Additional Soft Component Research Articles

Broad-Band X-Ray Analysis of NGC 3227

Abstract Previously in Project A we carried out analysis of the data obtained by observations of the 1.5 Seyfert galaxy NGC 3227—the two observations performed by XMM-Newton in 2000 and 2006 and six observations by Suzaku in 2008. In the current paper, Project B, our work was extended to joint XMM-Newton and NuSTAR observations in 2016 as well as a NuSTAR observation in 2017. In Project A a unified model was constructed. The model consists of a hard power law ($\Gamma _{\mathrm{ Hard}}=1.4{\!-\!}1.7$) which is interpreted as the Comptonized emission from the corona above an accretion disk. In the high flux states an additional soft excess component dominates, which is consistent with either a power law ($\Gamma _{\mathrm{ Soft}}=3.3{\!-\!}3.85$) or warm Comptonization. These emissions are absorbed by a partially covering material and warm absorbers. A reflection component and several emission lines are also present. In Project B, the model presented in Project A was applied to more recent observations. It was found that this model remains valid. Our conclusion is that this model is consistent, valid with the long observations from 2000 to 2017 and over the broad frequency band from 0.3 keV to 50 keV.

Obscuring Environment and X-Ray Variability of Compact Symmetric Objects Unveiled with XMM-Newton and NuSTAR

Compact symmetric objects (CSOs) show radio features such as jets, lobes, and hot spots, which are contained within their host galaxies, and likely represent a recent radio activity. A subpopulation of CSOs with high intrinsic X-ray column densities has been inferred from shallow, soft X-ray band exposures, and observed to cluster in the linear radio size versus 5 GHz radio power plane, which suggests that a dense circumnuclear medium may dramatically influence the growth of compact radio structures. Here, we report on the first detection of two CSOs, 2021+614 and J1511+0518, at energies above 10 keV with NuSTAR. We model the NuSTAR data jointly with the new XMM-Newton data of J1511+0518, and with the archival XMM-Newton data of 2021+614. A toroidal reprocessor model fits the data well and allows us to robustly confirm the X-ray properties of the CSO absorbers and continuum. In both sources, we find intrinsic X-ray absorbing column densities in excess of 1023 cm−2, hard photon indices of the primary emission, Γ ∼ 1.4–1.7, Fe Kα line emission, and variability of the intrinsic X-ray flux density on the timescale of years. The studied X-ray continua are dominated by the primary power-law emission at energies above 3 keV, and by the scattered component at energies below 3 keV. An additional soft X-ray component, modeled with a hot, collisionally ionized plasma with temperature kT ∼ 1 keV, is required by the XMM-Newton data in J1511+0518, which is corroborated by the tentative evidence for the extension in the archival Chandra image of the source.

Investigating the Accretion Nature of Binary Supermassive Black Hole Candidate SDSS J025214.67−002813.7

Abstract We present results of a multiwavelength analysis of SDSS J025214.67−002813.7, a system that has been previously classified as a binary active galactic nucleus (AGN) candidate based on periodic signals detected in the optical light curves. We use available radio−X-ray observations of the system to investigate the true accretion nature. Analyzing new observations from XMM-Newton and NuSTAR, we characterize the X-ray emission and search for evidence of circumbinary accretion. Although the 0.5–10 keV spectrum shows evidence of an additional soft emission component, possibly due to extended emission from hot nuclear gas, we find the spectral shape is consistent with that of a single AGN. Compiling a full multiwavelength spectral energy distribution (SED), we also search for signs of circumbinary accretion, such as a “notch” in the continuum due to the presence of minidisks. We find that the radio–optical emission agrees with the SED of a standard, radio-quiet, AGN; however, there is a large deficit in emission blueward of ∼1400 Å. Although this deficit in emission can plausibly be attributed to a binary AGN system, we find that the SED of SDSS J0252−0028 is better explained by emission from a reddened, single AGN. However, future studies of the expected hard X-ray emission associated with binary AGNs (especially in the unequal-mass regime) will allow for more rigorous analyses of the binary AGN hypothesis.

