Comptonizing Plasma Research Articles

Long term variability of Cygnus X-1

The Neutron Star Interior Composition Explorer (NICER) monitoring campaign of Cyg X-1 allows us to study its spectral-timing behavior at energies < 1 keV across all states. The hard state power spectrum can be decomposed into two main broad Lorentzians with a transition at around 1 Hz. The lower-frequency Lorentzian is the dominant component at low energies. The higher-frequency Lorentzian begins to contribute significantly to the variability above 1.5 keV and dominates at high energies. We show that the low- and high-frequency Lorentzians likely represent individual physical processes. The lower-frequency Lorentzian can be associated with a (possibly Comptonized) disk component, while the higher-frequency Lorentzian is clearly associated with the Comptonizing plasma. At the transition of these components, we discover a low-energy timing phenomenon characterized by an abrupt lag change of hard (≳2 keV) with respect to soft (≲1.5 keV) photons, accompanied by a drop in coherence, and a reduction in amplitude of the second broad Lorentzian. The frequency of the phenomenon increases with the frequencies of the Lorentzians as the source softens and cannot be seen when the power spectrum is single-humped. A comparison to transient low-mass X-ray binaries shows that this feature does not only appear in Cyg X-1, but that it is a general property of accreting black hole binaries. In Cyg X-1, we find that the variability at low and high energies is overall highly coherent in the hard and intermediate states. The high coherence shows that there is a process at work which links the variability, suggesting a physical connection between the accretion disk and Comptonizing plasma. This process fundamentally changes in the soft state, where strong red noise at high energies is incoherent to the variability at low energies.

FRAMEx. II. Simultaneous X-Ray and Radio Variability in Active Galactic Nuclei—The Case of NGC 2992

Abstract Using simultaneous Very Long Baseline Array and Neil Gehrels Swift Observatory X-ray Telescope observations of the active galactic nucleus (AGN) in NGC 2992 over a six-month observing campaign, we observed a large drop in core 5 cm radio luminosity, by a factor of >3, in tandem with a factor of >5 increase in 2–10 keV X-ray luminosity. While NGC 2992 has long been an important object for studies of X-ray variability, our study is the first simultaneous X-ray and radio variability campaign for this object. We observe that the X-ray spectral index does not change over the course of the flare, consistent with a change in the bulk amount of Comptonizing plasma, potentially due to a magnetic reconnection event in the accretion disk. The drop in apparent radio luminosity can be explained by a change in free–free absorption, which we calculate to correspond to an ionized region with a physical extent and electron density consistent with the broad-line region (BLR). Our results are consistent with magnetic reconnection events in the dynamic accretion disk creating outbursts of ionizing material, increasing Compton up-scattering of UV accretion disk photons and feeding material into the BLR. These findings present an important physical picture for the dynamical relationship between X-ray and radio emission in AGNs.

Broadband Variability Study of Maxi J1631-479 in Its Hard-intermediate State Observed with Insight-HXMT

Abstract We report the energy-resolved broadband timing analysis of the black hole X-ray transient MAXI J1631-479 during its 2019 outburst from February 11 to April 9, using data from the Insight−Hard X-ray Modulation Telescope (Insight-HXMT), which caught the source from its hard-intermediate state to the soft state. Thanks to the large effective area of Insight-HXMT at high energies, we are able to present the energy dependence of fast variability up to ∼100 keV. Type-C quasi-periodic oscillations (QPOs) with a frequency varying between 4.9 and 6.5 Hz are observed in the 1–100 keV energy band. While the QPO fractional rms increases with photon energy from 1 keV to ∼10 keV and remains more or less constant from ∼10 keV to ∼100 keV, the rms of the flat-top noise first increases from 1 keV to ∼8 keV and then drops to less than 0.1% above ∼30 keV. We suggest that the disappearance of the broadband variability above 30 keV could be caused by the nonthermal acceleration in the Comptonizing plasma. At the same time, the QPOs could be produced by the precession of either a small-scale jet or a hot inner flow model.

