Hard X-ray Domain Research Articles

Gas phase Photoemission studies in the hard X-ray domain

Recent results obtained in gas phase photoemission studies are presented in this article with particular emphasis on recoil, Double Core Hole and Post Collision Interaction. These three important effects are not specific to the gas phase and could have more general applications in condensed matter studies.

High-brightness self-seeded X-ray free-electron laser covering the 3.5 keV to 14.6 keV range

A self-seeded X-ray free-electron laser (XFEL) is a promising approach to realize bright, fully coherent free-electron laser (FEL) sources in the hard X-ray domain that have been a long-standing issue with longitudinal coherence remaining challenging. At the Pohang Accelerator Laboratory XFEL, we have demonstrated a hard X-ray self-seeded XFEL with a peak brightness of 3.2 × 1035 photons s–1 mm–2 mrad–2 0.1% bandwidth (BW)–1 at 9.7 keV. The bandwidth (0.19 eV) is about 1/70 times as wide (close to the Fourier transform limit) and the peak spectral brightness is 40 times higher than in self-amplified spontaneous emission (SASE), with substantial improvements in the stability of self-seeding and noticeably suppressed pedestal effects. We could reach an excellent self-seeding performance at a photon energy of 3.5 keV (lowest) and 14.6 keV (highest) with the same stability as the 9.7 keV self-seeding. The bandwidth of the 14.6 keV seeded FEL was 0.32 eV, and the peak brightness was 1.3 × 1035 photons s–1 mm–2 mrad–2 0.1%BW–1. We show that the use of seeded FEL pulses with higher reproducibility and a cleaner spectrum results in serial femtosecond crystallography data of superior quality compared with data collected using SASE mode.

Polydisperse inter-electrode plasma of Pd nanoclusters as a random cavity for x-ray spontaneous emission bursts

It is demonstrated experimentally that polydisperse Pd plasma of low energy (∼1 J) nanosecond vacuum discharge operating in a virtual cathode regime becomes an effective stochastic cavity in hard x-ray domain. A diffusion model of stochastic propagation of spontaneous x-ray radiation in the volume of randomly located reflecting clusters is developed also. The model provides qualitative explanation of both the experimentally observed effects of high-intensity bursts and partial ‘trapping’ of x-ray radiation. The x-ray burst emerges when photons are stored inside the plasma volume followed by instant release at the end of the discharge while the trapping regime corresponds to the slow developing x-ray diffusion with the characteristic time larger than the discharge duration. The results obtained are compared qualitatively with well-known concept of Letokhov’s random laser.

2003–2018 Monitoring of the Crab Nebula Polarization in Hard X-Rays with INTEGRAL SPI

Abstract We analyzed 16 yr of observations dedicated to the Crab (pulsar + nebula) with the SPectrometer on International Gamma-Ray Astrophysics Laboratory instrument to investigate its polarization properties. We find that the source presents a substantially polarized emission (PF = 24%) in the hard X-ray domain, with the electric vector aligned with the pulsar spin axis, which is in agreement with other results at various wavelengths. The stability of the polarization characteristics with energy and over the 16 yr covered by the data is remarkable, completing the standard candle status of the source in the spectral domain. The polarization measurements imply that the synchrotron emission is the dominant mechanism of photon production from radio to hard X-rays. The high level of polarized emission points out the steadiness of the source, in particular, of the magnetic field configuration and geometry.

