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Pulsating ultraluminous X-ray sources are thought to be X-ray bright, accreting, magnetized neutron stars. Their measured soft (<20 keV) X-ray luminosity can exceed the Eddington luminosity for a neutron star by a few orders of magnitude. Although several scenarios have been proposed to explain the different components observed in the X-ray spectrum and the characteristics of the X-ray pulse profile of these systems, detailed quantitative calculations are still missing. In particular, the observed soft X-ray pulse profiles are almost sinusoidal and show an increase in the pulsed fraction (from 8 up to even 30 per cent) with increasing energy. In this work, we discuss how emission originating from an optically thick envelope, expected to be formed during super-Eddington accretion, can match the observed spectra and pulse profiles. [ABSTRACT FROM AUTHOR]

Detailed studies of the Sun have shown that sunspots and solar flares are closely correlated. Photometric data from Kepler / K2 has allowed similar studies to be carried out on other stars. Here, we utilize Transiting Exoplanet Survey Satellite (TESS) photometric 2-min cadence of 167 low-mass stars from Sectors 1 to 3 to investigate the relationship between star-spots and stellar flares. From our sample, 90 per cent show clear rotational modulation likely due to the presence of a large, dominant star-spot and we use this to determine a rotational period for each star. Additionally, each low-mass star shows one or more flares in its light curve and using Gaia Data Release 2 parallaxes and SkyMapper magnitudes we can estimate the energy of the flares in the TESS band-pass. Overall, we have 1834 flares from the 167 low-mass stars with energies from 6.0 × 1029 to 2.4 × 1035 erg. We find none of the stars in our sample show any preference for rotational phase, suggesting the lack of a correlation between the large, dominant star-spot, and flare number. We discuss this finding in greater detail and present further scenarios to account for the origin of flares on these low-mass stars. [ABSTRACT FROM AUTHOR]

Classification of stellar spectra from voluminous spectra is a very important and challenging task. In order to better classify stellar spectra, inspired by the principle of deep convolutional neural network (CNN), we propose a supervised algorithm for stellar spectra classification based on 1D stellar spectra convolutional neural network (1D SSCNN). In 1D SSCNN, we modify the traditional 2D convolutional neural network into 1D network to adapt to the spectral classification. On the basis of using convolution algorithm, the spectral features are extracted and used for classification. We first use the stellar spectra data to train a 1D SSCNN to obtain a well-trained model, and then we apply the well-trained model to classify the unknown spectra. To evaluate the performance of the proposed algorithms, we apply 1D SSCNN to classify three spectral types: F-type spectra, G-type spectra, and K-type spectra and 10 subclasses of K-type spectra: A0-type, A5-type, F0-type, F5-type, G0-type, G5-type, K0-type, K5-type, M0-type, and M5-type spectra from Sloan Digital Sky Survey (SDSS). Our 1D SSCNN algorithm obtain higher classification accuracy compared with support vector machine (SVM), random forest (RF), and artificial neural network (ANN). [ABSTRACT FROM AUTHOR]

We present an analysis of K2 short cadence data of 34 M dwarfs which have spectral types in the range M0–L1. Of these stars, 31 showed flares with a duration between ∼10 and 90min. Using distances obtained from Gaia DR2 parallaxes, we determined the energy of the flares to be in the range ∼1.2 × 1029–6 × 1034 erg. In agreement with previous studies we find rapidly rotating stars tend to show more flares, with evidence for a decline in activity in stars with rotation periods longer than ∼10 d. The rotational modulation seen in M dwarf stars is widely considered to result from a starspot which rotates in and out of view. Flux minimum is therefore the rotation phase where we view the main starspot close to the stellar disc centre. Surprisingly, having determined the rotational phase of each flare in our study we find none show any preference for rotational phase. We outline three scenarios which could account for this unexpected finding. The relationship between rotation phase and flare rate will be explored further using data from wide surveys such as NGTS and TESS. [ABSTRACT FROM AUTHOR]