Hybrid Comptonization and Electron–Positron Pair Production in the Black-hole X-Ray Binary MAXI J1820+070

Abstract We study X-ray and soft gamma-ray spectra from the hard state of the accreting black-hole binary MAXI J1820+070. We perform an analysis of two joint spectra from NuSTAR and INTEGRAL, covering the range of 3–650 keV, and of an average joint spectrum over the rise of the hard state, covering the 3–2200 keV range. The spectra are well modeled by Comptonization of soft seed photons. However, the distributions of the scattering electrons are not purely thermal; we find they have substantial high-energy tails, well modeled as power laws. The photon tail in the average spectrum is detected well beyond the threshold for electron–positron pair production, 511 keV. This allows us to calculate the rate of the electron–positron pair production and put a lower limit on the size of the source from pair equilibrium. At the fitted Thomson optical depth of the Comptonizing plasma, the limit is about 4 gravitational radii. If we adopt the sizes estimated by us from the reflection spectroscopy of >20 gravitational radii, the fractional pair abundance becomes much less than unity. The low pair abundance is confirmed by the lack of both an annihilation feature and of a pair absorption cutoff above 511 keV in the average spectrum.

Complex optical/UV and X-ray variability in Seyfert 1 galaxy Mrk 509

AbstractWe performed a detailed spectral and timing analysis of a Seyfert 1 galaxy Mrk 509 using data from theNeil Gehrels Swiftobservatory that spanned over$\sim$13 years between 2006 and 2019. To study the variability properties from the optical/UV to X-ray emission, we used a total of 275 pointed observations in this work. The average spectrum over the entire duration exhibits a strong soft X-ray excess above the power law continuum. The soft X-ray excess is well described by two thermal components with temperatures of kT$_{\rm BB1}\sim$120 eV and kT$_{\rm BB2}\sim$460 eV. The warm thermal component is likely due to the presence of an optically thick and warm Comptonizing plasma in the inner accretion disk. The fractional variability amplitude is found to be decreasing with increasing wavelength, i.e., from the soft X-ray to UV/optical emission. However, the hard X-ray (2–8 keV) emission shows very low variability. The strength of the correlation within the UV and the optical bands (0.95–0.99) is found to be stronger than the correlation between the UV/optical and X-ray bands (0.40–0.53). These results clearly suggest that the emitting regions of the X-ray and UV/optical emission are likely distinct or partly interacting. Having removed the slow variations in the light curves, we find that the lag spectrum is well described by the 4/3 rule for the standard Shakura–Sunyaev accretion disk when we omit X-ray lags. All these results suggest that the real disk is complex, and the UV emission is likely reprocessed in the accretion disk to give X-ray and optical emission.

NuSTAR Survey of Obscured Swift/BAT-selected Active Galactic Nuclei. II. Median High-energy Cutoff in Seyfert II Hard X-Ray Spectra

Abstract Broadband X-ray spectroscopy of the X-ray emission produced in the coronae of active galactic nuclei (AGNs) can provide important insights into the physical conditions very close to their central supermassive black holes. The temperature of the Comptonizing plasma that forms the corona is manifested through a high-energy cutoff that has been difficult to directly constrain even in the brightest AGN because it requires high-quality data at energies above 10 keV. In this paper we present a large collection of coronal cutoff constraints for obscured AGNs based on a sample of 130 AGNs selected in the hard X-ray band with Swift/BAT and observed nearly simultaneously with NuSTAR and Swift/XRT. We find that under a reasonable set of assumptions regarding partial constraints the median cutoff is well constrained to 290 ± 20 keV, where the uncertainty is statistical and given at the 68% confidence level. We investigate the sensitivity of this result to our assumptions and find that consideration of various known systematic uncertainties robustly places the median cutoff between 240 and 340 keV. The central 68% of the intrinsic cutoff distribution is found to be between about 140 and 500 keV, with estimated uncertainties of 20 and 100 keV, respectively. In comparison with the literature, we find no clear evidence that the cutoffs in obscured and unobscured AGNs are substantially different. Our analysis highlights the importance of carefully considering partial and potentially degenerate constraints on the coronal high-energy cutoff in AGNs.