Understanding the interaction of soft and hard magnetic components in NdFeB with first-order reversal curves

First-order reversal curve (FORC) measurements are a powerful tool to study magnetization reversal processes and interactions in heterogeneous systems with broad coercivity distributions. In NdFeB hard magnets an additional soft magnetic component is often observed possibly originating from damaged surface grains. Here we use FORC to study the reversal processes and interactions in these permanent magnets at different temperatures between 50 and 350 K. The measured reversal curves reveal a strongly coupled switching of the soft and hard magnetic components above 250 K. Below this temperature the two components are decoupled and switch almost independently. This decrease in effective interaction at lower temperatures is also observed in the FORC diagrams by a relative reduction in intensity of the so called interaction peak. This result proves that FORC is a powerful method, contributing to a better understanding of magnetization reversal processes and interactions in permanent magnets.

Combined Analysis of X-Ray Spectra of NGC 3227

Abstract The 1.5 Seyfert galaxy NGC 3227 has been observed by several X-ray missions. We carried out a combined analysis of the data obtained by more recent major observations of this source—two observations performed by XMM-Newton in 2000 and 2006 and six observations performed by Suzaku in 2008. A unified model, which is consistent with all eight of the observations by the two satellites with large intensity and spectral changes, was constructed. The model consists of a hard power law with the spectral index of ΓHard = 1.4–1.7, which is interpreted as the Comptonized emission from the corona above an accretion disk. In the high-flux states, an additional soft excess component dominates, which is consistent with a model with either a steeper power law with ΓSoft = 3.3–3.85 or the warm Comptonization component. These emissions from the central engine are absorbed by a partial covering material and warm absorbers. A reflection component and several emission lines are also present. We examined the relationship between the intrinsic luminosity and the absorbers’ physical parameters such as the column density, which suggests that the source expanded significantly during the bright states when the soft excess is greatly enhanced.

Extreme quiescent variability of the transient neutron star low-mass X-ray binary EXO 1745−248 in Terzan 5

EXO 1745-248 is a transient neutron-star low-mass X-ray binary that resides in the globular cluster Terzan 5. We studied the transient during its quiescent state using 18 Chandra observations of the cluster acquired between 2003 and 2016. We found an extremely variable source, with a luminosity variation in the 0.5-10 keV energy range of $\sim3$ orders of magnitude (between $3\times10^{31}$ erg s$^{-1}$ and $2\times10^{34}$ erg s$^{-1}$) on timescales from years down to only a few days. Using an absorbed power-law model to fit its quiescent spectra, we obtained a typical photon index of $\sim1.4$, indicating that the source is even harder than during outburst and much harder than typical quiescent neutron stars if their quiescent X-ray spectra are also described by a single power-law model. This indicates that EXO 1745-248 is very hard throughout the entire observed X-ray luminosity range. At the highest luminosity, the spectrum fits better when an additional (soft) component is added to the model. All these quiescent properties are likely related to strong variability in the low-level accretion rate in the system. However, its extreme variable behavior is strikingly different from the one observed for other neutron star transients that are thought to still accrete in quiescence. We compare our results to these systems. We also discuss similarities and differences between our target and the transitional millisecond pulsar IGR J18245-2452, which also has hard spectra and strong variability during quiescence.

