X-ray Flux Level Research Articles

NICER Monitoring of Supersoft X-Ray Sources

We monitored four supersoft sources—two persistent ones, CAL 83 and MR Vel, and the recent novae YZ Ret (Nova Ret 2020), and V1674 Her (Nova Her 2021)—with NICER. The two persistent supersoft X-ray sources (SSS) were observed with unvaried X-ray flux level and spectrum, respectively, 13 and 20 yr after the last observations. Short-period modulations of the SSS appear where the spectrum of the luminous central source was fully visible (in CAL 83 and V1674 Her) and were absent in YZ Ret and MR Vel, in which the flux originated in photoionized or shocked plasma, while the white dwarf (WD) was not observable. We thus suggest that the pulsations occur on, or very close to, the WD surface. The pulsations of CAL 83 were almost unvaried after 15 yr, including an irregular drift of the ≃67 s period by 2.1 s. Simulations, including previous XMM-Newton data, indicate actual variations in period length within hours, rather than an artifact of the variable amplitude of the pulsations. Large amplitude pulsations with a period of 501.53 ± 0.30 s were always detected in V1674 Her, as long as the SSS was observable. This period seems to be due to rotation of a highly magnetized WD. We cannot confirm the maximum effective temperature of (≃145,000 K) previously inferred for this nova, and discuss the difficulty in interpreting its spectrum. The WD appears to present two surface zones, one of which does not emit SSS flux.

The intra-day optical monitoring of BL Lacerate object 1ES 1218+304 at its highest X-ray flux level

We here report a monitor of the BL Lac object 1ES 1218+304 in both B- and R-bands by the GWAC-F60A telescope in eight nights, when it was triggered to be at its highest X-ray flux in history by the VERITAS Observatory and Swift follow-ups. Both ANOVA and χ 2-test enable us to clearly reveal an intra-day variability in optical wavelengths in seven out of the eight nights. A bluer-when-brighter chromatic relationship has been clearly identified in five out of the eight nights, which can be well explained by the shock-in-jet model. In addition, a quasi-periodic oscillation phenomenon in both bands could be tentatively identified in the first night. A positive delay between the two bands has been revealed in three out of the eight nights, and a negative one in the other nights. The identified minimum time delay enables us to estimate the M BH = 2.8 × 107 M ⊙that is invalid.

Metastable Helium Absorptions with 3D Hydrodynamics and Self-consistent Photochemistry. I. WASP-69b, Dimensionality, X-Ray and UV Flux Level, Spectral Types, and Flares

Abstract The metastable helium (He*) lines near 10830 Å are ideal probes of atmospheric erosion—a common phenomenon of close-in exoplanet evolution. A handful of exoplanet observations yielded well-resolved He* absorption features in transits, yet they were mostly analyzed with 1D isothermal models prescribing mass-loss rates. This work devises 3D hydrodynamics coevolved with ray-tracing radiative transfer and nonequilibrium thermochemistry. Starting from the observed stellar/planetary properties with reasonable assumptions about the host’s high-energy irradiation, we predict from first principles the mass-loss rate, the temperature and ionization profiles, and 3D outflow kinematics. Our simulations well reproduce the observed He* line profiles and light curves of WASP-69b. We further investigate the dependence of He* observables on simulation conditions and host radiation. The key findings are as follows: (1) Simulations reveal a photoevaporative outflow (∼0.55 M ⊕ Gyr−1 ≃ 1011 g s−1) for WASP-69b without a prominent comet-like tail, consistent with the symmetric transit shape. (2) 3D simulations are mandatory for hydrodynamic features, including Coriolis force, advection, and kinematic line broadening. (3) EUV (>13.6 eV) photons dominate photoevaporative outflows and populate He* via recombination; FUV is also detrimental by destroying He*; X-ray plays a secondary role. (4) K stars hit the sweet spot of EUV/far-UV balance for He* line observation, while G and M stars are also worthy targets. (5) Stellar flares create characteristic responses in the He* line profiles.