New insights on the puzzling LMXB 1RXS J180408.9-342058: the intermediate state, the clocked type-I X-ray bursts, and much more

ABSTRACT 1RXS J180408.9–342058 is a low-mass X-ray binary hosting a neutron star, which shows X-ray activity at very different mass-accretion regimes, from very faint to almost the Eddington luminosity. In this work, we present a comprehensive X-ray study of this source using data from the Neil Gehrels Swift Observatory, NuSTAR, and INTEGRAL/JEM-X. In order to follow the spectral evolution, we analysed the 2015 outburst using Swift data and three NuSTAR observations. Besides the canonical hard and soft spectral states, we identified the rarely observed intermediate state. This was witnessed by the appearance of the accretion disc emission in the spectrum (at kTdisc ∼0.7 keV) and the simultaneous cooling of the hot corona. In addition, we also unveiled a hard tail above 30 keV in this state. In the hard state, a thermal Comptonization model with two seed photons populations (kTs,1 ∼ 1.5 keV and kTs,2 ∼ 0.4 keV, respectively) and a hot Comptonizing plasma, represents the physically best motivated scenario to describe the data. We also estimated a reflection fraction below 20 per cent in all states, while no constraints on the inclination and only lower limits on the inner disc radius could be inferred. Finally, we studied a number of type-I X-ray bursts displayed from the source, one of them at the Eddington limit (observed with JEM-X). Their characteristics, combined with the clocked behaviour observed during the intermediate state, point out H/He composition for the accreted material, which makes unlikely the helium dwarf nature for the companion.

BAT AGN Spectroscopic Survey – XII. The relation between coronal properties of active galactic nuclei and the Eddington ratio

The bulk of the X-ray emission in Active Galactic Nuclei (AGN) is produced very close to the accreting supermassive black hole (SMBH), in a corona of hot electrons which up scatters optical and ultraviolet photons from the accretion flow. The cutoff energy ($E_{\rm C}$) of the primary X-ray continuum emission carries important information on the physical characteristics of the X-ray emitting plasma, but little is currently known about its potential relation with the properties of accreting SMBHs. Using the largest broad-band (0.3-150 keV) X-ray spectroscopic study available to date, we investigate how the corona is related to the AGN luminosity, black hole mass and Eddington ratio ($\lambda_{\rm Edd}$). Assuming a slab corona the median values of the temperature and optical depth of the Comptonizing plasma are $kT_{\rm e}=105 \pm 18$ keV and $\tau=0.25\pm0.06$, respectively. When we properly account for the large number of $E_{\rm C}$ lower limits, we find a statistically significant dependence of the cutoff energy on the Eddington ratio. In particular, objects with $ \lambda_{\rm Edd}>0.1$ have a significantly lower median cutoff energy ($E_{\rm C}=160\pm41$ keV) than those with $\lambda_{\rm Edd}\leq 0.1$ ($E_{\rm C}=370\pm51$ keV). This is consistent with the idea that radiatively compact coronae are also cooler, because they tend to avoid the region in the temperature-compactness parameter space where runaway pair production would dominate. We show that this behaviour could also straightforwardly explain the suggested positive correlation between the photon index ($\Gamma$) and the Eddington ratio, being able to reproduce the observed slope of the $\Gamma-\lambda_{\rm Edd}$ trend.