TWO DISTINCT-ABSORPTION X-RAY COMPONENTS FROM TYPE IIn SUPERNOVAE: EVIDENCE FOR ASPHERICITY IN THE CIRCUMSTELLAR MEDIUM

ABSTRACT We present multi-epoch X-ray spectral observations of three Type IIn supernovae (SNe), SN 2005kd, SN 2006jd, and SN 2010jl, acquired with Chandra, XMM-Newton, Suzaku, and Swift. Previous extensive X-ray studies of SN 2010jl have revealed that X-ray spectra are dominated by thermal emission, which likely arises from a hot plasma heated by a forward shock propagating into a massive circumstellar medium (CSM). Interestingly, an additional soft X-ray component was required to reproduce the spectra at a period of ∼1–2 years after the SN explosion. Although this component is likely associated with the SN, its origin remained an open question. We find a similar, additional soft X-ray component from the other two SNe IIn as well. Given this finding, we present a new interpretation for the origin of this component; it is thermal emission from a forward shock essentially identical to the hard X-ray component, but directly reaches us from a void of the dense CSM. Namely, the hard and soft components are responsible for the heavily and moderately absorbed components, respectively. The co-existence of the two components with distinct absorptions as well as the delayed emergence of the moderately absorbed X-ray component could be evidence for asphericity of the CSM. We show that the X-ray spectral evolution can be qualitatively explained by considering a torus-like geometry for the dense CSM. Based on our X-ray spectral analyses, we estimate the radius of the torus-like CSM to be on the order of ∼5 × 1016 cm.

SPECTRAL SOFTENING BETWEEN OUTBURST AND QUIESCENCE IN THE NEUTRON STAR LOW-MASS X-RAY BINARY SAX J1750.8-2900

Tracking the spectral evolution of transiently accreting neutron stars between outburst and quiescence probes relatively poorly understood accretion regimes. Such studies are challenging because they require frequent monitoring of sources with luminosities below the thresholds of current all-sky X-ray monitors. We present the analysis of over 30 observations of the neutron star low-mass X-ray binary SAX J1750.8-2900 taken across four years with the X-ray telescope aboard Swift. We find spectral softening with decreasing luminosity both on long ($\sim$1 year) and short ($\sim$days to week) timescales. As the luminosity decreases from $4\times10^{36}$ erg s$^{-1}$ to $ \sim1\times10^{35} $ erg s$^{-1}$ (0.5-10 keV), the power law photon index increases from from 1.4 to 2.9. Although not statistically required, our spectral fits allow an additional soft component that displays a decreasing temperature as the luminosity decreases from $4 \times 10^{36} $ to $6 \times 10^{34}$ erg s$^{-1}$. Spectral softening exhibited by SAX J1750.8-2900 is consistent both with accretion emission whose spectral shape steepens with decreasing luminosity and also with being dominated by a changing soft component, possibly associated with accretion onto the neutron star surface, as the luminosity declines.

Time-resolved spectral analysis of prompt emission from long gamma-ray bursts with GeV emission

We performed detailed time-resolved spectroscopy of bright long gamma-ray bursts (GRBs) which show significant GeV emissions (GRB 080916C, GRB 090902B and GRB 090926A). In addition to the standard Band model, we also use a model consisting of a black body and a power law to fit the spectra. We find that for the latter model there are indications of an additional soft component in the spectra. While previous studies have shown that such models are required for GRB 090902B, here we find that a composite spectral model consisting of two blackbodies and a power law adequately fits the data of all the three bright GRBs. We investigate the evolution of the spectral parameters and find several interesting features that appear in all three GRBs, like (a) temperatures of the blackbodies are strongly correlated with each other, (b) fluxes in the black body components are strongly correlated with each other, (c) the temperatures of the black body trace the profile of the individual pulses of the GRBs, and (d) the characteristics of power law components like the spectral index and the delayed onset bear a close similarity to the emission characteristics in the GeV regions. We discuss the implications of these results and the possibility of identifying the radiation mechanisms during the prompt emission of GRBs.

