Soft X-ray Component Research Articles

An additional soft X-ray component in the dim low/hard state of black hole binaries

We test the truncated disc models using multiwavelength (optical/ultraviolet/X-ray) data from the 2005 hard state outburst of the black hole Swift J1753.5−0127. This system is both fairly bright and has fairly low interstellar absorption, so gives one of the best data sets to study the weak, cool disc emission in this state. We fit these data using models of an X-ray illuminated disc to constrain the inner disc radius throughout the outburst. Close to the peak, the observed soft X-ray component is consistent with being produced by the inner disc, with its intrinsic emission enhanced in temperature and luminosity by reprocessing of hard X-ray illumination in an overlap region between the disc and corona. This disc emission provides the seed photons for Compton scattering to produce the hard X-ray spectrum, and these hard X-rays also illuminate the outer disc, producing the optical emission by reprocessing. However, the situation is very different as the outburst declines. The optical is probably cyclo-synchrotron radiation, self-generated by the flow, rather than tracing the outer disc. Similarly, limits from reprocessing make it unlikely that the soft X-rays are directly tracing the inner disc radius. Instead they appear to be from a new component. This is seen more clearly in a similarly dim low/hard state spectrum from XTE J1118+480, where the 10 times lower interstellar absorption allows a correspondingly better view of the ultraviolet/extreme ultraviolet (EUV) emission. The very small emitting area implied by the relatively high temperature soft X-ray component is completely inconsistent with the much larger, cooler, ultraviolet component which is well fit by a truncated disc. We speculate on the origin of this component, but its existence as a clearly separate spectral component from the truncated disc in XTE J1118+480 shows that it does not simply trace the inner disc radius, so cannot constrain the truncated disc models.

Beginning of the super‐soft phase of the classical nova V2491 Cygni

AbstractWe present the results of soft X‐ray studies of the classical nova V2491 Cygni using the Suzaku observatory. On day 29 after outburst, a soft X‐ray component with a peak at ∼0.5 keV has appeared, which is tantalising evidence for the beginning of the super‐soft X‐ray emission phase. We show that an absorbed blackbody model can describe the observed spectra, yielding a temperature of 57 eV, neutral hydrogen column density of 2 × 1021 cm–2, and a bolometric luminosity of ∼1036 erg s–1. However, at the same time, we also found a good fit with an absorbed thin‐thermal plasma model, yielding a temperature of 0.1 keV, neutral hydrogen column density of 4 × 1021 cm–2, and a volume emission measure of ∼1058 cm–3. Owing to low spectral resolution and low signal‐to‐noise ratio below 0.6 keV, the statistical parameter uncertainties are large, but the ambiguity of the two very different models demonstrates that the systematic errors are the main point of concern. The thin‐thermal plasma model implies that the soft emission originates from optically thin ejecta, while the blackbody model suggests that we are seeing optically thick emission from the white dwarf (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Scintillation fibers and nanoscintillators for improving the spatial, spectrometric, and time resolution of radiation detectors

Experimental examples demonstrate a significant improvement of the most important parameters (sensitivity; spatial, spectrometric, and time resolution; and radiation hardness) of radiation detectors with sensitive elements based on fiber and nanocrystalline scintillators instead of conventional bulk single crystals. This improvement is related to several specific features of the new scintillator types: improved homogeneity of activator distribution, larger soft X-ray component in the secondary emission due to the high frequency of hot electron collisions with the nanoparticle surface, formation of high-quality optical nanocavities, and enhanced annihilation of radiation defects at their rapid motion to the surface.

The active nuclei ofz< 1.0 3CRR radio sources

We combine Chandra and XMM-Newton X-ray data from our previous papers with new X-ray observations and with Spitzer mid-infrared data in order to study the nature of the nuclei of radio galaxies and radio-loud quasars with z<1.0 from the 3CRR sample. The significant increase in sample size over our previous work, the reduction of bias in the sample as a result of new observations, and the availability of more mid-infrared data allow us to show conclusively that almost all objects classed as low-excitation radio galaxies in optical spectroscopic studies lack a radiatively efficient active nucleus. We show that the distribution of absorbing columns in the narrow-line radio galaxies differs from the population of X-ray-selected radio-quiet type-2 quasars and from that in local Seyfert 2s. We comment on the current evidence for the nature of the soft X-ray component in radio-galaxy nuclear spectra, concluding that a jet origin for this component is very hard to evade. Finally, we discuss the recently discovered `fundamental plane' of black hole activity, showing that care must be taken when placing radio-loud AGN on such diagnostic diagrams.