An X-Ray Study of the White Dwarf Binary AR Scorpii

Abstract We report a study of X-ray emission from the white dwarf/M-type star binary system AR Scorpii using archival data taken in 2016–2020. It has been known that the X-ray emission is dominated by optically thin thermal plasma emission and its flux level varies significantly over the orbital phase. The X-ray emission also contains a component that modulates with the beat frequency between the white dwarf’s spin frequency and orbital frequency. In this new analysis, the 2020 data taken by NICER shows that the X-ray emission modulates with the spin frequency as well as with the beat frequency, indicating that part of the X-ray emission comes from the white dwarf’s magnetosphere. It is found that the signal of the spin frequency appears only at a specific orbital phase, while the beat signal appears over the orbital phase. We interpret the X-ray emission modulating with the spin frequency and the beat frequency as a result of synchrotron emission from electrons with smaller and larger pitch angles, respectively. In the long-term evolution, the beat pulse profile averaged over the orbital phase changes from a single-peak structure in 2016/2018 to a double-peak structure in 2020. The observed X-ray flux levels measured in 2016/2017 are higher than those measured in 2018/2020. The plasma temperature and the amplitude of the orbital waveform might vary with time too. These results indicate that the X-ray emission from AR Scorpii evolves on a timescale of years. This long-term evolution would be explained by a superorbital modulation related to, for example, a precession of the white dwarf or a fluctuation of the system related to the activity of the companion star.

Slingshot prominences: nature’s wind gauges

Mass loss rates for the tenuous, hot winds of cool stars are extremely difficult to measure, yet they are a crucial ingredient in the stars' rotational evolution. We present a new method for measuring these mass loss rates in young, rapidly-rotating stars. These stars are known to support systems of "slingshot prominences" fed by hot wind material flowing up from the stellar surface into the summits of closed magnetic loop structures. The material gathers and cools near the co-rotation radius until its density becomes large enough that it is visible as a transient absorption feature in the hydrogen Balmer lines and strong resonance lines such as Ca II H \&\ K. Here we present the key insight that the sonic point usually lies well below the condensation region. The flow at the wind base is therefore unaffected by the presence of an overlying prominence, so we can use the observed masses and recurrence times of the condensations to estimate the mass flux in the wind. These measurements extend the relationship between mass loss rate per unit surface area and X-ray flux to span 5 orders of magnitude. They demonstrate no evidence of the suspected weakening of stellar mass loss rates at high X-ray flux levels.

The Sleeping Monster: NuSTAR Observations of SGR 1806–20, 11 Years After the Giant Flare

Abstract We report the analysis of five Nuclear Spectroscopic Telescope Array (NuSTAR) observations of SGR 1806−20 spread over a year from 2015 April to 2016 April, more than 11 years following its giant flare (GF) of 2004. The source spin frequency during the NuSTAR observations follows a linear trend with a frequency derivative Hz s−1, implying a surface dipole equatorial magnetic field G. Thus, SGR 1806−20 has finally returned to its historical minimum torque level measured between 1993 and 1998. The source showed strong timing noise for at least 12 years starting in 2000, with increasing one order of magnitude between 2005 and 2011, following its 2004 major bursting episode and GF. SGR 1806−20 has not shown strong transient activity since 2009, and we do not find short bursts in the NuSTAR data. The pulse profile is complex with a pulsed fraction of with no indication of energy dependence. The NuSTAR spectra are well fit with an absorbed blackbody, , plus a power law, . We find no evidence for variability among the five observations, indicating that SGR 1806−20 has reached a persistent and potentially its quiescent X-ray flux level after its 2004 major bursting episode. Extrapolating the NuSTAR model to lower energies, we find that the 0.5–10 keV flux decay follows an exponential form with a characteristic timescale days. Interestingly, the NuSTAR flux in this energy range is a factor of ∼2 weaker than the long-term average measured between 1993 and 2003, a behavior also exhibited in SGR 1900+14. We discuss our findings in the context of the magnetar model.