MAXI/GSC Discovery of the Black-Hole Candidate MAXI J1305–704

We present the first results on a new black-hole candidate, MAXI J1305–704, observed by MAXI/GSC. The new X-ray transient, named as MAXIJ1305–704, was first detected by the MAXI-GSC all-sky survey on 2012 April 9 in the direction to the outer Galactic bulge at $(l,b)=(304_\cdot^\circ 2,-7_\cdot^\circ 6)$. A Swift/XRT follow-up observation confirmed the uncatalogued point source, and localized it to the position at $(13^\textrm h06^\textrm m56_\cdot^\textrm s44,-70^\circ27^\prime4.^{\prime\prime}91)$. The source continued its activity for about five months, until 2012 August. The MAXI/GSC light curve in the 2–10 keV band and the variation of the hardness ratio of the 4–10 keV to the 2–4 keV flux revealed the hard-to-soft state transition on the the sixth day (April 15) in the brightening phase and the soft-to-hard transition on the ∼60th day (June 15) in the decay phase. The luminosity at the initial hard-to-soft transition was significantly higher than that at the soft-to-hard transition in the decay phase. The X-ray spectra in the hard state are represented by a single power-law model with a photon index of ∼2.0, while those in the soft state need such an additional soft component as represented by a multi-color disk blackbody emission with an inner disk temperature of ∼0.5–1.2 keV. All of the obtained features support the source identification of a Galactic black-hole binary located in the Galactic bulge.

XMM–Newton and Swift spectroscopy of the newly discovered very faint X-ray transient IGR J17494–3030

Abstract A growing group of low-mass X-ray binaries are found to be accreting at very faint X-ray luminosities of <1036 erg s−1 (2–10 keV). One such system is the new X-ray transient IGR J17494–3030. We present Swift and XMM–Newton observations obtained during its 2012 discovery outburst. The Swift observations trace the peak of the outburst, which reached a luminosity of ∼7 × 1035 (D/8 kpc)2 erg s−1 (2–10 keV). The XMM–Newton data were obtained when the outburst had decayed to an intensity of ∼8 × 1034 (D/8 kpc)2 erg s−1. The spectrum can be described by a power law with an index of Γ ∼ 1.7 and requires an additional soft component with a blackbody temperature of ∼0.37 keV (contributing ∼20 per cent to the total unabsorbed flux in the 0.5–10 keV band). Given the similarities with high-quality spectra of very faint neutron-star low-mass X-ray binaries, we suggest that the compact primary in IGR J17494–3030 is a neutron star. Interestingly, the source intensity decreased rapidly during the ∼12 h XMM–Newton observation, which was accompanied by a decrease in inferred temperature. We interpret the soft spectral component as arising from the neutron-star surface due to low-level accretion, and propose that the observed decline in intensity was the result of a decrease in the mass-accretion rate on to the neutron star.

GRO J1008−57: an (almost) predictable transient X-ray binary

A study of archival RXTE, Swift, and Suzaku pointed observations of the transient high mass X-ray binary GRO J1008-57 is presented. A new orbital ephemeris based on pulse arrival timing shows times of maximum luminosities during outbursts of GRO J1008-57 to be close to periastron at orbital phase -0.03. This makes the source one of a few for which outburst dates can be predicted with very high precision. Spectra of the source in 2005, 2007, and 2011 can be well described by a simple power law with high energy cutoff and an additional black body at lower energies. The photon index of the power law and the black body flux only depend on the 15-50 keV source flux. No apparent hysteresis effects are seen. These correlations allow to predict the evolution of the pulsar's X-ray spectral shape over all outbursts as a function of just one parameter, the source's flux. If modified by an additional soft component, this prediction even holds during GRO J1008-57's 2012 type II outburst.

THE BLACK HOLE SPIN AND SOFT X-RAY EXCESS OF THE LUMINOUS SEYFERT GALAXY FAIRALL 9

We present an analysis of all XMM-Newton and Suzaku X-ray spectra of the nearby luminous Seyfert galaxy Fairall 9. Confirming previous analyses, we find robust evidence for a broad iron line associated with X-ray reflection from the innermost accretion disk. By fitting a spectral model that includes a relativistically ionized reflection component, we examine the constraints on the inclination of the inner accretion disk and the black hole spin, and the complications introduced by the presence of a photoionized emission line system. Employing multi-epoch fitting, we attempt to obtain robust and concordant measures of the accretion disk parameters. We also clearly see a soft X-ray excess in Fairall 9. During certain epochs, the soft excess can be described with same disk reflection component that produces the iron line. However, there are epochs where an additional soft component is required. This can be attributed to either an additional highly-ionized, strongly blurred disk reflection component, or a new X-ray continuum component.