2XMMi J225036.9+573154 - a new eclipsing AM Her binary discovered usingXMM-Newton

We report the discovery of an eclipsing polar, 2XMMi J225036.9+573154, using XMM-Newton. It was discovered by searching the light curves in the 2XMMi catalogue for objects showing X-ray variability. Its X-ray light curve shows a total eclipse of the white dwarf by the secondary star every 174 mins. An extended pre-eclipse absorption dip is observed in soft X-rays at phi=0.8-0.9, with evidence for a further dip in the soft X-ray light curve at phi~0.4. Further, X-rays are seen from all orbital phases (apart from the eclipse) which makes it unusual amongst eclipsing polars. We have identified the optical counterpart, which is faint (r=21), and shows a deep eclipse (>3.5 mag in white light). Its X-ray spectrum does not show a distinct soft X-ray component which is seen in many, but not all, polars. Its optical spectrum shows Halpha in emission for a fraction of the orbital period.

SWIFTX-RAY AND ULTRAVIOLET MONITORING OF THE CLASSICAL NOVA V458 VUL (NOVA VUL 2007)

We describe the highly variable X-ray and UV emission of V458 Vul (Nova Vul 2007), observed by Swift between 1 and 422 days after outburst. Initially bright only in the UV, V458 Vul became a variable hard X-ray source due to optically thin thermal emission at kT = 0.64 keV with an X-ray band unabsorbed luminosity of 2.3 × 1034 erg s–1 during days 71-140. The X-ray spectrum at this time requires a low Fe abundance (0.2+0.3 –0.1 solar), consistent with a Suzaku measurement around the same time. On day 315 we find a new X-ray spectral component which can be described by a blackbody with temperature of kT = 23+9 –5 eV, while the previous hard X-ray component has declined by a factor of 3.8. The spectrum of this soft X-ray component resembles those typically seen in the class of supersoft sources (SSS) which suggests that the nova ejecta were starting to clear and/or that the white dwarf photosphere is shrinking to the point at which its thermal emission reaches into the X-ray band. We find a high degree of variability in the soft component with a flare rising by an order of magnitude in count rate in 0.2 days. In the following observations on days 342.4-383.6, the soft component was not seen, only to emerge again on day 397. The hard component continued to evolve, and we found an anticorrelation between the hard X-ray emission and the UV emission, yielding a Spearman rank probability of 97%. After day 397, the hard component was still present, was variable, and continued to fade at an extremely slow rate but could not be analyzed owing to pile-up contamination from the bright SSS component.

Circular polarization survey of intermediate polars I. Northern targets in the range 17 h &lt; RA &lt; 23 h

Context. The origin, evolution, and ultimate fate of magnetic cataclysmic variables are poorly understood. It is largely the nature of the magnetic fields in these systems that leads to this poor understanding. Fundamental properties, such as the field strength and the axis alignment, are unknown in a majority of these systems. Aims. We undertake to put all the previous circular polarization measurements into context and systematically survey intermediate polars for signs of circular polarization, hence to get an indication of their true magnetic field strengths and try to understand the evolution of magnetic cataclysmic variables. Methods. We used the TurPol instrument at the Nordic Optical Telescope to obtain simultaneous UBVRIphoto-polarimetric observations of a set of intermediate polars, during the epoch 2006 July 31–August 2. Results. Of this set of eight systems two (1RXS J213344.1+510725 and 1RXS J173021.5–055933) were found to show significant levels of circular polarization, varying with spin phase. Five others (V2306 Cyg, AO Psc, DQ Her, FO Aqr, and V1223 Sgr) show some evidence for circular polarization and variation of this with spin phase, whilst AE Aqr shows little evidence for polarized emission. We also report the first simultaneous UBVRI photometry of the newly identified intermediate polar 1RXS J173021.5–055933. Conclusions. Circular polarization may be ubiquitous in intermediate polars, albeit at a low level of one or two percent or less. It is stronger at longer wavelengths in the visible spectrum. Our results lend further support to the possible link between the presence of soft X-ray components and the detectability of circular polarization in intermediate polars.

