In terms of X-ray properties, we find no clear systematic differences between MSPs in globular clusters and in the field of the Galaxy. We discuss the implications of these results on the present understanding of the X-ray emission properties of MSPs.

X-ray emission properties of BLAGN in the XMM-2dF Wide Angle Survey

AbstractWe present the preliminary results of the X-ray spectral analysis of one of the largest samples of X-ray selected BLAGN assembled so far from the XMM-2dF Wide Angle Survey. The sample, with 641 spectroscopically identified BLAGN, provides a unique resource to carry out a statistical analysis of the emission properties of these objects over a broad range of X-ray luminosities and redshifts. The X-ray spectra of the majority of the objects were best fitted with a power law with a near constant mean spectral photon index. No obvious trend of this spectral parameter with X-ray luminosity or redshift was found.We measured the mean photon index of our objects to be ∼1.96 ± 0.05 with an intrinsic dispersion σ=0.22±0.03. X-ray absorption was detected in ∼8% of the sources, with no preferred luminosity or redshift and having typical values of the absorbing column density ≤1022cm−2.

On the X‐Ray Emission from Massive Star Clusters and Their Evolving Superbubbles

Here we discuss the X-ray emission properties from the hot thermalized plasma that results from the collisions of individual stellar winds and supernovae ejecta within rich and compact star clusters. We propose a simple analytical way of estimating the X-ray emission generated by super star clusters and derive an expression that indicates how this X-ray emission depends on the main cluster parameters. Our model predicts that the X-ray luminosity from the star cluster region is highly dependent on the star cluster wind terminal speed, a quantity related to the temperature of the thermalized ejecta. We have also compared the X-ray luminosity from the super stellar cluster (SSC) plasma with the luminosity of the interstellar bubbles generated from the mechanical interaction of the high-velocity star cluster winds with the interstellar medium (ISM). We found that the hard (2.0-8.0 keV) X-ray emission is usually dominated by the hotter SSC plasma, whereas the soft (0.3-2.0 keV) component is dominated by the bubble plasma. This implies that compact and massive star clusters should be detected as pointlike, hard X-ray sources embedded into extended regions of soft diffuse X-ray emission. We also compared our results with predictions from the population synthesis models that take into consideration binary systems and found that in the case of young, massive, and compact super star clusters the X-ray emission from the thermalized star cluster plasma may be comparable to or even larger than that expected from the high-mass X-ray binary (HMXB) population.

The Galactic Black Hole Transient H1743−322 during Outburst Decay: Connections between Timing Noise, State Transitions, and Radio Emission

Multi-wavelength observations of Galactic black hole transients during outburst decay are instrumental for our understanding of the accretion geometry and the formation of outflows around black hole systems. H1743-322, a black hole transient observed intensely in X-rays and also covered in the radio band during its 2003 decay, provides clues about the changes in accretion geometry during state transitions and also the general properties of X-ray emission during the intermediate and the low-hard states. In this work, we report on the evolution of spectral and temporal properties in X-rays and the flux in the radio band with the goal of understanding the nature of state transitions observed in this source. We concentrate on the transition from the thermal dominant state to the intermediate state that occurs on a timescale of one day. We show that the state transition is associated with a sudden increase in power-law flux. We determine that the ratio of the power-law flux to the overall flux in the 3--25 keV band must exceed 0.6 to observe strong timing noise. Even after the state transition, once this ratio was below 0.6, the system transited back to the thermal dominant state for a day. We show that the emission from the compact radio core does not turn on during the transition from the thermal dominant state to the intermediate state but does turn on when the source reaches the low-hard state, as seen in 4U 1543-47 and GX 339-4. We find that the photon index correlates strongly with the QPO frequency and anti-correlates with the rms amplitude of variability. We also show that the variability is more likely to be associated with the power-law emission than the disk emission.

Unravelling the nature of HD 81032 — A new RS CVn binary

BVR photometric and quasi-simultaneous optical spectroscopic observations of the star HD 81032 have been carried out during the years 2000-2004. A photometric period of 18.802 ± 0.07d has been detected for this star. A large group of spots with a migration period of 7.43 ± 0.07 years is inferred from the first three years of the data. Hα and CaII H and K emissions from the star indicate high chromospheric activity. The available photometry in the BVRIJHK bands is consistent with the spectral type of K0IV previously found for this star. We have also examined the spectral energy distribution of HD 81032 for the presence of an infrared colour excess using the 2MASS JHK and IRAS photometry, but found no significant excess in any band above the normal values expected for a star with this spectral type. We have also analyzed the X-ray emission properties of this star using data obtained by the ROSAT X-ray observatory during its All-Sky Survey phase. An X-ray flare of about 12 hours duration was detected during the two days of X-ray coverage obtained for this star. Its X-ray spectrum, while only containing 345 counts, is inconsistent with a single-temperature component solar-abundance coronal plasma model, but implies either the presence of two or more plasma components, nonsolar abundances, or a combination of both of these properties. All of the above properties of HD 81032 suggest that it is a newly identified, evolved RS CVn binary.