A CHALLENGING VIEW OF THE 2015 SUMMER V404 CYG OUTBURST AT HIGH ENERGY WITH INTEGRAL/SPI: THE FINALE*

ABSTRACT During its strong outburst of 2015 June/July, the X-ray transient V404 Cygni (=GS2023+338) was observed up to a level of 50 Crab in the hard X-ray domain. We focus here on a particularly intense episode preceeding a definitive decline of the source activity. We benefit from large signal-to-noise ratios to investigate the source spectral variability, on a timescale of five minutes. A hardness–intensity study of three broad bands reveals clearly different behaviors at low and high energy (below and above ∼100 keV). In particular, on two occasions, the source intensity varies by a factor of 3–4 in amplitude while keeping the same spectral shape. On the other hand, at the end of the major flare, the emission presents a clear anticorrelation between flux and hardness. These behaviors strongly suggest the presence of two spectral components related to emission processes varying in a largely independent way. The first component (E < 100–150 keV) is classically identified with a Comptonizing thermal electron population, and requires either an unusual seed photon population or a specific geometry with strong absorbing/reflecting material. The second component is modeled by a cutoff power-law, which could correspond to a second hotter Comptonizing population or another mechanism (synchrotron, non-thermal Comptonization...). In the framework of such a model, hardness–intensity and flux–flux diagrams clearly demonstrate that the source evolution follows a well-organized underlying scheme. They reveal unique information about the hard X-ray emission processes and connections between them.

Optimising a balloon-borne polarimeter in the hard X-ray domain: From the PoGOLite Pathfinder to PoGO+

PoGOLite is a balloon-borne hard X-ray polarimeter dedicated to the study of point sources. Compton scattered events are registered using an array of plastic scintillator units to determine the polarisation of incident X-rays in the energy range 20–240 keV. In 2013, a near circumpolar balloon flight of 14 days duration was completed after launch from Esrange, Sweden, resulting in a measurement of the linear polarisation of the Crab emission. Building on the experience gained from this Pathfinder flight, the polarimeter is being modified to improve performance for a second flight in 2016. Such optimisations, based on Geant4 Monte Carlo simulations, take into account the source characteristics, the instrument response and the background environment which is dominated by atmospheric neutrons. This paper describes the optimisation of the polarimeter and details the associated increase in performance. The resulting design, PoGO+, is expected to improve the Minimum Detectable Polarisation (MDP) for the Crab from 19.8% to 11.1% for a 5 day flight. Assuming the same Crab polarisation fraction as measured during the 2013 flight, this improvement in MDP will allow a 5σ constrained result. It will also allow the study of the nebula emission only (Crab off-pulse) and Cygnus X-1 if in the hard state.

FIRST INTEGRAL OBSERVATIONS OF V404 CYGNI DURING THE 2015 OUTBURST: SPECTRAL BEHAVIOR IN THE 20–650 KeV ENERGY RANGE

In June 2015, the source V404 Cygni (= GS2023+38) underwent an extraordinary outburst. We present the results obtained during the first revolution dedicated to this target by the INTEGRAL mission, and focus on the spectral behavior in the hard X-ray domain, using both SPI and IBIS instruments. The source exhibits extreme variability, and reaches fluxes of several tens of Crab. However, the emission between 20 and 650 keV can be understood in terms of two main components, varying on all the observable timescales, similar to what is observed in the persistent black hole system Cyg X-1. The low energy component (up to ~ 200 keV) presents a rather unusual shape, probably due to the intrinsic source variability. Nonetheless, a satisfactory description is obtained with a Comptonization model, if an unusually hot population of seed photons ($kT_0$ ~ 7 keV) is introduced. Above this first component, a clear excess extending up to 400-600 keV leads us to investigate a scenario where an additional (cutoff) power law could correspond to the contribution of the jet synchrotron emission, as proposed in Cyg X-1. A search for an annihilation feature did not provide any firm detection, with an upper limit of 2 x $10^{-4} ph/cm^2 s$ (2 \sigma) for a narrow line centered at 511 keV, on the averaged obtained spectrum.