Revealing the fastest component of the DG Tauri outflow through X-rays

Context. Some T Tauri stars show a peculiar X-ray spectrum that can be modelled by two components with different absorbing column densities. Aims. We seek to explain the soft X-ray component in DG Tau, the best studied of these sources, with an outflow model, taking observations at other wavelengths into consideration. Methods. We constrain the outflow properties through spectral fitting and employ simple semi-analytical formulae to describe properties of a shock wave that heats up the X-ray emitting region. Results. The X-ray emission is consistent with its arising from the fastest and innermost component of the optically detected outflow. Only a small fraction of the total mass loss is required for this X-ray emitting component. Our favoured model requires shock velocities between 400 and 500 km s -1 . For a density >10 5 cm -3 all dimensions of the shock cooling zone are only a few AU, so even in optical observations this cannot be resolved. Conclusions. This X-ray emission mechanism in outflows may also operate in other, less absorbed T Tauri stars, in addition to corona and accretion spots.

Two new intermediate polars with a soft X-ray component

Aims. We analyze the first X-ray observations with XMM-Newton of 1RXS J070407.9+262501 and 1RXS 180340.0+401214, in order to characterize their broad-band temporal and spectral properties, also in the UV/optical domain, and to confirm them as intermediate polars. Methods. For both objects, we performed a timing analysis of the X-ray and UV/optical light curves to detect the white dwarf spin pulsations and study their energy dependence. For 1RXS 180340.0+401214 we also analyzed optical spectroscopic data to determine the orbital period. X-ray spectra were analyzed in the 0.2–10.0 keV range to characterize the emission properties of both sources. Results. We find that the X-ray light curves of both systems are energy dependent and are dominated, below 3–5 keV, by strong pulsations at the white dwarf rotational periods (480 s for 1RXS J070407.9+262501 and 1520.5 s for 1RXS 180340.0+401214). In 1RXS 180340.0+401214 we also detect an X-ray beat variability at 1697 s which, together with our new optical spectroscopy, favours an orbital period of 4.4 h that is longer than previously estimated. Both systems show complex spectra with a hard (temperature up to 40 keV) optically thin and a soft (kT ∼ 85–100 eV) optically thick components heavily absorbed by material partially covering the X-ray sources. Conclusions. Our observations confirm the two systems as intermediate polars and also add them as new members of the growing group of “soft” systems which show the presence of a soft X-ray blackbody component. Differences in the temperatures of the blackbodies are qualitatively explained in terms of reprocessing over different sizes of the white dwarf spot. We suggest that systems showing cooler soft X-ray blackbody components also possess white dwarfs irradiated by cyclotron radiation.

The nature of the soft X-ray source in DG Tauri

The classical T Tauri star DG Tau shows all typical signatures of X-ray activity and, in particular, harbors a resolved X-ray jet. DG Tau’s jet is one of the most well studied jets of young stellar objects, having been observed for more than 25 years by a variety of instruments. We demonstrate that its soft and hard X-ray components are separated spatially by approximately 0.2 arcsec by deriving the spatial offset between both components from the event centroids of the soft and hard photons utilizing the intrinsic energyresolution of the Chandra ACIS-S detector. We also demonstrate that this offset is physical and cannot be attributed to an instrumental origin or to low counting statistics. Furthermore, the location of the derived soft X-ray emission peak coincides with emission peaks observed for optical emission lines, suggesting that both soft X-rays and optical emission have the same physical origin.

Soft X-ray components in the hard state of accreting black holes

Recent observations of two black hole candidates (GX 339-4 and J1753.5-0127) in the low-hard state ($L_{\rm{X}}/L_{\rm{Edd}}$ $\simeq$ 0.003-0.05) suggest the presence of a cool accretion disk very close to the innermost stable orbit of the black hole. This runs counter to models of the low-hard state in which the cool disk is truncated at a much larger radius. We study the interaction between a moderately truncated disk and a hot inner flow. Ion-bombardment heats the surface of the disk in the overlap region between a two-temperature advection-dominated accretion flow and a standard accretion disk, producing a hot ($kT_{\rm{e}}$ $\simeq$ 70 keV) layer on the surface of the cool disk. The hard X-ray flux from this layer heats the inner parts of the underlying cool disk, producing a soft X-ray excess. Together with interstellar absorption these effects mimic the thermal spectrum from a disk extending to the last stable orbit. The results show that soft excesses in the low-hard state are a natural feature of truncated disk models.