XMM-Newton and INTEGRAL observations of the very faint X-ray transient IGR J17285−2922/XTE J1728−295 during the 2010 outburst

We report the first broad-band (0.5–150 keV) simultaneous X-ray observations of the very faint X-ray transient IGR J17285−2922/XTE J1728−295 performed with the XMM–Newton and INTEGRAL satellites during its last outburst, started on 2010 August 28. XMM–Newton observed the source on 2010 September 9–10 for 22 ks. INTEGRAL observations were part of the publicly available Galactic bulge program, and overlapped with the times covered by XMM–Newton. The broad-band spectroscopy resulted in a best fit with an absorbed power law displaying a photon index Γ of 1.61 ± 0.01, an absorbing column density NH of (5.10 ± 0.05) × 1021 cm−2 and a flux of 2.4 × 10−10 erg cm−2 s−1 (1–100 keV), corrected for the absorption. The data did not require either a spectral cut-off (Ec > 50 keV) or an additional soft component. The slopes of the power law fitting the XMM–Newton and INTEGRAL separate spectra were compatible, within the uncertainties. The timing analysis does not show evidence either for X-ray pulsations or for type I X-ray bursts. The broad-band X-ray spectrum as well as the power-density spectrum is indicative of a low-hard state in a low-mass X-ray binary, although nothing conclusive can be said about the nature of the compact object (neutron star or black hole). The results we report here allow us to conclude that IGR J17285−2922 is a low-mass X-ray binary, located at a distance greater than 4 kpc.

Swift observations of the Be/X-ray transient system 1A 1118−615

We report results of Swift observations for the high-mass Be/X-ray binary system 1A 1118−615, during an outburst stage in 2009 January and at a flaring stage in 2009 March. Using the epoch-folding method, we successfully detected a pulsed period of 407.69(2) s in the outburst of January and of 407.26(1) s after the flare detection in March. We find that the spectral detection for the source during outburst can be described by a blackbody model with a high temperature (kT ∼ 1–3 keV) and a small radius (R ∼ 1 km), indicating that the emission results from the polar cap of the neutron star. On the other hand, the spectra obtained after the outburst can further be described by adding an additional component with a lower temperature (kT ∼ 0.1–0.2 keV) and a larger emission radius (R ∼ 10–500 km), which indicates the emission from around the inner region of an accretion disc. We find that the thermal emission from the hotspot of the accreting neutron star dominates the radiation in outburst; the existence of both this X-ray contribution and the additional soft component suggests that the polar cap and the accretion disc emission might co-exist after the outburst. Because the two-blackbody signature at the flaring stage is a unique feature of 1A 1118−615, our spectral results may provide a new insight to interpret the X-ray emission for the accreting neutron star. The time separation between the three main outbursts of this system is ∼17 yr and it might be related to the orbital period. We derive and discuss the associated physical properties by assuming the elongated orbit for this specific Be/X-ray transient.

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.

Broad-Band Temporal and Spectral Variation of 36 Active Galactic Nuclei Observed with Suzaku

Abstract 36 AGN data observed by Suzaku were compiled to examine the temporal variability in different energy bands covered by soft X-ray imaging systems (XRT $+$ XIS: 0.5–10 keV) and Hard X-ray Detector (HXD: 15–50 keV) aboard Suzaku. Three band light curves were obtained in L (0.5–2 keV), M (2–10 keV), and H (15–50 keV) with a time binning of 5760 s. The L-band flux change was almost proportional to that of the M-band without offsets in most of type-1 AGNs. The slope ($=$$dL/dM$) of the correlation changed by more than a factor of 10 from Seyfert 2 to narrow-line Seyfert 1 galaxies (NLS1). The power-law component can explain the slope of Seyfert 1 galaxies with a typical index, $\Gamma$, of 1.9. For NLS1 s with steeper slopes, an additional soft excess component is needed, whose variation should be proportional to that of the power-law component. It gives a new constraint to models for the soft excess component. The H-band flux change is a linear function of the M-band flux with a significant offset in the H-band. The slope can be explained by the power-law component with an absorption column from 10$^{22{-}24} $cm$^{-2}$. The offset can be attributed to the reflection component, which is stable on a time scale of days.

