Increased X-ray Flux Research Articles

The Multiwavelength Environment of Second Bologna Catalog Sources

We present the first results of the Chandra Cool Targets (CCT) survey of the Second Bologna Catalog (B2CAT) of powerful radio sources, aimed at investigating the extended X-ray emission surrounding these sources. For the first 33 sources observed in the B2CAT CCT survey, we performed both imaging and spectral X-ray analysis, producing multiband Chandra images, and compared these images with radio observations. To evaluate the presence of extended emission in the X-rays, we extracted surface flux profiles comparing them with simulated ACIS point-spread functions. We detected X-ray nuclear emission for 28 sources. In addition, we detected eight regions of increased X-ray flux originating from radio hot spots or jet knots, and a region of decreased flux, possibly associated with an X-ray cavity. We performed X-ray spectral analysis for 15 nuclei and found intrinsic absorption significantly larger than the Galactic values in four of them. We detected significant extended X-ray emission in five sources, and fitted their spectra with thermal models with gas temperatures ∼2 keV. In the case of B2.1 0742+31, the surrounding hot gas is compatible with the intracluster medium of low-luminosity clusters of galaxies, while the X-ray diffuse emission surrounding the highly disturbed wide-angle-tailed radio galaxy B2.3 2254+35 features a luminosity similar to those of relatively bright galaxy groups, although its temperature is similar to those of low-luminosity galaxy clusters. These results highlight the power of low-frequency radio selection, combined with short Chandra snapshot observations, for investigating the properties of X-ray emission from radio sources.

Revealing Thermal Comptonization of Accretion Disk Photons in IC 4329A with AstroSat

Abstract We present five simultaneous UV/X-ray observations of IC 4329A by AstroSat performed over a 5 month period. We utilize the excellent spatial resolution of the Ultra-Violet Imaging Telescope on board AstroSat to reliably separate the intrinsic active galactic nucleus (AGN) flux from the host galaxy emission and correct for the Galactic and internal reddening, as well as the contribution from the narrow- and broad-line regions. We detect large-amplitude UV variability, which is unusual for a large black hole mass AGN like IC 4329A, over such a small period. In fact, the fractional variability amplitude is larger in the UV band than in the X-ray band. This demonstrates that the observed UV variability is intrinsic to the disk and not due to X-ray illumination. The joint X-ray spectral analyses of five sets of Soft X-ray Telescope and Large Area X-ray Proportional Counter spectral data reveal a soft X-ray excess component, a narrow iron line (with no indication of a significant Compton hump), and a steepening power law (ΔΓ ∼ 0.21) with increasing X-ray flux. The soft excess component could arise due to thermal Comptonization of the inner disk photons in a warm corona with kT e ∼ 0.26 keV. The UV emission we detect acts as the primary seed photons for the hot corona, which produces the broadband X-ray continuum. The X-ray spectral variability is well described by the cooling of this corona from kT e ∼ 42 to ∼32 keV with increasing UV flux, while the optical depth remains constant at τ ∼ 2.3.

X-ray induced demagnetization of single-molecule magnets

Low-temperature x-ray magnetic circular dichroism measurements on the endohedral single-molecule magnet DySc2N@C80 at the Dy M4,5 edges reveal a shrinking of the opening of the observed hysteresis with increasing x-ray flux. Time-dependent measurements show that the exposure of the molecules to x-rays resonant with the Dy M5 edge accelerates the relaxation of magnetization more than off-resonant x-rays. The results cannot be explained by a homogeneous temperature rise due to x-ray absorption. Moreover, the observed large demagnetization cross sections indicate that the resonant absorption of one x-ray photon induces the demagnetization of many molecules.

NuSTAR DISCOVERY OF A CYCLOTRON LINE IN KS 1947+300

We present a spectral analysis of three simultaneous Nuclear Spectroscopy Telescope Array and Swift/XRT observations of the transient Be-neutron star binary KS 1947+300 taken during its outburst in 2013/2014. These broadband observations were supported by Swift/XRT monitoring snapshots every three days, which we use to study the evolution of the spectrum over the outburst. We find strong changes of the power-law photon index, which shows a weak trend of softening with increasing X-ray flux. The neutron star shows very strong pulsations with a period of P ≈ 18.8 s. The 0.8–79 keV broadband spectrum can be described by a power law with an exponential cutoff and a blackbody component at low energies. During the second observation we detect a cyclotron resonant scattering feature at 12.5 keV, which is absent in the phase-averaged spectra of observations 1 and 3. Pulse phase-resolved spectroscopy reveals that the strength of the feature changes strongly with pulse phase and is most prominent during the broad minimum of the pulse profile. At the same phases the line also becomes visible in the first and third observation at the same energy. This discovery implies that KS 1947+300 has a magnetic field strength of B ≈ 1.1 × 1012(1 + z) G, which is at the lower end of known cyclotron line sources.