We present the results of a series of short radio observations of six ultracool dwarfs made using the upgraded Very Large Array in S (2-4GHz) and C (4-7GHz) bands. LSR J1835+3259 exhibits a 100 per cent right-hand circularly polarized burst that shows intense narrow-band features with a fast negative frequency drift of about -30 MHz s-1. They are superimposed on a fainter broad-band emission feature with a total duration of about 20 min, bandwidth of about 1 GHz, centred at about 3.5 GHz, and a slow positive frequency drift of about 1 MHz s-1. This makes it the first such event detected below 4 GHz and the first one exhibiting both positive and negative frequency drifts. Polarized radio emission is also seen in 2MASS J00361617+1821104 and NLTT 33370, while LP 349-25 and TVLM 513-46546 have unpolarized emission and BRI B0021-0214 was not detected. We can reproduce the main characteristics of the burst from LSR J1835+3259 using a model describing the magnetic field of the dwarf as a tilted dipole. We also analyse the origins of the quiescent radio emission and estimate the required parameters of the magnetic field and energetic electrons. Although our results are non-unique, we find a set of models that agree well with the observations. [ABSTRACT FROM AUTHOR]

The 'Magnificent Seven' (M7) are a group of radio-quiet isolated neutron stars discovered in the soft X-rays through their purely thermal surface emission. Owing to the large inferred magnetic fields (B ≈ 1013 G), radiation from these sources is expected to be substantially polarized, independently of the mechanism actually responsible for the thermal emission. A large observed polarization degree (PD) is, however, expected only if quantum-electrodynamic (QED) polarization effects are present in the magnetized vacuum around the star. The detection of a strong linearly polarized signal would therefore provide the first observational evidence of QED effects in the strong-field regime. While polarization measurements in the soft X-rays are not feasible yet, optical polarization measurements are within reach also for quite faint targets, like the M7 which have optical counterparts with magnitudes ≈26-28. Here, we report on the measurement of optical linear polarization for the prototype, and brightest member, of the class, RX J1856.5-3754 (V ~ 25.5), the first ever for one of the M7, obtained with the Very Large Telescope. We measured a PD = 16.43 ± 5.26 per cent and a polarization position angle PA = ... ± ..., computed east of the North Celestial Meridian. The PD that we derive is large enough to support the presence of vacuum birefringence, as predicted by QED. [ABSTRACT FROM AUTHOR]

The observed polarization properties of thermal radiation from isolated, cooling neutron stars depend on both the emission processes at the surface and the effects of the magnetized vacuum which surrounds the star. Here, we investigate the polarized thermal emission from X-ray dim isolated neutron stars, taking RX J1856.5-3754 as a representative case. The physical conditions of the star outermost layers in these sources is still debated, and so we consider emission from a magnetized atmosphere and a condensed surface, accounting for the effects of vacuum polarization as the radiation propagates in the star magnetosphere. We have found that, for a significant range of viewing geometries, measurement of the phase-averaged polarization fraction and phase-averaged polarization angle at both optical and X-ray wavelengths allow us to determine whether this neutron star has an atmosphere or a condensed surface. Our results may therefore be relevant in view of future developments of soft X-ray polarimeters. [ABSTRACT FROM AUTHOR]

We revisit the discovery outburst of the X-ray transient XTE J1550−564 during which relativistic jets were observed in 1998 September, and review the radio images obtained with the Australian Long Baseline Array, and light curves obtained with the Molonglo Observatory Synthesis Telescope and the Australia Telescope Compact Array. Based on Hi spectra, we constrain the source distance to between 3.3 and 4.9 kpc. The radio images, taken some 2 d apart, show the evolution of an ejection event. The apparent separation velocity of the two outermost ejecta is at least and may be as large as ; when relativistic effects are taken into account, the inferred true velocity is . The flux densities appear to peak simultaneously during the outburst, with a rather flat (although still optically thin) spectral index of −0.2. [ABSTRACT FROM AUTHOR]

Compton scattering within the accretion column of magnetic cataclysmic variables (mCVs) can induce a net polarization in the X-ray emission. We investigate this process using Monte Carlo simulations and find that significant polarization can arise as a result of the stratified flow structure in the shock-ionized column. We find that the degree of linear polarization can reach levels up to ∼8 per cent for systems with high accretion rates and low white dwarf masses, when viewed at large inclination angles with respect to the accretion column axis. These levels are substantially higher than previously predicted estimates using an accretion column model with uniform density and temperature. We also find that for systems with a relatively low-mass white dwarf accreting at a high accretion rate, the polarization properties may be insensitive to the magnetic field, since most of the scattering occurs at the base of the accretion column where the density structure is determined mainly by bremsstrahlung cooling instead of cyclotron cooling. [ABSTRACT FROM AUTHOR]