Energy dependent time delays of kHz oscillations due to thermal Comptonization

We study the energy dependent photon variability from a thermal Comptonizing plasma that is oscillating at kHz frequencies. In particular, we solve the linearized time-dependent Kompaneets equation and consider the oscillatory perturbation to be either in the soft photon source or in the heating rate of the plasma. For each case, we self consistently consider the energy balance of the plasma and the soft photon source. The model incorporates the possibility of a fraction of the Comptonized photons impinging back into the soft photon source. We find that when the oscillation is due to the soft photon source, the variation of the fractional root mean sqaure (rms) is nearly constant with energy and the time-lags are hard. However, for the case when the oscillation is due to variation in the heating rate of the corona, and when a significant fraction of the photons impinge back into the soft photon source, the rms increases with energy and the time-lags are soft. As an example, we compare the results with the ∼850 Hz oscillation observed on 1996 March 3 for 4U 1608−52 and show that both the observed soft time-lags as well as the rms versus energy can be well described by such a model where the size of the Comptonizing plasma is ∼1 km. Thus, modelling of the time-lags as due to Comptonization delays, can provide tight constraints on the size and geometry of the system. Detailed analysis would require well-constrained spectral parameters.

The X-ray spectrum of the newly discovered accreting millisecond pulsar IGR J17511−3057

We report on a 70 ks XMM–Newton Target of Opportunity (ToO) observation of the newly discovered accreting millisecond pulsar, IGR J17511−3057. Pulsations at 244.833 9512(1) Hz are observed throughout the outburst with an rms-pulsed fraction of 14.4(3) per cent. Pulsations have been used to derive a precise solution for the Porb= 12 487.51(2) s binary system. The measured mass function indicates a main-sequence companion star with a mass between 0.15 and 0.44 M⊙. The XMM–Newton 0.5–11 keV spectrum of IGR J17511−3057 can be modelled by at least three components, which we interpret, from the softest to the hardest, as multi-coloured disc emission, thermal emission from the neutron star surface and thermal Comptonization emission. Spectral fit of the XMM–Newton data and of the Rossi X-ray Timing Explorer (RXTE) data, taken in a simultaneous temporal window, well constrain the Comptonization parameters: the electron temperature, kTe= 51+6−4 keV, is rather high, while the optical depth (τ= 1.34+0.03−0.06) is moderate. The energy dependence of the pulsed fraction supports the interpretation of the cooler thermal component as coming from the accretion disc, and indicates that the Comptonizing plasma surrounds the hot spots on the neutron star surface, which in turn provides the seed photons. Signatures of reflection, such as a broadened iron Kα emission line and a Compton hump at ∼30 keV, are also detected. We derive from the smearing of the reflection component an inner disc radius of ≳40 km for a 1.4 M⊙ neutron star, and an inclination between 38° and 68°. XMM–Newton also observed two type I X-ray bursts, whose fluence and recurrence time suggest that the bursts are ignited in a nearly pure helium environment. No photospheric radius expansion is observed, thus leading to an upper limit on the distance to the source of 10 kpc. A lower limit of 6.5 kpc can be also set if it is assumed that emission during the decaying part of the burst involves the whole neutron star surface. Pulsations are observed during the burst decay with an amplitude similar to the persistent emission. They are also compatible with being phase locked to pre-burst pulsations, suggesting that the location on the neutron star surface where they are formed does not change much during bursts.

VARIABILITY AND SPECTRAL MODELING OF THE HARD X-RAY EMISSION OF GX 339–4 IN A BRIGHT LOW/HARD STATE

We study the high-energy emission of the Galactic black hole candidate GX 339-4 using INTEGRAL/SPI and simultaneous RXTE/PCA data. By the end of January 2007, when it reached its peak luminosity in hard X-rays, the source was in a bright hard state. The SPI data from this period show a good signal to noise ratio, allowing a detailed study of the spectral energy distribution up to several hundred keV. As a main result, we report on the detection of a variable hard spectral feature (>150 keV) which represents a significant excess with respect to the cutoff power law shape of the spectrum. The SPI data suggest that the intensity of this feature is positively correlated with the 25 - 50 keV luminosity of the source and the associated variability time scale is shorter than 7 hours. The simultaneous PCA data, however, show no significant change in the spectral shape, indicating that the source is not undergoing a canonical state transition. We analyzed the broad band spectra in the lights of several physical models, assuming different heating mechanisms and properties of the Comptonizing plasma. For the first time, we performed quantitative model fitting with the new versatile Comptonization code BELM, accounting self-consistently for the presence of a magnetic field. We show that a magnetized medium subject to pure non-thermal electron acceleration provides a framework for a physically consistent interpretation of the observed 4 - 500 keV emission. Moreover, we find that the spectral variability might be triggered by the variations of only one physical parameter, namely the magnetic field strength. Therefore, it appears that the magnetic field is likely to be a key parameter in the production of the Comptonized hard X-ray emission.