ASTRO-H CdTe detectors proton irradiation at PIF

Asbstract The French Atomic Energy Commission (CEA), with the support of the European Space Agency (ESA), is partner of the Soft Gamma-Ray Detector (SGD) and the Hard X-ray Imager (HXI) onboard the 6th Japanese X-ray scientific satellite ASTRO-H (JAXA) initiated by the Institute of Space and Astronautical Science (ISAS). Both scientific instruments, one hosting a series of Compton Gamma Cameras and the other being a focal plane of a grazing incidence mirror telescope in the hard X-ray domain, are equipped with Cadmium Telluride based detectors. ASTRO-H will be operated in a Low Earth Orbit with a 31° inclination at ~550 km altitude, thus passing daily through the South Atlantic Anomaly radiation belt, a specially harsh environment where the detectors are suffering the effect of the interaction with trapped high energy protons. As CdTe detector performance might be affected by the irradiation, we investigate the effect of the accumulated proton fluence on their spectral response. To do so, we have characterized and irradiated representative samples of SGD and HXI detector under different conditions. The detectors in question, from ACRORAD, are single-pixels having a size of 2 mm by 2 mm and 750 µm thick. The Schottky contact is either made of an Indium or Aluminum for SGD and HXI respectively. We ran the irradiation test campaign at the Proton Irradiation Facility (PIF) at PSI, and ESA approved equipment to evaluate the radiation hardness of flight hardware. We simulated the proton flux expected on the sensors over the entire mission, and secondary neutrons flux due to primary proton interactions into the surrounding BGO active shielding. We eventually characterized the detector response evolution, emphasizing each detector spectral response as well as its stability by studying the so-called Polarization effect. The latter is provoking a spectral response degradation against time as a charge accumulation process occurs in Schottky type CdTe sensors. In this paper, we report on the test campaigns at PIF and will show up our experimental setup. We will pursue describing the irradiation conditions associated with our GEANT 4 predictions and finally, we report the main results of our campaigns concluding that the proton effect does not severely affect the CdTe response neither the detector stability while the secondary neutrons might be more active to reduce the performance on the long run.

A Swift view on IGR J19149+1036

IGR J19149+1036 is a high-mass X-ray binary detected by INTEGRAL in 2011 in the hard X-ray domain. We have analysed the Burst Alert Telescope (BAT) survey data of the first 103 months of the Swift mission detecting this source at a significance level of ∼30 standard deviations. The timing analysis on the long-term BAT light curve reveals the presence of a strong sinusoidal intensity modulation of 22.25 ± 0.05 d, that we interpret as the orbital period of this binary system. A broad-band (0.3–150 keV) spectral analysis was performed combining the BAT spectrum and the X-Ray Telescope (XRT) spectra from the pointed follow-up observations. The spectrum is adequately modelled with an absorbed power law with a high-energy cutoff at ∼24 keV and an absorption cyclotron feature at ∼31 keV. Correcting for the gravitational redshift, the inferred magnetic field at the neutron star surface is Bsurf ∼ 3.6 × 1012 G.

Twelve-hour spikes from the Crab Pevatron

Aims. The Crab nebula displayed a large gamma-ray flare on September 18, 2010. To more closely understand the origin of this phenomenon, we analyze the INTEGRAL (20-500 keV) and FERMI (0.1-300 GeV) data collected almost simultaneously during the flare. Methods. We divide the available data into three different sets, corresponding to the pre-flare period, the flare, and the subsequent quiescence. For each period, we perform timing and spectral analyses to differentiate between the contributions of the pulsar and from the surrounding nebula to the gamma-ray luminosity. Results. No significant variations in the pulse profile and spectral characteristics are detected in the hard X-ray domain. In contrast, we identify three separate enhancements in the gamma-ray flux lasting for about 12 hours and separated by an interval of about two days from each other. The spectral analysis shows that the flux enhancement, confined below ~ 1 GeV, can be modelled by a power-law with a high energy exponential cut-off, where either the cut-off energy or the model normalization increased by a factor of ~ 5 relative to the pre-flare emission. We also confirm that the gamma-ray flare is not pulsed. Conclusions. The timing and spectral analysis indicate that the gamma-ray flare is due to synchrotron emission from a very compact Pevatron located in the region of interaction between the pulsar wind and the surrounding nebula. These are the highest electron energies ever measured in a cosmic accelerator. The spectral properties of the flare are interpreted in the framework of a relativistically moving emitter and/or a harder emitting electron population.