X-ray spectral evolution of the extragalactic Z source LMC X-2

We present the results obtained by a detailed study of the extragalactic Z source, LMC X-2, using broad band Suzaku data and a large ($ \sim 750$ ksec) data set obtained with the proportional counter array (PCA) onboard RXTE. The PCA data allows for studying the complete spectral evolution along the horizontal, normal and flaring branches of the Z-track. Comparison with previous study show that the details of spectral evolution (like variation of Comptonizing electron temperature), is similar to that of GX 17+2 but unlike that of Cyg X-2 and GX 349+2. This suggests that Z sources are heterogeneous group with perhaps LMC X-2 and GX 17+2 being member of a subclass. However non monotonic evolution of the Compton y-parameter seems to be generic to all sources. The broad band {\it Suzaku} data reveals that the additional soft component of the source modelled as a disk blackbody emission is strongly preferred over one where it is taken to be a blackbody spectrum. This component as well as the temperature of seed photons do not vary when source goes into a flaring mode and the entire variation can be ascribed to the Comptonizing cloud. The bolometric unabsorbed luminosity of the source is well constrained to be $ \sim 2.23 \times 10^{38}$ ergs/sec which if the source is Eddington limited implies a neutron star mass of 1.6 M$_\odot$. We discuss the implications of these results.

SWIFTOBSERVATIONS OF HARD X-RAY EMITTING WHITE DWARFS IN SYMBIOTIC STARS

The X-ray emission from most accreting white dwarfs (WDs) in symbiotic binary stars is quite soft. Several symbiotic WDs, however, produce strong X-ray emission at energies greater than ~20 keV. The Swift BAT instrument has detected hard X-ray emission from 4 such accreting WDs in symbiotic stars: RT Cru, T CrB, CD -57 3057, and CH Cyg. In one case (RT Cru), Swift detected X-rays out to greater than 50 keV at a > 5 sigma confidence level. Combining data from the XRT and BAT detectors, we find that the 0.3-150 keV spectra of RT Cru, T CrB, and CD -57 3057 are well described by emission from a single-temperature, optically thin thermal plasma, plus an unresolved 6.4-6.9 keV Fe line complex. The X-ray spectrum of CH Cyg contains an additional bright soft component. For all 4 systems, the spectra suffer high levels of absorption from material that both fully and partially covers the source of hard X-rays. The XRT data did not show any of the rapid, periodic variations that one would expect if the X-ray emission were due to accretion onto a rotating, highly magnetized WD. The X-rays were thus more likely from the accretion-disk boundary layer around a massive, non-magnetic WD in each binary. The X-ray emission from RT Cru varied on timescales of a few days. This variability is consistent with being due to changes in the absorber that partially covers the source, suggesting localized absorption from a clumpy medium moving into the line of sight. The X-ray emission from CD -57 3057 and T CrB also varied during the 9 months of Swift observations, in a manner that was also consistent with variable absorption.

SUZAKU VIEW OF POWERFUL GAMMA-RAY QUASARS

We present the results from multiwavelength campaigns of three powerful gamma-ray quasars, PKS 1510-089, RBS 315 and Swift J0746.3+2548, recently organized with Suzaku. The Suzaku observation provided one of the highest S/N X-ray spectra ever reported between 0.3 and 50 keV. For these quasars, the X-ray spectrum is well represented by an extremely hard power-law with photon index Γ ≃ 1.2, but is augmented by an additional soft component apparently below 1 keV for PKS 1510-089, whereas a strong deficit of soft photons is observed in RBS 315. We model the broadband spectra of these powerful quasars and argue that the power of the jet is dominated by protons but with the number of electrons/positrons exceeding the number of protons by a factor ≃ 10. We also argue that an extremely hard X-ray spectra may result from a double power-law form of the injected electrons, with the break energy γ br ≃ 1000 corresponding to the anticipated threshold of diffusive shock acceleration.