The first broad-band X-ray study of the Supergiant Fast X-ray Transient SAX J1818.6���1703 in outburst

The Supergiant Fast X‐ray Transient (SFXT) SAX J1818.6 1703 underwent an outburst on 2009 May 6 and was observed with Swift. We report on these observations which, for the first time, allow us to study the broad-band spectrum from sof t to hard X‐rays of this source. No X‐ray spectral information was available on this source before the Swift monitoring. The spectrum can be deconvolved well with models usually adopted to describe the emission from High Mass X‐ray Binary X‐ray pulsars, and is characterized by a very high absorption, a flat power law (photon index �0.1‐0.5) and a cutoff at about 7‐12 keV. Alternatively, the SAX J1818.6 1703 emission can be described with a Comptonized emission from a cold and optically thick corona, with an electron temperature kTe=5‐7 keV, a hot seed photon temperature, kT0, of 1.3‐1.4 keV, and an optical depth for the Comptonizing plasma, � , of about 10. The 1‐100 keV luminosity at the peak of the flare is 3 ×10 36 erg s 1 (assuming the optical counterpart distance of 2.5 kpc). These properties of SAX J1818.6 1703 resemble those of the prototype of the SFXT class, XTE J1739‐302. The monitoring with Swift/XRT reveals an outburst duration of about 5 days, similarly to ot her members of the class of SFXTs, confirming SAX J1818.6 1703 as a member of this class.

Spectral and Fourier analyses of X-ray quasi-periodic oscillations in accreting black holes

We study energy dependencies of quasi-periodic oscillations (QPOs) from a number of black hole X-ray binaries. The selected sources were observed by RXTE at time periods close to state transitions and showed QPOs in the 1–10 Hz range. We have constructed QPO rms energy spectra, which provide information about underlying physical process leading to QPO generation. These spectra show an interesting anticorrelation with the time-averaged spectra. The QPO rms spectra are harder than the time-averaged spectra when the latter are soft, while they are softer than the time-averaged spectra when the latter are hard. We then discuss these observational results in the context of simple spectral variability models. Hard QPO spectra can be produced by quasi-periodic modulations of the heating rate of the Comptonizing plasma, while soft QPO spectra result from modulations of the cooling rate by soft photons.

GRS 1915+105: the distance, radiative processes and energy-dependent variability

We present an exhaustive analysis of five broad-band observations of GRS 1915+105 in two variability states, chi and omega, observed simultaneously by the PCA, HEXTE and OSSE. We find all the spectra well fitted by Comptonization of disc blackbody photons, with very strong evidence for the presence of a nonthermal electron component in the Comptonizing plasma. Both the energy and the power spectra in the chi state are typical to the very high/intermediate state of black-hole binaries. The spectrum of the omega state is characterized by a strong blackbody component Comptonized by thermal electrons and a weak nonthermal tail. We then calculate rms spectra (fractional variability as functions of energy) for the PCA data. We accurately model the rms spectra by coherent superposition of variability in the components implied by the spectral fits, namely a less variable blackbody and more variable Comptonization. The latter dominates at high energies, resulting in a flattening of the rms at high energies in most of the data. This is also the case for the spectra of the QPOs present in the chi state. Then, some of our data require a radial dependence of the rms of the disc blackbody. We also study the distance to the source, and find 11 kpc as the most likely value, contrary to a recent claim of a much lower value.

