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At least 685 records · Page 38
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DOE OSTI2015

REPEATED, DELAYED TORQUE VARIATIONS FOLLOWING X-RAY FLUX ENHANCEMENTS IN THE MAGNETAR 1E 1048.1–5937

We report on two years of flux and spin evolution monitoring of 1E 1048.1–5937, a 6.5 s X-ray pulsar identified as a magnetar. Using Swift X-Ray Telescope data, we observed an X-ray outburst consisting of an increase in the persistent 1-10 keV flux by a factor of 6.3 ± 0.2, beginning on 2011 December 31 (MJD 55926). Following a delay of ∼100 days, the magnetar entered a period of large torque variability, with ν-dot reaching a factor of 4.55 ± 0.05 times the nominal value, before decaying in an oscillatory manner over a timescale of months. We show by comparing to previous outbursts from the source that this pattern of behavior may repeat itself with a quasi-period of ∼1800 days. We compare this phenomenology to periodic torque variations in radio pulsars, finding some similarities that suggest a magnetospheric origin for the behavior of 1E 1048.1–5937.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

MAPPING THE SURFACE OF THE MAGNETAR 1E 1048.1–5937 IN OUTBURST AND QUIESCENCE THROUGH PHASE-RESOLVED X-RAY SPECTROSCOPY

We model the pulse profiles and the phase-resolved spectra of the anomalous X-ray pulsar 1E 1048.1–5937 obtained with XMM-Newton to map its surface temperature distribution during an active and a quiescent epoch. We develop and apply a model that takes into account the relevant physical and geometrical effects on the neutron star surface, magnetosphere, and spacetime. Using this model, we determine the observables at infinity as a function of pulse phase for different numbers and sizes of hot spots on the surface. We show that the pulse profiles extracted from both observations can be modeled with a single hot spot and an antipodal cool component. The size of the hot spot changes from ≈80° in 2007, three months after the onset of a dramatic flux increase, to ≈30° during the quiescent observation in 2011, when the pulsed fraction returned to the pre-outburst ≈65% level. For the 2007 observation, we also find that a model consisting of a single 0.4 keV hot spot with a magnetic field strength of 1.8 × 10{sup 14} G accounts for the spectra obtained at three different pulse phases but underpredicts the flux at the pulse minimum, where the contribution to the emission from the cooler component is non-negligible. The inferred temperature of the spot stays approximately constant between different pulse phases, in agreement with a uniform temperature, single hot spot model. These results suggest that the emitting area grows significantly during outbursts but returns to its persistent and significantly smaller size within a timescale of a few years.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Wideband dynamic radio spectra of two ultra-cool dwarfs

A number of radio-loud ultra-cool dwarf (UCD) stars exhibit both continuous broadband and highly polarized pulsed radio emission. In order to determine the nature of the emission and the physical characteristics in the source region, we have made multi-epoch, wideband spectral observations of TVLM 0513-46 and 2M 0746+20. We combine these observations with archival radio data to fully characterize both the temporal and spectral properties of the radio emission. The continuum spectral energy distribution can be well modeled using gyrosynchrotron emission from mildly relativistic electrons in a dipolar field. The pulsed emission exhibits a variety of time-variable characteristics, including frequency drifts, frequency cutoffs, and multiple pulses per period. For 2M 0746+20 we determine a pulse period consistent with previously determined values. We modeled locations of pulsed emission using an oblique rotating magnetospheric model with beamed electron-cyclotron maser (ECM) sources. The best-fit models have narrow ECM beaming angles aligned with the local source magnetic field direction, except for one isolated burst from 2M 0746+20. For TVLM 0513-46, the best-fit rotation axis inclination is nearly orthogonal to the line of sight. For 2M 0746+20 we found a good fit using a fixed inclination i=36{sup ∘}, determined from optical observations. For both stars the ECM sources are located near feet of magnetic loops with radial extents 1.2R{sub s}–2.7R{sub s} and surface fields 2.2–2.5 kG. These results support recent suggestions that radio over-luminous UCDs have a global “weak field” non-axisymmetric magnetic topologies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