Compton scattering of X-rays in the bulk flow of the accretion column in magnetic cataclysmic variables (mCVs) can significantly shift photon energies. We present Monte Carlo simulations based on a non-linear algorithm demonstrating the effects of Compton scattering on the H-, He-like and neutral Fe Kα lines produced in the post-shock region of the accretion column. The peak line emissivities of the photons in the post-shock flow are taken into consideration and frequency shifts due to Doppler effects are also included. We find that line profiles are most distorted by Compton scattering effects in strongly magnetized mCVs with a low white dwarf mass and high mass accretion rate and which are viewed at an oblique angle with respect to the accretion column. The resulting line profiles are most sensitive to the inclination angle. We have also explored the effects of modifying the accretion column width and using a realistic emissivity profile. We find that these do not have a significant overall effect on the resulting line profiles. A comparison of our simulated line spectra with high-resolution Chandra/HETGS observations of the mCV GK Per indicates that a wing feature redward of the 6.4-keV line may result from Compton recoil near the base of the accretion column. [ABSTRACT FROM AUTHOR]

We present anXMM–Newtonobservation of the Seyfert–LINER (low-ionization nuclear emission-line region) galaxy NGC 7213. The RGS soft X-ray spectrum is well fitted with a power law plus soft X-ray collisionally ionized thermal plasma. We confirm the presence of Fe i, Fe xxvand Fe xxviKα emission in the EPIC spectrum and set tighter constraints on their equivalent widths ofand 24+10−13 eV, respectively. We compare the observed properties together with the inferred mass accretion rate of NGC 7213 with those of other Seyfert and LINER galaxies. We find that NGC 7213 has intermediate X-ray spectral properties lying between those of the weak active galactic nucleus found in the LINER M81 and higher-luminosity Seyfert galaxies. There appears to be a continuous sequence of X-ray properties from the Galactic Centre through LINER galaxies to Seyferts, probably determined by the amount of material available for accretion in the central regions. [ABSTRACT FROM AUTHOR]

We present soft X-ray spectroscopy of the black hole binary LMC X-3 from the XMM–Newton Reflection Grating Spectrometer. The observations span the full range of spectral states seen in LMC X-3. The spectra are completely dominated by continuum emission, and the neutral absorbing column measured from the O i edge is consistent with the Galactic interstellar column density towards LMC X-3. We find no evidence for variability of the neutral absorbing column. We also constrain the ionized column density using the upper limits to the equivalent width of the O ii–O viii K-shell resonance lines: we find that the equivalent hydrogen column density of gas in which O is partially ionized is . From this upper limit we can rule out a line-driven stellar wind as the power source for the X-ray emission of LMC X-3 except when it is faint. At wavelengths longward of the peak emission the spectral shape is well described by a multitemperature disc–blackbody spectrum; the power-law component, which dominates at shorter wavelengths, does not continue longward of the disc–blackbody peak. This implies that the multitemperature disc–blackbody component supplies the seed photons that are Compton upscattered in the hot corona, consistent with the standard paradigm for black hole X-ray binary spectral states. [ABSTRACT FROM AUTHOR]

Abstract The X-ray-bright Seyfert 1 galaxy III Zw 2 was observed with XMM–Newton in 2000 July. Its X-ray spectrum can be described by a power law of photon index Γ= 1.7 and an extremely broad (FWHM∼ 140 000 km s[sup -1] ) Fe Kα line at 6.44 keV. The iron line has an equivalent width of ∼800 eV. To study the long-term X-ray behaviour of the source we have analysed 25 yr of data, from 1975 to 2000. There is no evidence of significant intrinsic absorption within the source or of a soft X-ray excess in the XMM or archival data. We do not detect rapid X-ray variability (a few × 10[sup 3] s ) during any of the individual observations; however, on longer time-scales (a few years) the X-ray light curve shows 10-fold flux variations. We infer a black hole mass of ∼10[sup 9] M[sub ⊙] (from Hβ FWHM) for III Zw 2 which is much higher than some previous estimates. A comparison of X-ray variability with light curves at other wavelengths over a 25-yr period reveals correlated flux variations from radio to X-ray wavelengths. We interpret the variable radio to optical emission as synchrotron radiation, self-absorbed in the radio/millimetre region, and the X-rays mainly as a result of Compton up-scattering of low-energy photons by the population of high-energy electrons that give rise to the synchrotron radiation. [ABSTRACT FROM AUTHOR]