Investigating the feasibility of photon‐counting K‐edge imaging at high x‐ray fluxes using nonlinearity corrections

Pulse pileup occurring at high x-ray fluxes can severely degrade the energy resolution provided by a photon counting detector, which can represent a problem in spectroscopic CT when performing quantitative material discrimination tasks. As the effects of pileup can be most easily seen as a degradation of a detector's count rate linearity at high fluxes, it has been proposed previously to quantify and correct these nonlinearities. While this strategy has been applied successfully to materials without K-edges, it is currently unknown if this still prevails when using medical contrast agents. The purpose of this study is to close this gap. A Medipix2MXR Hexa detector was employed, featuring a pixel pitch of 165 [micro sign]m and a 1 mm thick CdTe sensor. A phantom containing various concentrations of iodine and gadolinium contrast agents was subject to energy selective CT acquisitions, using a pulsable x-ray source operated at 70 kVp. These acquisitions were obtained at low and high photon fluxes of 1.0 × 10(6) and 1.3 × 10(7) mm(-2) s(-1), respectively. Nonlinearity corrections were applied to the high-flux projections and for each pixel separately. The results were compared to the results at low photon fluxes. At high fluxes, a general reduction of the reconstructed attenuation coefficients was observed, which could be partially recovered using the correction strategy applied. The spectroscopic separation of iodine from the phantom material, however, degraded with increasing x-ray flux. In contrast to this, gadolinium could still be discriminated almost as well as in the low flux case. Nonlinearity corrections applied to high flux measurements can help to recover attenuation coefficients normally obtained at low fluxes for low-Z materials, which do not exhibit an absorption edge in the relevant energy range. However, as a result of a significant change of the x-ray spectrum, the spectroscopic contrast normally observed for iodine was found to vanish with increasing x-ray flux. In other words, the authors' results indicate that nonlinearity corrections may be feasible only when the K-edge of interest is sufficiently high compared to the mean photon energy, and that spectroscopic CT at high x-ray fluxes may suffer from less limitations when using high-Z materials as contrast agents. A future study should aim to confirm these findings under clinical conditions.

DETECTION OF VHE γ-RAYS FROM HESS J0632+057 DURING THE 2011 FEBRUARY X-RAY OUTBURST WITH THE MAGIC TELESCOPES

The very high energy (VHE) \gamma-ray source HESS J0632+057 has recently been confirmed to be a \gamma-ray binary. The optical counterpart is the Be star MWC 148, and a compact object of unknown nature orbits it every ~321 d with a high eccentricity of ~0.8. We monitored HESS J0632+057 with the stereoscopic MAGIC telescopes from 2010 October to 2011 March and detected significant VHE \gamma-ray emission during 2011 February, when the system exhibited an X-ray outburst. We find no \gamma-ray signal in the other observation periods when the system did not show increased X-ray flux. Thus HESS J0632+057 exhibits \gamma-ray variability on timescales of the order of one to two months possibly linked to the X-ray outburst that takes place about 100 days after the periastron passage. Furthermore our measurements provide for the first time the \gamma-ray spectrum down to about 140 GeV and indicate no turnover of the spectrum at low energies. We compare the properties of HESS J0632+057 with the similar \gamma-ray binary LS I +61 303, and discuss on the possible origin of the multi-wavelength emission of the source

EPISODIC X-RAY EMISSION ACCOMPANYING THE ACTIVATION OF AN ERUPTIVE PROMINENCE: EVIDENCE OF EPISODIC MAGNETIC RECONNECTION

We present an X-ray imaging and spectroscopic study of a partially occulted C7.7 flare on 2003 April 24 observed by RHESSI that accompanied a prominence eruption observed by TRACE. (1) The activation and rise of the prominence occurs during the preheating phase of the flare. The initial X-ray emission appears as a single coronal source at one leg of the prominence and it then splits into a double source. Such a source splitting happens three times, each coinciding with an increased X-ray flux and plasma temperature, suggestive of fast reconnection in a localized current sheet and an enhanced energy release rate. In the late stage of this phase, the prominence displays a helical structure. These observations are consistent with the tether-cutting and/or kink instability model for triggering solar eruptions. (2) The eruption of the prominence takes place during the flare impulsive phase. Since then, there appear signatures predicted by the classical CSHKP model of two-ribbon flares occurring in a vertical current sheet trailing an eruption. These signatures include an EUV cusp and current-sheet-like feature (or ridge) above it. There is also X-ray emission along the EUV ridge both below and above the cusp, which in both regions appears closer to the cusp at higher energies in the thermal regime. This trend is reversed in the nonthermal regime. (3) Spectral analysis indicates thermal X-rays from all sources throughout the flare, while during the impulsive phase there is additional nonthermal emission which primarily comes from the coronal source below the cusp. This source also has a lower temperature, a higher emission measure, and a much harder nonthermal spectrum than the upper sources.