Physics of accretion in the millisecond pulsar XTE J1751 − 305

We have undertaken an extensive study of X-ray data from the accreting millisecond pulsar XTE J1751-305 observed by RXTE and XMM-Newton during its 2002 outburst. In all aspects this source is similar to a prototypical millisecond pulsar SAX J1808.4-3658, except for the higher peak luminosity of 13 per cent of Eddington, and the optical depth of the hard X-ray source larger by factor ~2. Its broad-band X-ray spectrum can be modelled by three components. We interpret the two soft components as thermal emission from a colder (kT ~ 0.6 keV) accretion disc and a hotter (~1 keV) spot on the neutron star surface. We interpret the hard component as thermal Comptonization in plasma of temperature ~40 keV and optical depth of ~1.5 in a slab geometry. The plasma is heated by the accretion shock as the material collimated by the magnetic field impacts on to the surface. The seed photons for Comptonization are provided by the hotspot, not by the disc. The Compton reflection is weak and the disc is probably truncated into an optically thin flow above the magnetospheric radius. Rotation of the emission region with the star creates an almost sinusoidal pulse profile with rms amplitude of 3.3 per cent. The energy-dependent soft phase lags can be modelled by two pulsating components shifted in phase, which is naturally explained by a different character of emission of the optically thick spot and optically thin shock combined with the action of the Doppler boosting.

Detection of Hard X‐Ray Pulsations and a Strong Iron Kβ Emission Line during an Extended Low State of GX 1+4

We present here results obtained from a detailed timing and spectral analysis of three BeppoSAX observations of the binary X-ray pulsar GX 1+4 carried out in 1996 August, 1997 March, and 2000 August. In the middle of the 2000 August observation, the source was in a rare low-intensity state that lasted for about 30 hr. Although the source does not show pulsations in the soft X-ray band (1.0-5.5 keV) during the extended low state, pulsations are detected in the 5.5-10.0 keV energy band of the MECS detector and in the hard X-ray energy bands (15-150 keV) of the PDS instrument. Comparing the 2-10 keV flux during this low state with the previously reported low states in GX 1+4, we suggest that the propeller regime in GX 1+4 occurs at a lower mass accretion rate than reported earlier. Broadband (1.0-150 keV) pulse-averaged spectroscopy reveals that the best-fit model comprises a Comptonized continuum along with an iron Kα emission line. A strong iron Kβ emission line is detected for the first time in GX 1+4 during the extended low state of the 2000 observation with an equivalent width of ~550 eV. The optical depth and temperature of the Comptonizing plasma are found to be identical during the high- and low-intensity states, whereas the hydrogen column density and the temperature of the seed photons are higher during the low state. We also present results from pulse-phase-resolved spectroscopy during the high- and low-flux episodes.

High‐Redshift Radio‐quiet Quasars: Exploring the Parameter Space of Accretion Models. I. Hot Semispherical Flow

Two families of models are currently considered to describe an accretion flow onto black holes and production of the observed X-ray radiation: (1) a standard cold accretion disk with a hot corona above it and (2) an outer truncated accretion disk with a hot semispherical inner flow. We compute spectra in the scenario with a hot inner flow surrounded by a truncated accretion disk covered by a hot corona and test the results on a sample of high-redshift (z > 4) quasars observed with Chandra. We find that in order to reproduce the ratio of optical to X-ray fluxes (the αox parameter), the optical depth of the Comptonizing plasma has to be rather low (τ = 0.02-0.25 in the corona above the disk, and τ = 0.10-0.70 in the hot inner flow). This, together with the observed X-ray photon indices, implies either a high temperature in a thermal plasma (kTe = 90-500 keV) or a nonthermal electron distribution in the plasma. We put an upper limit on the disk truncation radius, rtr ≤ 40RS. The modeled accretion rate is high, > 0.2Edd, which may suggest that high-z radio-quiet quasars are analogs of X-ray binaries in their high or very high state.