TOWARD AN EMPIRICAL THEORY OF PULSAR EMISSION. XI. UNDERSTANDING THE ORIENTATIONS OF PULSAR RADIATION AND SUPERNOVA “KICKS”

Two entwined problems have remained unresolved since pulsars were discovered nearly 50 yr ago: the orientation of their polarized emission relative to the emitting magnetic field and the direction of putative supernova “kicks” relative to their rotation axes. The rotational orientation of most pulsars can be inferred only from the (“fiducial”) polarization angle of their radiation, when their beam points directly at the Earth and the emitting polar fluxtube field is ∥ to the rotation axis. Earlier studies have been unrevealing owing to the admixture of different types of radiation (core and conal, two polarization modes), producing both ∥ or ⊥ alignments. In this paper we analyze some 50 pulsars having three characteristics: core radiation beams, reliable absolute polarimetry, and accurate proper motions (PMs). The “fiducial” polarization angle of the core emission, we then find, is usually oriented ⊥ to the PM direction on the sky. The primary core emission is polarized ⊥ to the projected magnetic field in Vela and other pulsars where X-ray imaging reveals the orientation. This shows that the PMs usually lie ∥ to the rotation axes on the sky. Two key physical consequences then follow: first, to the extent that supernova “kicks” are responsible for pulsar PMs, they are mostly ∥ to the rotation axis; and, second, most pulsar radiation is heavily processed by the magnetospheric plasma such that the lowest altitude “parent” core emission is polarized ⊥ to the emitting field, propagating as the extraordinary (X) mode.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Evolution of finite viscous disks with time-independent viscosity

We find Green functions for the accretion disk with fixed outer radius and time-independent viscosity. With the Green functions, a viscous evolution of the disk with any initial conditions can be described. Two types of inner boundary conditions are considered: the zero stress tensor and the zero accretion rate. The variable mass inflow at the outer radius can also be included. The well-known exponential decline of the accretion rate is a part of the solution with the inner zero stress tensor. The solution with the zero central accretion rate is applicable to disks around stars whose magnetosphere's boundary exceeds the corotation radius. Using the solution, the viscous evolution of disks in some binary systems can be studied. We apply the solution with zero inner stress tensor to outbursts of short-period X-ray transients during the time around the peak. It is found that for the Kramers’ regime of opacity and the initial surface density proportional to the radius, the rise time to the peak is t{sub rise}≈0.15 r{sub out}{sup 2}/ν{sub out} and the e-folding time of the decay is t{sub exp}≈0.45 r{sub out}{sup 2}/ν{sub out}. Comparison to non-stationary α-disks shows that both models with the same value of viscosity at the outer radius produce similar behavior on the viscous time-scale. For six bursts in X-ray novae, which exhibit fast-rise–exponential-decay and are fitted by the model, we find a way to restrict the turbulent parameter α.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Revisiting the Local Leo Cold Cloud and revised constraints on the Local Hot Bubble

The Local Leo Cold Cloud (LLCC, at a distance of 11–24 pc) was studied in its relation to the Local Hot Bubble (LHB) and the result suggested that much of the observed 1/4 keV emission in that direction originates in front of the cloud. This placed a strong constraint on the distribution of X-ray emission within the LHB and called into question the assumption of a uniform distribution of X-ray emitting plasma within the Local Cavity. However, recent work has quantified the contribution of heliospheric solar wind charge exchange (SWCX) emission to the diffuse X-ray background measured by the ROSAT All-Sky Survey (RASS) at 1/4 keV, and led to the consistency of pressure measurements between the LHB and the local cloud component of the complex of local interstellar clouds (CLICs) surrounding the Sun. In this paper we revisit the LLCC and improve the previous analysis by using higher resolution RASS data, a serendipitous ROSAT pointed observation, a rigorous treatment of the band-averaged X-ray absorption cross section, and models for the heliospheric and magnetospheric SWCX contributions. We find that the foreground emission to the cloud is in excess of the expected heliospheric (interplanetary plus near Earth) SWCX contribution but that it is marginally consistent with the range of possible LHB plasma path lengths between the LLCC and the CLICs given the currently understood plasma emissivity.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