The X-ray variability of Mrk 421

The results of monitoring the flaring activity of Mrk 421 with the XMM-Newton and RXTE orbiting observatories are presented. The ASM/RXTE data show that, during most of the observational period, 2002–2006, the source was in a very active state, with a mean 1.5–12 keV flux equals ∼17 mCrab. Analysis of daily and weekly averaged data shows strong X-ray variability of Mrk 421 on time scales from days to months. The long-term light curve demonstrates a rich variety of variability patterns and a complex hierarchical flare structure, in most cases superimposed on longer trends of rising and decreasing source activity. Quantitative estimates of the variability of Mrk 421 shows evidence of correlations between the source intensity in X-rays and the variability parameters, both for separate years and epochs and for most of the observational period. The soft and hard X-ray components (according to XMM-Newton data) are correlated with each other, and spectral changes (according to ASM/RXTE data) are characterized by increasing hardness with brightening of the object in X-rays. Moderate spectral variability at individual epochs and evidence for an energy dependence of the source variability are also observed. The 2–12 keV fluxes from Mrk 421 detected by XMM-Newton are consistent with the maximum 2–10 keV flux detected by ASCA ∼10 years ago. An analysis of simultaneous day-time-scale variability of Mrk 421 in X-rays (ASM/RXTE) and very-high-energy γ-rays (GT-48 Cherenkov telescope of the Crimean Astrophysical Observatory) in periods of increased activity of the source in December 2002 and April 2004 is also presented.

XMM‐NewtonObservations of the Red Type 1.8 Quasar 2M1049+5837: Reflection from Cold and Warm (Ionized) Matter

Chandra observations of the type 1.8, low-redshift (z = 0.115), red (J − KS ~ 2) quasar 2MASSJ 104943+583750 (2M1049+5837) indicated an unusually hard X-ray spectrum (HR ~ 0.6), suggesting obscuration of NH ~ 4 × 1022 cm−2 and a flat Γ ~ 0.4 power-law slope. A higher signal-to-noise ratio XMM-Newton observation reported here reveals a more complex spectrum, being extremely hard above ~2 keV, Γ ~ − 0.6, with a well-defined soft excess similar to the bright, nearby Seyfert 2 galaxy Mrk 3. Such extremely hard quasar spectra cannot be a dominant contributor to the cosmic X-ray background (CXRB; Γ ~ 1.4). Modeling of 2M1049+5837 shows the observed 2-10 keV spectrum to be the sum of a strongly absorbed normal Γ ~ 1.8 underlying quasar X-ray continuum, and a cold reflection component (R ~ 2). The strong attenuation of the intrinsic X-ray continuum by a sub-Compton-thick line-of-sight column, NH ~ 3.4 × 1023 cm−2, reveals a soft X-ray emission component whose spectrum indicates reprocessing/emission from, possibly extended, photoionized gas. The luminosity of the soft X-ray component is similar to Seyfert 2 galaxies, an order of magnitude less than comparable type 1 AGNs, suggesting partial obscuration of core-bright emission. The optical emission is complex, with distinct red and blue scattered light components believed to originate in two extended regions visible in HST imaging data. The unusual combination of properties: optical and X-ray obscuration of the nuclear emission, partially obscured broad but unobscured narrow emission lines, and partially obscured soft X-ray excess, implies an intermediate viewing angle, over or through the edge of an obscuring disk + wind or torus. 2M1049+5837 again demonstrates both the ambiguity of low signal-to-noise X-ray spectra/hardness ratios and the potential of 2MASS-selected red sources for exploring the complex obscuration and geometry of the nuclear regions in AGNs.