Using a parametrized function for the mass loss at the base of the post-shock region, we have constructed a formulation for magnetically confined accretion flows which avoids singularities, such as the infinity in density, at the base associated with all previous formulations. With the further inclusion of a term allowing for the heat input into the base from the accreting white dwarf, we are also able to obtain the hydrodynamic variables to match the conditions in the stellar atmosphere. (We do not, however, carry out a mutually consistent analysis for the match.) Changes to the emitted X-ray spectra are negligible unless the thickness of mass leakage region at the base approaches or exceeds one per cent of the height of the post-shock region. In this case the predicted spectra from higher-mass white dwarfs will be harder, and fits to X-ray data will predict lower white dwarf masses than previous formulations. [ABSTRACT FROM AUTHOR]

Sharp dips observed in the pulse profiles of three X-ray pulsars (GX 1+4, RX J0812.4-3114 and A 0535+26) have previously been suggested to arise from partial eclipses of the emission region by the accretion column occurring once each rotation period. We present pulse-phase spectroscopy from Rossi X-ray Timing Explorer satellite observations of GX 1+4 and RX J0812.4-3114, which for the first time confirms this interpretation. The dip phase corresponds to the closest approach of the column axis to the line of sight, and the additional optical depth of photons escaping from the column in this direction gives rise to both the decrease in flux and increase in the fitted optical depth measured at this phase. Analysis of the arrival time of individual dips in GX 1+4 provides the first measurement of azimuthal wandering of a neutron star accretion column. The column longitude varies stochastically with a standard deviation ranging between 2° and 6° depending on the source luminosity. Measurements of the phase width of the dip both from mean pulse profiles and from the individual eclipses demonstrate that the dip width is proportional to the flux. The variation is consistent with that expected if the azimuthal extent of the accretion column depends only upon the Keplerian velocity at the inner disc radius, which varies as a consequence of the accretion rate M·. [ABSTRACT FROM AUTHOR]

We present a general formulation for stability analyses of radiative shocks with multiple cooling processes, longitudinal and transverse perturbations, and unequal electron and ion temperatures. Using the accretion shocks of magnetic cataclysmic variables as an illustrative application, we investigate the shock instabilities by examining the eigenfunctions of the perturbed hydrodynamic variables. We also investigate the effects of varying the condition at the lower boundary of the post-shock flow from a zero-velocity fixed wall to several alternative types of boundaries involving the perturbed hydrodynamic variables, and the variations of the emission from the post-shock flow under different modes of oscillations. We found that the stability properties for flow with a stationary-wall lower boundary are not significantly affected by perturbing the lower boundary condition, and they are determined mainly by the energy-transport processes. Moreover, there is no obvious correlation between the amplitude or phase of the luminosity response and the stability properties of the system. Stability of the system can, however, be modified in the presence of transverse perturbation. The luminosity responses are also altered by transverse perturbation. [ABSTRACT FROM AUTHOR]

We have detected the optical counterpart of the proposed double degenerate polar RX J1914+24. The I-band light curve is modulated on the 9.5-min period seen in X-rays. There is no evidence for any other periods. No significant modulation is seen in J. The infrared colours of RX J1914+24 are not consistent with a main-sequence dwarf secondary star. Our ASCA spectrum of RX J1914+24 is typical of a heavily absorbed polar and our ASCA light curve also shows only the 9.5-min period. We find that the folded I band and X-ray light curves are out of phase. We attribute the I-band flux to the irradiated face of the donor star. The long-term X-ray light curve shows a variation in the observed flux of up to an order of magnitude. These observations strengthen the view that RX J1914+24 is indeed the first double degenerate polar to be detected. In this light, we discuss the synchronizing mechanisms in such a close binary and other system parameters. [ABSTRACT FROM AUTHOR]