A low-mass main-sequence star and accretion disk in the very faint X-ray transient M15 X-3

We present near-simultaneous Chandra/Hubble Space Telescope (HST) observations of the very faint (L{sub x} < 10{sup 36} erg s{sup −1}) X-ray transient source M15 X-3, as well as unpublished archival Chandra observations of M15 X-3. The Chandra observations constrain the luminosity of M15 X-3 to be <10{sup 34} erg s{sup −1} in all observed epochs. The X-ray spectrum shows evidence of curvature, and prefers a fit to a broken power law with break energy E{sub break}=2.7{sub −0.6}{sup +0.4} keV, and power-law indices of Γ{sub 1}=1.3{sub −0.2}{sup +0.1} and Γ{sub 2}=1.9{sub −0.2}{sup +0.2} over a single power law. We fit our new F438W (B), F606W (broad V), and F814W (I) HST data on the blue optical counterpart with a model for an accretion disk and a metal-poor main-sequence star. From this fit, we determine the companion to be consistent with a main-sequence star of mass 0.440{sub −0.060}{sup +0.035} M{sub ⊙} in a ∼4 hr orbit. X-ray irradiation of the companion is likely to be a factor in the optical emission from the system, which permits the companion to be smaller than calculated above, but larger than 0.15 M{sub ⊙} at the 3σ confidence level. M15 X-3 seems to be inconsistent with all suggested hypotheses explaining very faint transient behavior, except for magnetospherically inhibited accretion.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

SHORT- AND LONG-TERM RADIO VARIABILITY OF YOUNG STARS IN THE ORION NEBULA CLUSTER AND MOLECULAR CLOUD

We have used the Karl G. Jansky Very Large Array (VLA) to carry out multi-epoch radio continuum monitoring of the Orion Nebula Cluster (ONC) and the background Orion Molecular Cloud (OMC; 3 epochs at Q band and 11 epochs at Ka band). Our new observations reveal the presence of 19 radio sources, mainly concentrated in the Trapezium Cluster and the Orion Hot Core (OHC) regions. With the exception of the Becklin–Neugebauer object and source C (which we identify here as dust emission associated with a proplyd) the sources all show radio variability between the different epochs. We have found tentative evidence of variability in the emission from the massive object related to source I. Our observations also confirm radio flux density variations of a factor >2 on timescales of hours to days in five sources. One of these flaring sources, OHC-E, has been detected for the first time. We conclude that the radio emission can be attributed to two different components: (i) highly variable (flaring) non-thermal radio gyrosynchrotron emission produced by electrons accelerated in the magnetospheres of pre-main-sequence low-mass stars and (ii) thermal emission due to free–free radiation from ionized gas and/or heated dust around embedded massive objects and proplyds. Combining our sample with other radio monitoring at 8.3 GHz and the X-ray catalog provided by Chandra, we have studied the properties of the entire sample of radio/X-ray stars in the ONC/OMC region (51 sources). We have found several hints of a relation between the X-ray activity and the mechanisms responsible for (at least some fraction of) the radio emission. We have estimated a radio flaring rate of ∼0.14 flares day{sup −1} in the dense stellar cluster embedded in the OHC region. This suggests that radio flares are more common events during the first stages of stellar evolution than previously thought. The advent of improved sensitivity with the new VLA and ALMA will dramatically increase the number of stars in young clusters detected at radio wavelengths, which will help us improve our understanding of the origin and nature of the radio emission.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

PHOTOSPHERIC ABUNDANCES OF POLAR JETS ON THE SUN OBSERVED BY HINODE

Many jets are detected at X-ray wavelengths in the Sun's polar regions, and the ejected plasma along the jets has been suggested to contribute mass to the fast solar wind. From in situ measurements in the magnetosphere, it has been found that the fast solar wind has photospheric abundances while the slow solar wind has coronal abundances. Therefore, we investigated the abundances of polar jets to determine whether they are the same as that of the fast solar wind. For this study, we selected 22 jets in the polar region observed by Hinode/EUV Imaging Spectroscopy (EIS) and X-ray Telescope (XRT) simultaneously on 2007 November 1–3. We calculated the First Ionization Potential (FIP) bias factor from the ratio of the intensity between high (S) and low (Si, Fe) FIP elements using the EIS spectra. The values of the FIP bias factors for the polar jets are around 0.7–1.9, and 75% of the values are in the range of 0.7–1.5, which indicates that they have photospheric abundances similar to the fast solar wind. The results are consistent with the reconnection jet model where photospheric plasma emerges and is rapidly ejected into the fast wind.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