The changing accretion states of the intermediate polar MU Camelopardalis

Aims. We study the timing and spectral properties of the intermediate polar MU Camelopardalis (1RXS J062518.2+733433) to determine the accretion modes and the accretion geometry from multi-wavelength, multi-epoch observational data. Methods. Light curves in different observed energy ranges (optical, UV, X-ray) are extracted. The timescales of variability in these light curves are determined using Analysis of Variance. Phase-resolved X-ray spectra are created with respect to the most prominent detected periodicities and each fitted with an identical model, to quantify the differences in the fitted components. Results. The published tentative value for the spin period is unambiguously identified with the rotation period of the white dwarf. We detect a distinct soft X-ray component that can be reproduced well by a black body. The analysis of data obtained at different epochs demonstrates that the system is changing its accretion geometry from disk-dominated to a combination of disk- plus stream-dominated, accompanied with a significant change in brightness at optical wavelengths.

XMM-Newton observations of the eclipsing polar V2301 Oph

We present XMM-Newton observations of the eclipsing polar V2301 Oph which cover nearly 2.5 binary orbital cycles and 2 eclipses. This polar is believed to have the lowest magnetic field strength (7 MG) of any known polar. We find evidence for structure in the X-ray eclipse profile which shows a `standstill' feature lasting 26+/-4 sec. This allows us to place an upper limit on the mass of the white dwarf of ~1.2 Msun. We find no evidence for QPOs in the frequency range 0.02-10 Hz. This coupled with the absence of QPOs in RXTE data suggest that, if present, any oscillations in the shock front have a minimal effect on the resultant X-ray flux. We find no evidence for a distinct soft X-ray component in its spectrum - it therefore joins another 7 systems which do not show this component. We suggest that those systems which are asynchronous, have low mass transfer rates, or have accretion occurring over a relatively large fraction of the white dwarf are more likely to show this effect. We find that the specific mass transfer rate has to be close to 0.1 g cm^-2 s^-1 to predict masses which are consistent with that derived from our eclipse analysis. This maybe due to the fact that the low magnetic field strength allows accretion to take place along a wide range of azimuth.

The XMM-SSC survey of hard-spectrum XMM–Newton sources – I. Optically bright sources

We present optical and X-ray data for a sample of serendipitous XMM-Newton sources that are selected to have 0.5-2 versus 2-4.5 keV X-ray hardness ratios which are harder than the X-ray background. The sources have 2-4.5 keV X-ray flux , and in this paper we examine a subsample of 42 optically bright (r sources; this subsample is 100 per cent spectroscopically identified. All but one of the optical counterparts are extragalactic, and we argue that the single exception, a Galactic M star, is probably a coincidental association rather than the correct identification of the X-ray source. The X-ray spectra of all the sources are consistent with heavily absorbed power laws (21.8 10(44) erg s(-1), and two of these sources have optical spectra which are dominated by narrow emission lines, that is, are type 2 QSOs. Only a small fraction of the sources (7/42) show broad optical emission lines, and all of these have N(H) 100 times the Galactic gas-to-dust ratio are rare in AGN absorbers (at most a few per cent of the population), and may be restricted to broad absorption line QSOs. Seven objects appear to have an additional soft X-ray component in addition to the heavily absorbed power law; all seven are narrow emission-line objects with z 4 per cent of broad-line AGN (BLAGN) have ionized absorbers that attenuate their soft X-ray flux by > 50 per cent. In at least one of the X-ray-absorbed BLAGN in our sample the X-ray spectrum requires an ionized absorber, consistent with this picture.

The Hαsurges and EUV jets from magnetic flux emergences and cancellations

We analyzed multi-wavelength observations of three surges with a recurrent period of about 70 min in Ha, EUV, and soft X-ray, which occurred in the quiet-sun region on 2000 November 3. These homologous surges were associated with small flares at the same base, but their exact footpoints were spatially separated from the flare. Each surge consisted of a cool Ha component and a hot, EUV or soft X-ray component, which showed different evolutions not only in space but also in time. The EUV jets had slightly converging shapes, underwent more complicate development, showed clearly twisting structures, and appeared to open to space. The Ha surges, however, were smaller and only traced the edges of the jets. They always occurred later than the jets but had dark EUV counterparts appearing in the bright jets. These surge activities were closely associated with two emerging bipoles and their driven flux cancellations at the base region, and were consistent with the magnetic reconnection surge model. The possible cause of the delay between the surges and jets, of the dark structures in the jets are discussed, along with the possible role of flux cancellations in generation of these surges.