ABSOLUTE TIMING OF THE CRAB PULSAR WITH THEINTEGRAL/SPI TELESCOPE

We have investigated the pulse shape evolution of the Crab pulsar emission in the hard X-ray domain of the electromagnetic spectrum. In particular, we have studied the alignment of the Crab pulsar phase profiles measured in the hard X-rays and in other wavebands. To obtain the hard X-ray pulse profiles, we have used six year (2003-2009, with a total exposure of about 4 Ms) of publicly available data of the SPI telescope on-board of the INTEGRAL observatory, folded with the pulsar time solution derived from the Jodrell Bank Crab Pulsar Monthly Ephemeris. We found that the main pulse in the hard X-ray 20-100 keV energy band is leading the radio one by $8.18\pm0.46$ milliperiods in phase, or $275\pm15 \mu s$ in time. Quoted errors represent only statistical uncertainties.Our systematic error is estimated to be $\sim 40 \mu s$ and is mainly caused by the radio measurement uncertainties. In hard X-rays, the average distance between the main pulse and interpulse on the phase plane is $0.3989\pm0.0009$. To compare our findings in hard X-rays with the soft 2-20 keV X-ray band, we have used data of quasi-simultaneous Crab observations with the PCA monitor on-board the Rossi X-Ray Timing Explorer (RXTE) mission. The time lag and the pulses separation values measured in the 3-20 keV band are $0.00933\pm0.00016$ (corresponding to $310\pm6 \mu s$) and $0.40016\pm0.00028$ parts of the cycle, respectively. While the pulse separation values measured in soft X-rays and hard X-rays agree, the time lags are statistically different. Additional analysis show that the delay between the radio and X-ray signals varies with energy in the 2 - 300 keV energy range. We explain such a behaviour as due to the superposition of two independent components responsible for the Crab pulsed emission in this energy band.

Cygnus X-3 transition from the ultrasoft to the hard state

Aims: The nature of Cygnus X-3 is still not understood well. This binary system might host a black hole or a neutron star. Recent observations by INTEGRAL have shown that Cygnus X-3 was again in an extremely ultrasoft state. Here we present our analysis of the transition from the ultrasoft state, dominated by blackbody radiation at soft X-rays plus non-thermal emission in the hard X-rays, to the low hard state. Methods: INTEGRAL observed Cyg X-3 six times during three weeks in late May and early June 2007. Data from IBIS/ISGRI and JEM-X1 were analysed to show the spectral transition. Results: During the ultrasoft state, the soft X-ray spectrum is well-described by an absorbed (NH = 1.5E22 1/cm**2) black body model, whereas the X-ray spectrum above 20 keV appears to be extremely low and hard (Gamma = 1.7). During the transition, the radio flux rises to a level of >1 Jy, and the soft X-ray emission drops by a factor of 3, while the hard X-ray emission rises by a factor of 14 and becomes steeper (up to Gamma = 4). Conclusions: The ultrasoft state apparently precedes the emission of a jet, which is apparent in the radio and hard X-ray domain.

2003–2005INTEGRALandXMM-Newtonobservations of 3C 273

The aim of this paper is to study the evolution of the broadband spectrum of one of the brightest and nearest quasars 3C 273. We analyze the data obtained during quasi-simultaneous INTEGRAL and XMM monitoring of the blazar 3C 273 in 2003--2005 in the UV, X-ray and soft gamma-ray bands and study the results in the context of the long-term evolution of the source. The 0.2-100 keV spectrum of the source is well fitted by a combination of a soft cut-off power law and a hard power law. No improvement of the fit is achieved if one replaces the soft cut-off power law by either a blackbody, or a disk reflection model. During the observation period the source has reached the historically softest state in the hard X-ray domain with a photon index $\Gamma=1.82\pm 0.01$. Comparing our data with available archived X-ray data from previous years, we find a secular evolution of the source toward softer X-ray emission (the photon index has increased by $\Delta\Gamma\simeq 0.3-0.4$ over the last thirty years). We argue that existing theoretical models have to be significantly modified to account for the observed spectral evolution of the source.