Multisource inverse-geometry CT. Part I. System concept and development

This paper presents an overview of multisource inverse-geometry computed tomography (IGCT) as well as the development of a gantry-based research prototype system. The development of the distributed x-ray source is covered in a companion paper [V. B. Neculaes et al., "Multisource inverse-geometry CT. Part II. X-ray source design and prototype," Med. Phys. 43, 4617-4627 (2016)]. While progress updates of this development have been presented at conferences and in journal papers, this paper is the first comprehensive overview of the multisource inverse-geometry CT concept and prototype. The authors also provide a review of all previous IGCT related publications. The authors designed and implemented a gantry-based 32-source IGCT scanner with 22 cm field-of-view, 16 cm z-coverage, 1 s rotation time, 1.09 × 1.024 mm detector cell size, as low as 0.4 × 0.8 mm focal spot size and 80-140 kVp x-ray source voltage. The system is built using commercially available CT components and a custom made distributed x-ray source. The authors developed dedicated controls, calibrations, and reconstruction algorithms and evaluated the system performance using phantoms and small animals. The authors performed IGCT system experiments and demonstrated tube current up to 125 mA with up to 32 focal spots. The authors measured a spatial resolution of 13 lp/cm at 5% cutoff. The scatter-to-primary ratio is estimated 62% for a 32 cm water phantom at 140 kVp. The authors scanned several phantoms and small animals. The initial images have relatively high noise due to the low x-ray flux levels but minimal artifacts. IGCT has unique benefits in terms of dose-efficiency and cone-beam artifacts, but comes with challenges in terms of scattered radiation and x-ray flux limits. To the authors' knowledge, their prototype is the first gantry-based IGCT scanner. The authors summarized the design and implementation of the scanner and the authors presented results with phantoms and small animals.

Simulation Evaluation of Quantitative Myocardial Perfusion Assessment from Cardiac CT.

Contrast enhancement on cardiac CT provides valuable information about myocardial perfusion and methods have been proposed to assess perfusion with static and dynamic acquisitions. There is a lack of knowledge and consensus on the appropriate approach to ensure 1) sufficient diagnostic accuracy for clinical decisions and 2) low radiation doses for patient safety. This work developed a thorough dynamic CT simulation and several accepted blood flow estimation techniques to evaluate the performance of perfusion assessment across a range of acquisition and estimation scenarios. Cardiac CT acquisitions were simulated for a range of flow states (Flow = 0.5, 1, 2, 3 ml/g/min, cardiac output = 3,5,8 L/min). CT acquisitions were simulated with a validated CT simulator incorporating polyenergetic data acquisition and realistic x-ray flux levels for dynamic acquisitions with a range of scenarios including 1, 2, 3 sec sampling for 30 sec with 25, 70, 140 mAs. Images were generated using conventional image reconstruction with additional image-based beam hardening correction to account for iodine content. Time attenuation curves were extracted for multiple regions around the myocardium and used to estimate flow. In total, 2,700 independent realizations of dynamic sequences were generated and multiple MBF estimation methods were applied to each of these. Evaluation of quantitative kinetic modeling yielded blood flow estimates with an root mean square error (RMSE) of ∼0.6 ml/g/min averaged across multiple scenarios. Semi-quantitative modeling and qualitative static imaging resulted in significantly more error (RMSE = ∼1.2 and ∼1.2 ml/min/g respectively). For quantitative methods, dose reduction through reduced temporal sampling or reduced tube current had comparable impact on the MBF estimate fidelity. On average, half dose acquisitions increased the RMSE of estimates by only 18% suggesting that substantial dose reductions can be employed in the context of quantitative myocardial blood flow estimation. In conclusion, quantitative model-based dynamic cardiac CT perfusion assessment is capable of accurately estimating MBF across a range of cardiac outputs and tissue perfusion states, outperforms comparable static perfusion estimates, and is relatively robust to noise and temporal subsampling.

Comparison of blood flow models and acquisitions for quantitative myocardial perfusion estimation from dynamic CT