Simulations of X-ray spectral/timing properties in a propagation model of variability of accreting black holes

A phenomenological model of X-ray variability of accreting black holes is considered, where the variable emission is attributed to multiple active regions/perturbations moving radially towards the central black hole. The hard X-rays are produced by inverse Compton upscattering of soft photons coming from reprocessing/thermalization of the same hard X-rays. The heating rate of the Comptonizing plasma is assumed to scale with the rate of dissipation of gravitational energy while the supply of soft photons is assumed to diminish towards the centre. Two scenarios are considered: (1) an inner hot flow with outer truncated standard accretion disc and (2) an accretion disc with an active corona and a thick hot ionized skin. A variant of the model is also considered, which is compatible with the currently discussed multi-Lorentzian description of power spectral densities of X-ray light curves. In the inner hot flow scenario the model can reproduce the observed Fourier frequency resolved spectra observed in X-ray binaries, in particular the properties of the reprocessed component as functions of Fourier frequency. In the accretion disc with ionized skin scenario the reduction of soft photons owing to the ionized skin is insufficient to produce the observed characteristics.

X-ray and -ray spectra and variability of the black hole candidate GX 339--4

We analyse five observations of the X-ray binary GX 339‐4 by the soft γ -ray OSSE detector on board CGRO simultaneous with either Ginga or RXTE observations. The source was bright during four of them, with the luminosity of L ∼ 10 37 erg s −1 and the spectrum typical for hard states of accreting black holes, and it was in an off state during the fifth one, with L ∼ 10 35 erg s −1 . Our broad-band spectral fits show the mean electron energy of electrons in the Comptonizing plasma decreasing with increasing luminosity within the hard (bright) state. For the observation with the best statistics at soft γ -rays, ∼1/4 of energy in the Comptonizing plasma is probably carried by non-thermal electrons. Then, considering the efficiency of Comptonized hybrid synchrotron emission allows us to obtain an upper limit on the strength of the magnetic field in the X-ray source. Furthermore, this synchrotron emission is capable of producing the optical spectrum observed in an optically-high state of GX 339‐4. In the off state, the hard X-ray spectrum is consistent with being dominated by bremsstrahlung. The unusually strong Fe Kα line observed by the RXTE/PCA during that state is found not to be intrinsic to the source but to originate mostly in the Galactic diffuse emission.

Phase-resolved X-ray spectroscopy of the millisecond pulsar SAX J1808.4−3658

We present new results based on RXTE observations of the millisecond pulsar SAX J1808.4−3658, carried out during the decay of the 1998 April outburst. The X-ray spectrum can be fitted by a two-component model. We interpret the soft component as blackbody emission from a heated spot on the neutron star, and the hard component as coming from Comptonization in plasma heated by the accretion shock as the material collimated by the magnetic field impacts on to the neutron star surface. The hotspot is probably the source of seed photons for Comptonization. The hard component illuminates the disc, giving rise to a reflected spectrum. The amount of reflection indicates that the disc is truncated at fairly large radii (20–40Rg), consistent with the lack of relativistic smearing of the spectral features. The inferred evolution of the inner radius is not consistent with the magnetic field truncating the disc. Instead, it seems more likely that the inner disc radius is set by some much longer time-scale process, most probably connected to the overall evolution of the accretion disc. This disc truncation mechanism would then have to be generic in all low-mass accretion rate flows both in disc accreting neutron stars and black hole systems. The phase-resolved spectra show clearly that the blackbody and hard Comptonized spectra pulse independently. This obviously gives an energy-dependent phase-lag. Full general relativistic effects are not required to explain this. The soft blackbody component is optically thick, so its variability is dominated by its changing projected area, while the Doppler shifts (which are maximized 90° before the maximum in projected area) are somewhat stronger for the translucent column. We do not detect Compton reflection from the neutron star surface, though we predict that it should be present in the X-ray spectrum. This would give an unambiguous observational measure of M/R if there is any iron on the neutron star surface which is not completely ionized.