AN EXTRAORDINARY OUTBURST OF THE MAGNETAR SWIFT J1822.3–1606

The 2011 outburst of Swift J1822.3–1606 was extraordinary; periodic modulations at the spin period of the underlying neutron star were clearly visible, remarkably similar to what is observed during the decaying tail of magnetar giant flares. We investigated the temporal characteristics of X-ray emission during the early phases of the outburst. We performed a periodicity search with the spectral hardness ratio (HR) and found a coherent signal near the spin period of the neutron star, but with a lag of about 3 radians. Therefore, the HR is strongly anti-correlated with the X-ray intensity, which is also seen in the giant flares. We studied the time evolution of the pulse profile and found that it evolves from a complex morphology to a much simpler shape within about a month. Pulse profile simplification also takes place during the giant flares, but on a much shorter timescale of about a few minutes. We found that the amount of energy emitted during the first 25 days of the outburst is comparable to what was detected in minutes during the decaying tail of giant flares. Based on these similarities, we suggest that the triggering mechanisms of the giant flares and the magnetar outbursts are likely the same. We propose that the trapped fireball that develops in the magnetosphere at the onset of the outburst radiates away efficiently in minutes in magnetars exhibiting giant flares, while in other magnetars, such as Swift J1822.3–1606, the efficiency of radiation of the fireball is not as high and, therefore, lasts much longer.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

An empirical sequence of disk gap opening revealed by rovibrational CO

The fundamental rovibrational band of CO near 4.7 μm is a sensitive tracer of the presence and location of molecular gas in the planet-forming region of protoplanetary disks at 0.01–10 AU. We present a new analysis of a high-resolution spectral survey (R ∼ 96,000, or ∼3.2 km s{sup −1}) of CO rovibrational lines from protoplanetary disks spanning a wide range of stellar masses and of evolutionary properties. We find that the CO emission originates in two distinct velocity components. Line widths of both components correlate strongly with disk inclination, as expected for gas in Keplerian rotation. By measuring the line flux ratios between vibrational transitions F{sub v=2−1}/F{sub v=1−0}, we find that the two velocity components are clearly distinct in excitation. The broad component (FWHM=50−200 km s{sup −1}) probes the disk region near the magnetospheric accretion radius at ≈0.05 AU, where the gas is hot (800−1500 K). The narrow component (FWHM=10−50 km s{sup −1}) probes the disk at larger radii of 0.1–10 AU, where the gas is typically colder (200–700 K). CO excitation temperatures and orbital radii define an empirical temperature–radius relation as a power law with index −0.3 ± 0.1 between 0.05 and 3 AU. The broad CO component, co-spatial with the observed orbital distribution of hot Jupiters, is rarely detected in transitional and Herbig Ae disks, providing evidence for an early dissipation of the innermost disk. An inversion in the temperature profile beyond 3 AU is interpreted as a tracer of a regime dominated by UV pumping in largely devoid inner disks, and may be a signature of the last stage before the disk enters the gas-poor debris phase.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