On the Early‐Time X‐Ray Spectra ofSwiftAfterglows. I. Evidence for Anomalous Soft X‐Ray Emission

We have conducted a thorough and blind search for emission lines in >70 Swift X-ray afterglows of total exposure ~10^7s. We find that most afterglows are consistent with pure power-laws plus extinction. Significant outliers to the population exist at the 5-10% level and have anomalously soft, possibly thermal spectra. Four bursts are singled out via possible detections of 2-5 lines: GRBs 060218, 060202, 050822, and 050714B. Alternatively, a blackbody model with kT~0.1-0.5 keV can describe the soft emission in each afterglow. The most significant soft component detections in the full data set of ~2000 spectra correspond to GRB060218/SN2006aj, with line significances ranging up to \~20-sigma. A thermal plasma model fit to the data indicates that the flux is primarily due to L-shell transitions of Fe at ~ solar abundance. We associate (>4-sigma significant) line triggers in the 3 other events with K-shell transitions in light metals. We favor a model where the possible line emission in these afterglows arises from the mildly relativistic cocoon of matter surrounding the GRB jet as it penetrates and exits the surface of the progenitor star. The emitting material in each burst is at a similar distance \~10^12--10^13 cm, a similar density ~10^17 cm^-3, and subject to a similar flux of ionizing radiation. The lines may correlate with the X-ray flaring. For the blackbody interpretation, the soft flux may arise from break out of the GRB shock or plasma cocoon from the progenitor stellar wind, as recently suggested for GRB060218 (Campana et al. 2006). Due to the low z of GRB060218, bursts faint in Gamma-rays with fluxes dominated by this soft X-ray component could outnumber classical GRBs 100-1.

XMM-Newton observation of Mrk 110

Context. We report on the first XMM-Newton observation of the bright narrow-line Seyfert 1 galaxy Mrk 110. Aims. Our analysis is aimed to study the properties of the X-ray spectrum of Mrk 110 and compare them with those inferred from optical spectroscopy. Methods. We make use of detailed timing and spectral analysis as well as high resolution X-ray spectroscopy with the XMM-Newton gratings. Results. We find a narrow Fe K fluorescent line, a broad component ($FWHM\simeq$16 500 km s -1 ) of the O VII triplet, either due to infall motions or gravitational redshift effects in the vicinity of the central black hole, a Comptonized accretion disk layer, and a strong starburst component. Conclusions. We found that Mrk 110 has a complex X-ray spectrum, exhibiting relatively strong broadening of the O VII emission line, probably associated with X-ray emission from the Broad Line Region (BLR), which might be correlated with the optical gravitationally redshifted, asymmetric line profiles. Spectral fits including a Gaussian line or a discline give the same statistical significance. If the broad redshifted soft X-ray components are due to gravitational redshift effects, the distance of the line-emitting regions ranges between about 0.2 and 1 light day with respect to the central black hole. In addition, the EPIC pn spectrum shows a double power-law and a strong starburst component. One power-law component exhibits a photon index slope of $\rm 1.40^{1.50}_{1.38}$, while the second is much steeper with a power law slope of $\rm 2.50^{2.63}_{2.48}$. The second power-law is most probably due to thermal Comptonization of a hot electron layer above the accretion disk. Mrk 110 is another example of extragalactic sources showing Comptonization effects in the accretion disc and its properties are very similar to the narrow-line Seyfert 1 Galaxy Ton S 180.

A new interpretation of the remarkable X-ray spectrum of the symbiotic star CH Cyg

We have reanalysed the ASCA X-ray spectrum of the bright symbiotic star CH Cyg, which exhibits apparently distinct hard and soft X-ray components. Our analysis demonstrates that the soft X-ray emission can be interpreted as scattering of the hard X-ray component in a photo-ionised medium surrounding the white dwarf. This is in contrast to previous analyses in which the soft X-ray emission was fitted separately and assumed to arise independently of the hard X-ray component. We note the striking similarity between the X-ray spectra of CH Cyg and Seyfert 2 galaxies, which are also believed to exhibit scattering in a photo-ionised medium.