Myocardial blood flow (MBF) can be estimated from dynamic contrast enhanced (DCE) cardiac CT acquisitions, leading to quantitative assessment of regional perfusion. The need for low radiation dose and the lack of consensus on MBF estimation methods motivates this study to refine the selection of acquisition protocols and models for CT-derived MBF. DCE cardiac CT acquisitions were simulated for a range of flow states (MBF = 0.5, 1, 2, 3 ml (min g)−1, cardiac output = 3, 5, 8 L min−1). Patient kinetics were generated by a mathematical model of iodine exchange incorporating numerous physiological features including heterogenenous microvascular flow, permeability and capillary contrast gradients. CT acquisitions were simulated for multiple realizations of realistic x-ray flux levels. CT acquisitions that reduce radiation exposure were implemented by varying both temporal sampling (1, 2, and 3 s sampling intervals) and tube currents (140, 70, and 25 mAs). For all acquisitions, we compared three quantitative MBF estimation methods (two-compartment model, an axially-distributed model, and the adiabatic approximation to the tissue homogeneous model) and a qualitative slope-based method. In total, over 11 000 time attenuation curves were used to evaluate MBF estimation in multiple patient and imaging scenarios. After iodine-based beam hardening correction, the slope method consistently underestimated flow by on average 47.5% and the quantitative models provided estimates with less than 6.5% average bias and increasing variance with increasing dose reductions. The three quantitative models performed equally well, offering estimates with essentially identical root mean squared error (RMSE) for matched acquisitions. MBF estimates using the qualitative slope method were inferior in terms of bias and RMSE compared to the quantitative methods. MBF estimate error was equal at matched dose reductions for all quantitative methods and range of techniques evaluated. This suggests that there is no particular advantage between quantitative estimation methods nor to performing dose reduction via tube current reduction compared to temporal sampling reduction. These data are important for optimizing implementation of cardiac dynamic CT in clinical practice and in prospective CT MBF trials.

Ultraluminous X-ray source XMMUJ132218.3-164247 is in fact a type I Quasar

Context. The true nature of ultraluminous X-ray sources (ULX) is still currently vigorously debated. One of the most interesting possibilities is that these sources are intermediate-mass (M ∼ 103−105 M ) black holes. The most luminous ULX (L2−10 keV ∼ 1041 erg s−1) are the most suitable candidates for being genuine intermediate-mass black holes (IMBH). Aims. XMMUJ132218.3-164247 was proposed to be the most luminous ULX known so far. For this reason a firm assessment of its intrinsic nature is needed and pursued in this work. Methods. We precisely defined the position of XMMUJ132218.3-164247 using a short Chandra pointing of the field. Then, we obtained high quality optical spectra of the source with the FORS2 instrument on the VLT to define its nature and distance. We then used unpublished data obtained with XMM-Newton to investigate its nature and emission properties in more detail. Results. Features in its optical spectrum place XMMUJ132218.3-164247 at a redshift of z ∼ 1, implying that it is a background QSO instead of an ULX. We clearly detected some emission lines typical of a QSO, including OIII lines and a broad Mg II line that indicates that the source is a type I AGN. The X-ray spectrum of the source is well modeled by a simple power-law with absorption slightly in excess to the Galactic value. No emission feature at the energy of the FeKα is present in the data. Finally, the source has been detected at a X-ray flux level higher (by ∼40%) than previously measured and reported in its discovery work. Conclusions. XMMUJ132218.3-164247 is not a ULX but a type I QSO. It shows a standard X-ray spectrum and exhibits a variability pattern that is typical of QSOs. A very rough estimate of its black-hole mass yields a value of MBH ∼ 2 × 10 M .

Locating the TeV γ-rays from the shell regions of Cassiopeia A

AbstractWe have analyzed Chandra X-ray data from different parts of the shell of young supernova remnant (SNR) in the energy range of 0.7 - 8 keV. We observed that X-ray flux level varies over different shell regions of the source. Implications of X-ray observation will be discussed here. We also analyzed Fermi-LAT data in the energy range 0.5 - 50 GeV for the source. The differential spectrum obtained in this way fits with simple power-law. We also present here multi-wavelength modeling of the source considering archival radio and TeV data along with Chandra and Fermi-LAT data.

Correlated X-ray/ultraviolet/optical variability in the very low mass AGN NGC 4395

We report the results of a one year Swift X-ray/UV/optical programme monitoring the dwarf Seyfert nucleus in NGC 4395 in 2008-2009. The UV/optical flux from the nucleus was found to vary dramatically over the monitoring period, with a similar pattern of variation in each of the observed UV/optical bands (spanning 1900 - 5500 {\AA}). In particular, the luminosity of NGC 4395 in the 1900 {\AA} band changed by more than a factor of eight over the monitoring period. The fractional variability was smaller in the UV/optical bands than that seen in the X-rays, with the X-ray/optical ratio increasing with increasing flux. Pseudo-instantaneous flux measurements in the X-ray and each UV/optical band were well correlated, with cross correlation coefficients of >0.7, significant at 99.9 per cent confidence. Archival Swift observations from 2006 sample the intra-day X-ray/optical variability on NGC 4395. These archival data show a very strong correlation between the X-ray and b bands, with a cross-correlation coefficient of 0.84 (significant at >99 per cent confidence). The peak in the cross correlation function is marginally resolved and asymmetric, suggesting that X-rays lead the b band, but by 1 hour. In response to recent (August 2011) very high X-ray flux levels from NGC4395 we triggered Swift ToO observations, which sample the intra-hour X-ray/UV variability. These observations indicate, albeit with large uncertainties, a lag of the 1900 {\AA} band behind the X-ray flux of ~400 s. The tight correlation between the X-ray and UV/optical lightcurves, together with the constraints we place on lag time-scale are consistent with the UV/optical variability of NGC 4395 being primarily due to reprocessing of X-ray photons by the accretion disc.