The X-ray emission properties and the dichotomy in the central stellar cusp shapes of early-type galaxies

The Hubble Space Telescope has revealed a dichotomy in the central surface brightness profiles of early-type galaxies, which have subsequently been grouped into two families: core, boxy, anisotropic systems; and cuspy (‘power-law’), discy, rotating ones. Here we investigate whether a dichotomy is also present in the X-ray properties of the two families. We consider both their total soft emission (LSX,tot), which is a measure of the galactic hot gas content, and their nuclear hard emission (LHX,nuc), mostly coming from Chandra observations, which is a measure of the nuclear activity. At any optical luminosity, the highest LSX,tot values are reached by core galaxies; this is explained by their being the central dominant galaxies of groups, subclusters or clusters, in many of the log LSX,tot (erg s−1) ≳ 41.5 cases. The highest LHX,nuc values, similar to those of classical active galactic nuclei (AGNs), in this sample are hosted only by core or intermediate galaxies; at low luminosity AGN levels, LHX,nuc is independent of the central stellar profile shape. The presence of optical nuclei (also found by HST) is unrelated to the level of LHX,nuc, even though the highest LHX,nuc are all associated with optical nuclei. The implications of these findings for galaxy evolution and accretion modalities at the present epoch are discussed.

X‐Ray Emission from Orion Nebula Cluster Stars with Circumstellar Disks and Jets

We investigate the X-ray and near-infrared emission properties of a sample of pre-main-sequence (PMS) stellar systems in the Orion Nebula Cluster (ONC) that display evidence for circumstellar disks (proplyds) and optical jets in Hubble Space Telescope (HST) imaging. Our study uses X-ray data acquired during Chandra Orion Ultradeep Program (COUP) observations, as well as complementary optical and near-infrared data recently acquired with HST and the Very Large Telescope (VLT), respectively. Approximately 70% of ~140 proplyds were detected as X-ray sources in the 838 ks COUP observation of the ONC, including ~25% of proplyds that do not display central stars in HST imaging. In near-infrared imaging, the detection rate of proplyd central stars is >90%. Many proplyds display near-infrared excesses, suggesting disk accretion is ongoing onto the central, PMS stars. About 50% of circumstellar disks that are detected in absorption in HST imaging contain X-ray sources. For these sources, we find that X-ray absorbing column and apparent disk inclination are well correlated, providing insight into the disk scale heights and metal abundances of UV- and X-ray-irradiated protoplanetary disks. Approximately 2/3 of the ~30 proplyds and PMS stars exhibiting jets in Hubble images have COUP X-ray counterparts. These jet sources display some of the largest near-infrared excesses among the proplyds, suggesting that the origin of the jets is closely related to ongoing, PMS stellar accretion. One morphologically complex jet source, d181-825, displays a double-peaked X-ray spectral energy distribution with a prominent soft component that is indicative of strong shocks in the jet collimation region. A handful of similar objects also display X-ray spectra that are suggestive of shocks near the jet source. These results support models in which circumstellar disks collimate and/or launch jets from young stellar objects and, furthermore, demonstrate that star-disk-jet interactions may contribute to PMS X-ray emission.

The Sloan Digital Sky Survey Quasar Catalog. III. Third Data Release

We present the third edition of the Sloan Digital Sky Survey (SDSS) Quasar Catalog. The catalog consists of the 46,420 objects in the SDSS Third Data Release that have luminosities larger than Mi = -22 (in a cosmology with H0 = 70 km s-1 Mpc-1, ΩM = 0.3, and ΩΛ = 0.7), have at least one emission line with FWHM larger than 1000 km s-1 or are unambiguously broad absorption line quasars, are fainter than i = 15.0, and have highly reliable redshifts. The area covered by the catalog is ≈4188 deg2. The quasar redshifts range from 0.08 to 5.41, with a median value of 1.47; the high-redshift sample includes 520 quasars at redshifts greater than 4, of which 17 are at redshifts greater than 5. For each object the catalog presents positions accurate to better than 02 rms per coordinate, five-band (ugriz) CCD-based photometry with typical accuracy of 0.03 mag, and information on the morphology and selection method. The catalog also contains radio, near-infrared, and X-ray emission properties of the quasars, when available, from other large-area surveys. The calibrated digital spectra cover the wavelength region 3800–9200 A at a spectral resolution of 2000; the spectra can be retrieved from the public database using the information provided in the catalog. A total of 44,221 objects in the catalog were discovered by the SDSS; 28,400 of the SDSS discoveries are reported here for the first time.