DISSECTING MAGNETAR VARIABILITY WITH BAYESIAN HIERARCHICAL MODELS

Neutron stars are a prime laboratory for testing physical processes under conditions of strong gravity, high density, and extreme magnetic fields. Among the zoo of neutron star phenomena, magnetars stand out for their bursting behavior, ranging from extremely bright, rare giant flares to numerous, less energetic recurrent bursts. The exact trigger and emission mechanisms for these bursts are not known; favored models involve either a crust fracture and subsequent energy release into the magnetosphere, or explosive reconnection of magnetic field lines. In the absence of a predictive model, understanding the physical processes responsible for magnetar burst variability is difficult. Here, we develop an empirical model that decomposes magnetar bursts into a superposition of small spike-like features with a simple functional form, where the number of model components is itself part of the inference problem. The cascades of spikes that we model might be formed by avalanches of reconnection, or crust rupture aftershocks. Using Markov Chain Monte Carlo sampling augmented with reversible jumps between models with different numbers of parameters, we characterize the posterior distributions of the model parameters and the number of components per burst. We relate these model parameters to physical quantities in the system, and show for the first time that the variability within a burst does not conform to predictions from ideas of self-organized criticality. We also examine how well the properties of the spikes fit the predictions of simplified cascade models for the different trigger mechanisms.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Magnetohydrodynamics using path or stream functions

Magnetization in highly conductive plasmas is ubiquitous to astronomical systems. Flows in such media can be described by three path functions Λ{sub α}, or, for a steady flow, by two stream functions λ{sub κ} and an additional field such as mass density ρ, velocity v, or travel time Δt. While typical analyses of a frozen magnetic field B are problem-specific and involve nonlocal gradients of the fluid element position x(t), we derive the general, local (in Λ or λ space) solution B=(∂x/∂Λ{sub α}){sub t} B-tilde {sub α}ρ/ ρ-tilde , where Lagrangian constants denoted by a tilde are directly fixed at a boundary hypersurface H-tilde on which B is known. For a steady flow, ρ-tilde B/ρ=(∂x/∂λ{sub κ}){sub Δt} B-tilde {sub κ}+v B-tilde {sub 3}/ v-tilde ; here the electric field E∼( B-tilde {sub 2}∇λ{sub 1}− B-tilde {sub 1}∇λ{sub 2})/ ρ-tilde depends only on λ{sub κ} and the boundary conditions. Illustrative special cases include compressible axisymmetric flows and incompressible flows around a sphere, showing that viscosity and compressibility enhance the magnetization and lead to thicker boundary layers. Our method is especially useful for directly computing electric fields, and for addressing upstream magnetic fields that vary in spacetime. We thus estimate the electric fields above heliospheres and magnetospheres, compute the draping of magnetic substructure around a planetary body, and demonstrate the resulting inverse polarity reversal layer. Our analysis can be immediately incorporated into existing hydrodynamic codes that are based on stream or path functions, to passively evolve the electromagnetic fields in a simulated flow. Furthermore, in such a prescription the electromagnetic fields are frozen onto the grid, so it may be developed into a fully magnetohydrodynamic, efficient simulation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Analytic properties of force-free jets in the Kerr spacetime—I

The Blandford–Znajek (BZ) mechanism describes a process for extracting the rotation energy from a spinning black hole (BH) via magnetic field lines penetrating the event horizon of the central BH. In this paper, we present a perturbation approach to study force-free jets launched by the BZ mechanism, and then discuss its two immediate applications: (1) we present a high-order split-monopole perturbation solution to the BZ mechanism which accurately pins down the energy extraction rate E-dot and well describes the structure of the BH magnetosphere for the entire range of BH spins (0 ≤ a ≤ 1); (2) the approach yields an exact constraint for the monopole field configuration in Kerr spacetime, I=Ω(1−A{sub ϕ}{sup 2}), where A{sub ϕ} is the ϕ component of the vector potential of electromagnetic field, Ω is the angular velocity of the magnetic field lines, and I is the poloidal electric current. The constraint is of particular importance to benchmark the accuracy of numerical simulations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