The long-lasting activity of 3C 454.3

We present multiwavelength observations of 3C 454.3 from April 2008 to March 2010. The radio to optical data are mostly from the GASP-WEBT, UV and X-ray data from Swift, and gamma-ray data from the AGILE and Fermi satellites. We improved the calibration of optical-UV data from the UVOT and OM instruments and estimated the Lyalpha flux to disentangle the contributions from different components in this spectral region. The observations reveal prominent variability above 8 GHz. In the optical-UV band, the variability amplitude decreases with increasing frequency due to a steadier radiation from both a broad line region and an accretion disc. The optical flux reaches nearly the same levels in the 2008-2009 and 2009-2010 observing seasons; the mm one shows similar behaviour, whereas the gamma and X-ray flux levels rise in the second period. Two prominent gamma-ray flares in mid 2008 and late 2009 show a double-peaked structure, with a variable gamma/optical flux ratio. The X-ray flux variations seem to follow the gamma-ray and optical ones by about 0.5 and 1 d, respectively. We interpret the multifrequency behaviour in terms of an inhomogeneous curved jet, where synchrotron radiation of increasing wavelength is produced in progressively outer and wider jet regions, which can change their orientation in time. In particular, we assume that the long-term variability is due to this geometrical effect. By combining the optical and mm light curves to fit the gamma and X-ray ones, we find that the gamma (X-ray) emission may be explained by inverse-Comptonisation of synchrotron optical (IR) photons by their parent relativistic electrons (SSC process). A slight, variable misalignment between the synchrotron and Comptonisation zones would explain the increased gamma and X-ray flux levels in 2009-2010, as well as the change in the gamma/optical flux ratio during the outbursts peaks.

The Density of Coronal Plasma in Active Stellar Coronae

We have analyzed high-resolution X-ray spectra of a sample of 22 active stars observed with the High Energy Transmission Grating Spectrometer on Chandra in order to investigate their coronal plasma density. Densities were investigated using the lines of the He-like ions O VII, Mg XI, and Si XIII. Si XIII lines in all stars of the sample are compatible with the low-density limit (i.e., ne ≲ 1013 cm-3), casting some doubt on results based on lower resolution Extreme Ultraviolet Explorer (EUVE) spectra finding densities ne > 1013 cm-3. Mg XI lines betray the presence of high plasma densities up to a few times 1012 cm-3 for most of the sources with higher X-ray luminosity (≳1030 ergs s-1); stars with higher LX and LX/Lbol tend to have higher densities at high temperatures. Ratios of O VII lines yield much lower densities of a few times 1010 cm-3, indicating that the "hot" and "cool" plasma resides in physically different structures. In the cases of EV Lac, HD 223460, Canopus, μ Vel, TY Pyx, and IM Peg, our results represent the first spectroscopic estimates of coronal density. No trends in density-sensitive line ratios with stellar parameters effective temperature and surface gravity were found, indicating that plasma densities are remarkably similar for stars with pressure scale heights differing by up to 3 orders of magnitude. Our findings imply remarkably compact coronal structures, especially for the hotter (~7 MK) plasma emitting the Mg XI lines characterized by the coronal surface filling factor, f, ranging from 10-4 to 10-1, while we find f values from a few times 10-3 up to ~1 for the cooler (~2 MK) plasma emitting the O VII lines. We find that f approaches unity at the same stellar surface X-ray flux level as characterizes solar active regions, suggesting that these stars become completely covered by active regions. At the same surface flux level, f is seen to increase more sharply with increasing surface flux. These results appear to support earlier suggestions that hot 107 K plasma in active coronae arises from flaring activity and that this flaring activity increases markedly once the stellar surface becomes covered with active regions. Comparison of our measured line fluxes with theoretical models suggests that significant residual model inaccuracies might be present and, in particular, that cascade contributions to forbidden and intercombination lines resulting from dielectronic recombination might be to blame.