AChandraX‐Ray Investigation of the Violent Interstellar Medium: From Dwarf Starbursts to Ultraluminous Infrared Galaxies

We have analyzed observations with the Chandra X-ray Observatory of the diffuse emission by hot gas in 7 dwarf starburst galaxies, 6 edge-on starburst galaxies, and 9 Ultra Luminous Infrared Galaxies. These systems cover ranges of \~ 10^4 in X-ray luminosity and several thousand in star formation rate and K-band luminosity (a proxy for stellar mass). Despite this range in fundamental parameters, we find that the properties of the diffuse X-ray emission are very similar in all three classes of starburst galaxies. The spectrum of the diffuse emission is well fit by thermal emission from gas with kT ~ 0.25 to 0.8 keV and with several-times-solar abundance ratios of alpha elements to Fe. The ratio of the thermal X-ray to far-infrared luminosity is roughly constant, as is the characteristic surface brightness of the diffuse X-ray emission. The size of the diffuse X-ray source increases systematically with both far-infrared and K-Band luminosity. All three classes show strong morphological relationships between the regions of hot gas probed by the diffuse X-ray emission and the warm gas probed by optical line emission. These findings suggest that the same physical mechanism is producing the diffuse X-ray emission in the three types of starbursts, and are consistent with that mechanism being shocks driven by a galactic superwind'' powered by the kinetic energy collectively supplied by stellar winds and supernovae in the starburst.

Spectral properties of nonthermal X-ray emission from the shell-type SNR RX J1713.7–3946 as revealed by XMM-Newton

We present the results of our morphological and spectral study of properties of the supernova remnant RX J1713.7-3946 based on data obtained with XMM-Newton. Highly inhomogeneous structures, such as the bright spots, filaments and dark voids, noted by Uchiyama et al. (2003), appear in the entire bright western portion of the shell. In addition two narrow rims are found which run parallel to each other from north to south in the western part of the remnant. No complex structures are seen in the interior region of the remnant. The X-ray spectra everywhere can be well represented with a power-law function of photon index ranging within Gamma = 2.0-2.8. A clear difference of spectra between the central dim region and the bright western portion is seen at lower energies. This differences can be ascribed either to an additional thermal component with electron temperature, kTe= 0.56keV from the center or, alternatively, to an increase in column density of Delta NH ~ 0.3 x 10^22 cm^-2 in the western region. In the context of the recent discovery by the NANTEN telescope of a molecular cloud apparently interacting with the western part of the supernova remnant, the second possibility seems to be the more likely scenario.

X-Ray Properties of Lyman Break Galaxies in the Great Observatories Origins Deep Survey

We constrain the X-ray emission properties of Lyman break galaxies (LBGs) at z ≈ 3–6 using the ≈2 Ms Chandra Deep Field North and ≈1 Ms Chandra Deep Field South. Large samples of LBGs were discovered using the Hubble Space Telescope as part of the Great Observatories Origins Deep Survey (GOODS). Deep optical and X-ray imaging over the GOODS fields have allowed us to place the most significant constraints on the X-ray properties of LBGs to date. Mean X-ray properties of 449, 1734, 629, and 247 LBGs with z ∼ 3, 4, 5, and 6, respectively, were determined using stacking techniques. When stacked, we detect X-ray emission from LBGs at z ∼ 3 (∼7 σ) and from an optically bright subset (brightest 25%) of LBGs at z ∼ 4 (∼3 σ); the latter is the highest redshift detection yet for "normal" galaxies in the X-ray band. The effective exposure times for these stacked observations are ≈0.7 and 0.5 Gs, respectively. The derived average rest-frame 2.0–8.0 keV luminosities are 1.5 × 1041 and 1.4 × 1041 ergs s-1, respectively. X-ray emission from these LBGs is likely due to high-mass X-ray binaries and Type II supernovae; the corresponding star formation rates are ≈10–30 M⊙ yr-1. The X-ray–to–B-band mean luminosity ratio (LX/LB) at z ∼ 3 is somewhat elevated with respect to that measured for starburst galaxies in the local universe (significance ∼3 σ). When stacking full samples of LBGs at z ∼ 4, 5, and 6, we do not obtain significant detections (<3 σ) and derive rest-frame 2.0–8.0 keV luminosity upper limits (3 σ) of 0.9, 2.8, and 7.1 × 1041 ergs s-1, respectively. These upper limits constrain any widespread active galactic nucleus (AGN) activity in these objects to be modest at best. Furthermore, we find that ∼0.5% of our LBGs from z ≈ 3 to z ≈ 6 are detected individually in the X-ray band. These LBGs have spectral shapes and luminosities characteristic of moderate-power AGNs (e.g., Seyfert galaxies and quasars).