BROADBAND X-RAY SPECTRAL INVESTIGATIONS OF MAGNETARS, 4U 0142+61, 1E 1841–045, 1E 2259+586, AND 1E 1048.1–5937

We have generated an extended version of a rather simplified but physically oriented three-dimensional magnetar emission model, STEMS3D, to allow spectral investigations up to 100 keV. We then applied our model to the broadband spectra of four magnetars: 4U 0142+61, 1E 1841–045, 1E 2259+586, and 1E 1048.1–5937, using data collected with Swift/XRT or XMM-Newton in soft X-rays, and the Nuclear Spectroscopic Telescope Array in the hard X-ray band. We found that the hard X-ray emission of 4U 0142+61 was spectrally hard compared to earlier detections, indicating that the source was likely in a transition to or from a harder state. We find that the surface properties of the four magnetars are consistent with what we have obtained using only the soft X-ray data with STEMS3D, implying that our physically motivated magnetar emission model is a robust tool. Based on our broadband spectral investigations, we conclude that resonant scattering of the surface photons in the magnetosphere alone cannot account for the hard X-ray emission in magnetars; therefore, an additional non-thermal process, or a population of relativistic electrons is required. We also discuss the implication of the non-detection of persistent hard X-ray emission in 1E 1048.1–5937.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Plastic damping of Alfvén waves in magnetar flares and delayed afterglow emission

Magnetar flares generate Alfvén waves bouncing in the closed magnetosphere with energy up to ∼10{sup 46} erg. We show that on a timescale of 10 ms the waves are transmitted into the star and form a compressed packet of high energy density. This packet strongly shears the stellar crust and initiates a plastic flow, heating the crust and melting it hundreds of meters below the surface. A fraction of the deposited plastic heat is eventually conducted to the stellar surface, contributing to the surface afterglow months to years after the flare. A large fraction of heat is lost to neutrino emission or conducted into the core of the neutron star.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

ON THE POLARIZATION PROPERTIES OF MAGNETAR GIANT FLARE PULSATING TAILS

Three giant flares have been detected so far from soft gamma-ray repeaters, each characterized by an initial short hard spike and a pulsating tail. The observed pulsating tails are characterized by a duration of ∼100 s, an isotropic energy of ∼10{sup 44} erg, and a pulse period of a few seconds. The pulsating tail emission likely originates from the residual energy after the intense energy release during the initial spike, which forms a trapped fireball composed of a photon-pair plasma in a closed-field-line region of the magnetars. Observationally the spectra of pulsating tails can be fitted by the superposition of a thermal component and a power-law component, with the thermal component dominating the emission in the early and late stages of the pulsating-tail observations. In this paper, assuming that the trapped fireball is from a closed-field-line region in the magnetosphere, we calculate the atmospheric structure of the optically thick trapped fireball and the polarization properties of the trapped fireball. By properly treating the photon propagation in a hot, highly magnetized, electron–positron pair plasma, we tally photons in two modes (O mode and E mode) at a certain observational angle through Monte Carlo simulations. Our results suggest that the polarization degree depends on the viewing angle with respect to the magnetic axis of the magnetar, and can be as high as Π ≃ 30% in the 1–30 keV band, and Π ≃ 10% in the 30–100 keV band, if the line of sight is perpendicular to the magnetic axis.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

X-RAY EMISSION FROM MAGNETIC MASSIVE STARS

Magnetically confined winds of early-type stars are expected to be sources of bright and hard X-rays. To clarify the systematics of the observed X-ray properties, we have analyzed a large series of Chandra and XMM-Newton observations, corresponding to all available exposures of known massive magnetic stars (over 100 exposures covering ∼60% of stars compiled in the catalog of Petit et al.). We show that the X-ray luminosity is strongly correlated with the stellar wind mass-loss rate, with a power-law form that is slightly steeper than linear for the majority of the less luminous, lower- M-dot B stars and flattens for the more luminous, higher- M-dot O stars. As the winds are radiatively driven, these scalings can be equivalently written as relations with the bolometric luminosity. The observed X-ray luminosities, and their trend with mass-loss rates, are well reproduced by new MHD models, although a few overluminous stars (mostly rapidly rotating objects) exist. No relation is found between other X-ray properties (plasma temperature, absorption) and stellar or magnetic parameters, contrary to expectations (e.g., higher temperature for stronger mass-loss rate). This suggests that the main driver for the plasma properties is different from the main determinant of the X-ray luminosity. Finally, variations of the X-ray hardnesses and luminosities, in phase with the stellar rotation period, are detected for some objects and they suggest that some temperature stratification exists in massive stars' magnetospheres.

79 ASTRONOMY AND ASTROPHYSICS↗