The central engine of Seyfert 2 galaxies with and without polarized broad lines

Abstract We study the hard X-ray emission of two subsamples of Seyfert 2 galaxies with and without Polarized Broad Lines (PBL hereafter). In the hard X-ray domain, absorption effects do not significantly modify the intrinsic emission allowing us a direct access to the central engine. The purpose of this study is to compare the primary emission of these two subclasses in order to investigate the nature of their central engine and to test unified models according to which they both have a hidden Sy 1 nucleus. We compute the average hard X-ray spectra of Seyfert 2 galaxies with and without PBL observed with BeppoSAX/PDS (15-136 keV). The two spectra have a common general behavior at first sight but investigating deeper we find differences in the intrinsic properties of the two categories of Seyfert 2 galaxies. Sy 2 with PBL have physical conditions close to that of Sy 1 galaxies whereas Sy 2 without PBL differ substantially, suggesting that they may have a particular place in the scheme of Seyfert galaxies.

Investigating the central engine of Seyfert 2 galaxieswithandwithoutPolarized Broad Lines

We study the hard X-ray emission of two samples of Seyfert 2 galaxies with and without Polarized Broad Lines (PBL). In the hard X-ray domain, absorption effects do not significantly modify the intrinsic emission allowing us a direct access to the central engine. The purpose of this study is to compare the primary emission of the two Seyfert 2 subclasses in order to investigate the nature of their central engine and to test unified models according to which they both have a hidden Seyfert 1 nucleus. We compute the average hard X-ray spectra of Seyfert 2 galaxies with and without PBL observed with BeppoSAX/PDS (15-136 keV). The two spectra have a common general behavior at first sight, but investigating deeper we find differences in the intrinsic properties of the two categories of Seyfert 2 galaxies. Sy 2 with polarized broad lines have physical conditions close to those of Sy 1 galaxies whereas Sy 2 without PBL differ substantially, suggesting that they may have a particular place in the scheme of Seyfert galaxies.

Optical holography in the hard X-ray domain

Present status of our developments of x-ray holographic microscopes at SPring-8 BL20XU is described. A combination of the x-ray undulator and a zone plate enabled us to make a coherent x-ray source of around 0.1 μm size. Using this secondary source, two types of x-ray holographic microscopes were investigated. First, a Gabor microscope in point-projection geometry was tested. A tantalum 0.2 μm line-and-space pattern could be resolved. Second, using a zone plate as a beam splitter, a Fourier transform holographic microscope was tested. A tantalum 0.2 μm line-and-space pattern could be observed. Polystyrene beads of 2.8 μmand 0.8 μm in diameter could be observed. In Fourier transform holography, a reconstructed image of a specimen that is located out of the plane of the reference source is blurred. Numerical focusing of such an x-ray hologram could be successfully demonstrated.

Classical theory of resonant transition radiation in multilayer structures

A rigorous classical electromagnetic theory of the transition radiation in finite and infinite multilayer structures is presented. It makes the standard results of thin-film optics, such as the matrix formalism, accountable; it allows thus an exact treatment of the propagation of the waves induced by the electron. This method is applied to the particular case of the periodic structures to treat the resonant transition radiation (RTR). It is noted that the present theory gives, in the hard x-ray domain, results previously published. The reason for this approach is to make the numerical calculations rigorous and easy. The numerical results of our theory are compared to experimental RTR data obtained recently by Yamada et al. [Phys. Rev. A 59, 3673 (1999)] with a nickel-carbon multilayer structure.