A Study of Quasar Evolution in the X‐Ray Band with the Aid of Gravitational Lensing

We present results from a minisurvey of relatively high redshift (1.7 < z < 4), gravitationally lensed, radio-quiet quasars observed with the Chandra X-Ray Observatory and with XMM-Newton. The lensing magnification effect allows us to search for changes with redshift in quasar spectroscopic and flux variability properties, ranging over 3 orders of magnitude in intrinsic X-ray luminosity. It extends the study of quasar properties to unlensed X-ray flux levels as low as a few times 10-15 ergs cm-2 s-1 in the observed 0.4-8 keV band. For the first time, these observations of lensed quasars have provided medium to high signal-to-noise ratio X-ray spectra of a sample of relatively high redshift and low X-ray luminosity quasars. We find a possible correlation between the X-ray power-law photon index and the X-ray luminosity of the gravitationally lensed, radio-quiet quasar sample. The X-ray spectral slope steepens as the X-ray luminosity increases. This correlation is still significant when we combine our data with other samples of radio-quiet quasars with z > 1.5, especially in the low-luminosity range between 1043 and 1045.5 ergs s-1. This result is surprising, considering that such a correlation is not found for quasars with redshifts below 1.5. We suggest that this correlation can be understood in the context of the hot-corona model for X-ray emission from quasar accretion disks, under the hypothesis that the quasars in our sample accrete very close to their Eddington limits and that the observed luminosity range is set by the range of black hole masses (this hypothesis is consistent with recent predictions of semianalytic models for quasar evolution). The upper limits of X-ray variability for our relatively high redshift sample of lensed quasars are consistent with the known correlation between variability and luminosity observed in Seyfert 1 galaxies when this correlation is extrapolated to the larger luminosities of our sample.

X-ray emission from Saturn

We report the first unambiguous detection of X-ray emission originating from Saturn with a Chandra observation, duration 65.5 ksec with ACIS-S3. Beyond the pure detection we analyze the spatial distribution of X-rays on the planetary surface, the light curve, and some spectral properties. The detection is based on 162 cts extracted from the ACIS-S3 chip within the optical disk of Saturn. We found no evidence for smaller or larger angular extent. The expected background level is 56 cts, i.e., the count rate is (1.6 +- 0.2) 10^-3 cts/s. The extracted photons are rather concentrated towards the equator of the apparent disk, while both polar caps have a relative photon deficit. The inclination angle of Saturn during the observation was -27 degrees, so that the northern hemisphere was not visible during the complete observation. In addition, it was occulted by the ring system. We found a small but significant photon excess at one edge of the ring system. The light curve shows a small dip twice at identical phases, but rotational modulation cannot be claimed at a significant level. Spectral modeling results in a number of statistically, but not necessarily physically, acceptable models. The X-ray flux level we calculate from the best-fit spectral models is 6.8 10^-15 erg/cm^2/s (in the energy interval 0.1-2keV), which corresponds to an X-ray luminosity of 8.7 10^14 erg/s. A combination of scatter processes of solar X-rays requires a relatively high albedo favoring internal processes, but a definitive explanation remains an open issue.

NEXXUS: A comprehensive ROSAT survey of coronal X-ray emission among nearby solar-like stars

We present a final summary of all ROSAT X-ray observations of nearby stars. All available ROSAT observations with the ROSAT PSPC, HRI and WFC have been matched with the CNS4 catalog of nearby stars and the results gathered in the Nearby X-ray and XUV-emitting Stars data base, available via www from the Home Page of the Hamburger Sternwarte at the URL http://www.hs.uni-hamburg.de/DE/For/Gal/Xgroup/nexxus. New volume-limited samples of F/G-stars (dlim = 14 pc), K-stars (dlim = 12 pc), and M-stars (dlim = 6 pc) are constructed within which detection rates of more than 90% are obtained; only one star (GJ 1002) remains undetected in a pointed follow-up observation. F/G-stars, K-stars and M-stars have indistinguishable surface X-ray flux distributions, and the lower envelope of the observed distribution at FX ≈ 10 4 erg/cm 2 /s is the X-ray flux level observed in solar coronal holes. Large amplitude variations in X-ray flux are uncommon for solar-like stars, but maybe more common for stars near the bottom of the main sequence; a large amplitude flare is reported for the M star LHS 288. Long term X-ray light curves are presented for α Cen A/B and Gl 86, showing variations on time scales of weeks and demonstrating that α Cen B is a flare star.