The nature of X-ray spectral variability in Seyfert Galaxies

Abstract In many Seyfert galaxies the 2-20 keV X-ray spectrum typically softens with increasing X-ray flux. This effect has been explained by two different models; either pivoting of a single spectral component or relative changes in the normalisations of two components (a hard and a soft) which each have constant shape. We present a model-independent technique to analyse spectral variability, the ‘flux-flux’ plot, which shows conclusively that the two-component model is correct for some Seyfert galaxies (including the well studied MCG–6-30-15 ) but in other Seyferts spectral pivoting applies (e.g. NGC 4051). The nature of each of the spectral components in MCG–6-30-15 are briefly examined.

Delays in dwarf novae I: The case of SS Cygni

Using the disc instability model and a simple but physically reasonable model for the X-ray, extreme UV, UV and optical emission of dwarf novae we investigate the time lags observed between the rise to outburst at different wavelengths. We find that for ``normal'', i.e. fast-rise outbursts, there is good agreement between the model and observations provided that the disc is truncated at a few white dwarf radii in quiescence, and that the viscosity parameter alpha is ~0.02 in quiescence and ~0.1 in outburst. In particular, the increased X-ray flux between the optical and EUV rise and at the end of an outburst, is a natural outcome of the model. We cannot explain, however, the EUV delay observed in anomalous outbursts because the disc instability model in its standard alpha-prescription form is unable to produce such outbursts. We also find that the UV delay is, contrary to common belief, slightly longer for inside-out than for outside-in outbursts, and that it is not a good indicator of the outburst type.

A new macromolecular oscillation camera at CHESS.

Increasing X-ray flux and decreasing crystal size are two factors placing new demands on macromolecular diffraction cameras at synchrotrons. A new oscillation camera with high mechanical precision and fast rotation speed is described.

Protein Crystallography Using Capillary-Focused X-Rays

Tapered glass capillaries for X-ray beam concentration have been a topic of much interest for the synchrotron X-ray community in recent years. These optics have long held the promise of high-intensity microbeam generation for the “hard” X-ray energies used in crystallography and fluorescence analysis.X-ray concentration is achieved by exploiting the total external reflection property of glass surfaces for glancing angles of incidence. X-rays directed into the entrance aperture of the capillary are reflected toward an exit aperture of smaller dimensions, resulting in an increased X-ray flux per unit area at the exit aperture. Capillary designs with a true geometrical focus beyond the capillary exit are also possible.Our group has recently demonstrated a paraboloidally-tapered glass capillary optic which produced a focused X-ray beam using a monochromatised synchrotron source. The optic was designed to produce a focal region for singly-reflected X-rays at a point 40 mm from the end of the capillary. Such a focal region was observed, with a FWHM intensity gain of two orders of magnitude over the incident X-ray intensity from the channel-cut monochromator. Moreover, this gain was achieved for X-ray energies from 5 to 20 keV. We subsequently used a similar optic to obtain X-ray diffraction patterns from a crystal of hen egg-white lysozyme on image plates. The use of the capillary-focused beam yielded diffraction patterns 70 to 100 times faster than using an unfocused beam from the channel-cut monochromator alone. Placement of the crystal at different positions in the capillary-focused beam demonstrated the focusing of Bragg reflections and diffraction from a small volume of crystal.

The early bolometric evolution of Nova Cygni 1992

We report on combined ultraviolet and optical observations of early stages of the outburst of Nova Cygni 1992. We show that the initial optical rise and decline were produced by the dramatic increase in UV line opacity and its gradual subsequent lifting as the ejecta expand. The source of the M(sub v) - t(sub 3) relation is easily understood in light of these low-resolution UV spectra. The multiwavelength data confirm the theoretical prediction that a nova evolves at a constant bolometric luminosity in the early stages of outburst. We verify the prediction for this nova for at least the first month of the outburst. The detection of far-UV radiation with the Voyager Ultraviolet Spectrometer and the detection of an increasing X-ray flux with Roentgen Satellite (ROSAT) imply that this phase lasted for more than 6 months.

On the correlation between X-ray flux and hydrogen line intensities in quasars and active galactic nuclei

Hydrogen line intensities in quasars and active galactic nuclei are calculated as a function of the UV/X-ray luminosity ratio. The dependence of Ly-alpha/H-alpha (or Ly-alpha/H-beta) on L(uv)/Lx is found to be weak and not easily discernible in a large data sample if variations in the cloud column and nucleon density are large. Two causes account for the weak dependence. At high X-ray luminosities, Balmer continuum emission is the dominant coolant in the extended ionized zone (where X-ray heating occurs) of a broad-line cloud, and the Balmer line intensities increase very slowly with increasing X-ray flux. At low X-ray luminosities, both ionization from excited states of N I and Ly-beta continuum resonance fluorescence enable the gas to absorb continuum photons between about 11.5 and 13.6 eV, and the consequent heating, now relatively important, enhances H-alpha and H-beta emissions. 17 references.