Multiwavelength Observation of a New Black Hole Candidate: EXS 1737.9−2952

We report a multiwavelength analysis of an unusual high-energy transient: EXS 1737.9-2952. Due to the features this source exhibited in the hard X-ray domain similar to another source in the Galactic center region (1E1740.9 - 2952), and in order to study the molecular gas toward this X-ray source, we performed an observation of the EXS region in 1993 August, using the Swedish-ESO Submillimeter Telescope (SEST) located in La Silla (Chilean Andes). We observed a cloud, at the forbidden velocity of 135 km s-1, using 12CO (1-0) transitions, giving a maximum column density of 8 × 1021 cm-2. In 1994 we conducted other observations to search for higher density regions inside the cloud, using HCO+ and CS lines, but they were unsuccessful: we concluded that the cloud could be associated with the X-ray source and its mean density is of the order of ≈ 103 cm-3. In 1994 April, we performed a set of observations of the field containing EXS, at 20 cm and 6 cm, using the VLA in its A configuration, and found four possible radio candidates for an association with the EXS X-ray source, one of them (source 3) being extended, exhibiting a head-tail morphology, and a having a thermal spectrum with a spectral index ≈ -0.7. We reproduced our observation in 1994 November and December, using the C configuration at 6 cm, in order to investigate on possible variability and extension of these sources and found a marginal indication in the 20 cm image that source 3 may have a weak second component displaced about 15''. Nevertheless, this indication is too faint to associate this source definitely with EXS since no significant variation was detected. In addition, during the 1994 November-December observation, two more extended sources were detected but their association with EXS is unlikely. We also analyzed the Einstein/IPC image of the 5 σ EXS error box which does not exhibit, at the time of the observation, any significant low-energy X-ray counterpart to EXS. A nearby pulsar PSR 1737-30 in the ROSAT catalog is outside this error box. Finally, IRAS maps of the EXS region do not show any IR contribution at the location of the radio sources. We conclude that (1) EXS 1737.9-2952 is a high-energy transient, (2) a persistent counterpart at other wavelengths is not demonstrated, and (3) EXS, when flaring, as well as other GC gamma-ray sources, could possibly contribute to the 511 keV bulge emission.

Modeling the Continuum and Line Spectra of the Seyfert 2 Galaxy NGC 5252

The S0 galaxy NGC 5252 is one of the best examples of the anisotropic radiation theories believed to govern the physics of active galactic nuclei. Considerable amounts of observational data, from the radio to the hard X-ray domain, have been accumulated from the central nuclear region of this galaxy. This work makes use of all the available observational information in an attempt to model in a self-consistent manner the continuum and line emission spectra of the nuclear region and the extended emission-line region. It is suggested that both the nuclear continuum and the nuclear line emission originate in gaseous clouds that are exposed to the nuclear ionizing radiation and subjected to shocks. The best fit to the data in the nuclear region is obtained with a weighted-average model that includes radiation-dominated clouds with low velocities (100 km s-1), producing most of the emission lines and the low-frequency continuum, and high-velocity (~400 km s-1) shock-dominated clouds, where the soft X-ray continuum originates. The far-infrared emission is due to dust heated by both the nuclear radiation and shocks. Modeling of the extended emission-line region was performed on the basis of the derived nuclear model. That region was found to be characterized by densities of a few cm-3 and a cloud size of about 100 pc. Excitation via shocks is found negligible, whereas photoionization by the central power source widely accounts for the observed line spectrum.

Microwave, Soft and Hard X-Ray Observations of Solar Flares – a Self-Consistent Model of the Flare Site

High-resolution microwave observations of several flares performed with the Westerbork Synthesis Radio Telescope (WRST) on 3 and 4 July 1993 are compared with Yohkoh observations in the soft and hard X-ray domain. Only for one flare, among the six analyzed, was the hard X-ray spectrum between 20 and 200 keV available from the Wide Bragg Spectrometer, supplying the energy spectrum of non-thermal particles responsible for this radiation and for the radio emission. A complete model of this flare is derived which accounts for all available observations in the X-ray and radio wavelengths.