X-ray and optical counterparts of hard X-ray selected sources from the SHEEP survey: first results

We present follow-up observations of five hard X-ray sources from the ASCA 5–10 keV Search for the High Energy Extragalactic Population (SHEEP) survey, which has a limiting flux of ∼ 10−13 erg cm−2 s−1. Chandra data have been obtained to improve the X-ray positions from a few arcmin to <1 arcsec, which allows unambiguous optical identification. While the objects almost certainly house active galactic nuclei (AGNs) based on their X-ray luminosity, optical spectroscopy reveals a variety of properties. The identifications indicate that the SHEEP survey samples the same populations as deeper surveys which probe the origin of the X-ray background, but because the SHEEP sources are far brighter, they are more amenable to detailed follow-up work. We find a variety of classifications and properties, including a type II quasi-stellar object (QSO), a galaxy undergoing star formation and a broad-line AGN which has a very hard X-ray spectrum, indicating substantial absorption in the X-ray but none in the optical. Two objects have X-ray/optical flux ratios which, were they at an X-ray flux level typical of objects in Chandra deep surveys, would place them in the ‘optically faint’ category. They are both identified with broad line QSOs at z∼ 1. Clearly, this survey – which is relatively unbiased against obscured objects – is revealing a set of remarkable objects quite different to the familiar classes of AGNs found in previous optical and soft X-ray surveys.

Thermal Comptonization and Disk Thermal Reprocessing in NGC 3516

We present an application of the thermal Comptonization/disk reprocessing model recently proposed by Zdziarski, Lubiniski, and Smith. We show that the absence of strong optical variations in the presence of strong concurrent X-ray variations, similar to those found by Hubble Space Telescope (HST)/Rossi X-Ray Timing Explorer (RXTE) monitoring observations of NGC 3516, can be explained by changing the geometry of the Comptonizing plasma rather than the accretion disk itself. The total X-ray luminosity of the Comptonizing plasma must decrease as its spatial extent increases. In contrast, the disk inner radius must be roughly fixed in order not to produce optical/ultraviolet color variations stronger than observed. By including emission due to internal viscous dissipation in the disk, we can roughly match the optical and X-ray flux levels and variability amplitudes seen from NGC 3516 during the HST/RXTE campaign.

Ultraviolet Broad Absorption Features and the Spectral Energy Distribution of the Quasar PG 1351+64

We present a moderate-resolution (~20 km/s) spectrum of the mini broad-absorption-line QSO PG1351+64 between 915-1180 A, obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE). Additional spectra at longer wavelengths were also obtained with the HST and ground-based telescopes. Broad absorption is present on the blue wings of CIII 977, Ly-beta, OVI 1032,1038, Ly-alpha, NV 1238,1242, SiIV 1393,1402, and CIV 1548,1450. The absorption profile can be fitted with five components at velocities of ~ -780, -1049, -1629, -1833, and -3054 km/s with respect to the emission-line redshift of z = 0.088. All the absorption components cover a large fraction of the continuum source as well as the broad-line region. The OVI emission feature is very weak, and the OVI/Lyalpha flux ratio is 0.08, one of the lowest among low-redshift active galaxies and QSOs. The UV continuum shows a significant change in slope near 1050 A in the restframe. The steeper continuum shortward of the Lyman limit extrapolates well to the observed weak X-ray flux level. The absorbers' properties are similar to those of high-redshift broad absorption-line QSOs. The derived total column density of the UV absorbers is on the order of 10^21 cm^-2, unlikely to produce significant opacity above 1 keV in the X-ray. Unless there is a separate, high-ionization X-ray absorber, the QSO's weak X-ray flux may be intrinsic. The ionization level of the absorbing components is comparable to that anticipated in the broad-line region, therefore the absorbers may be related to broad-line clouds along the line of sight.