The structure of the hot downstream region below a radiative accretion shock, such as that of an accreting compact object, may oscillate because of a global thermal instability. The oscillatory behaviour depends on the functional forms of the cooling processes, the energy exchanges of electrons and ions in the shock-heated matter, and the boundary conditions. We analyse the stability of a shock with unequal electron and ion temperatures, where the cooling consists of thermal bremsstrahlung radiation which promotes instability, plus a competing process which tends to stabilize the shock. The effect of transverse perturbations is considered also. As an illustration, we study the special case in which the stabilizing cooling process is of order 3/20 in density and 5/2 in temperature, which is an approximation for the effects of cyclotron cooling in magnetic cataclysmic variables. We vary the efficiency of the second cooling process, the strength of the electron–ion exchange and the ratio of electron and ion pressures at the shock, to examine particular effects on the stability properties and frequencies of oscillation modes. [ABSTRACT FROM AUTHOR]

We carried out spectroscopic observations of the candidate black hole binary GX 339-4 during its low–hard and high–soft X-ray states. We have found that the spectrum is dominated by emission lines of neutral elements with asymmetric, round-topped profiles in the low–hard state. In the high–soft state, however, the emission lines from both neutral and ionized elements have unambiguously resolved double-peaked profiles. The detection of double-peaked emission lines in the high–soft state, with a larger peak separation for higher ionization lines, indicates the presence of an irradiatively heated accretion disc. The round-topped lines in the low–hard state are probably caused by a dense matter outflow from an inflated non-Keplerian accretion disc. Our data do not show velocity modulations of the line centres caused by the orbital motion of the compact object, neither do the line basewidths show substantial variations in each observational epoch. There are no detectable absorption lines from the companion star. All these features are consistent with those of a system with a low-mass companion star and low orbital inclination. [ABSTRACT FROM AUTHOR]

We carried out I, R, V and B photometric observations of the neutron star X-ray binary RXTE J2123-058 shortly after the end of the X-ray outburst in mid-1998. We adopt the low-mass binary model to interpret our observations. After folding our data on the 0.24 821-d orbital period, and correcting for the steady brightness decline following the outburst, we observed sinusoidal oscillations with hints of ellipsoidal modulations which became progressively more evident. Our data also show that the decline in brightness was faster in the V band than in the R and I bands. This suggests both the cooling of an irradiation-heated secondary star and the fading of an accretion disc over the nights of our observations. [ABSTRACT FROM AUTHOR]

ABSTRACT We calculate the temperature and density structure of the hot post-shock plasma in magnetically confined accretion flows, including the gravitational potential. This avoids the inconsistency of previous calculations which assume that the height of the shock is negligible. We assume a stratified accretion column with 1D flow along the symmetry axis. We find that the calculations predict a lower shock temperature than previous calculations, with a flatter temperature profile with height. We revise previous determinations of the masses of the white dwarf primary stars, and find that for higher mass white dwarfs there is a general reduction in derived masses when the gravitational potential is included. This is because the spectrum from such flows is harder than that of previous prescriptions at intermediate energies. [ABSTRACT FROM AUTHOR]

We present new optical and infrared (IR) observations of Cir X-1 taken near apastron. Both sets of spectra show asymmetric emission lines. Archival optical observations show that an asymmetric Hα emission line has been in evidence for the past 20 years, although the shape of the line has changed significantly. We present an eccentric (e∼0.7–0.9) low-mass binary model, where the system consists of a neutron star orbiting around a (sub)giant companion star of 3–5 M[sub ⊙]. We suggest that the broad components of the emission lines arise in a high-velocity, optically thick flow near the neutron star, while the narrow components of the optical and the IR lines arise near the companion star and a heated ejecta shell surrounding the binary respectively. In this model, the velocity of the narrow component reflects the space velocity of the binary; the implied radial velocity (+430 km s[sup -1] after correcting for Galactic rotation) is the highest velocity known for an X-ray binary. [ABSTRACT FROM AUTHOR]

We have analysed the kinematical parameters of Cir X-1 to constrain the nature of its companion star, the eccentricity of the binary and the pre-supernova parameter space. We argue that the companion is most likely to be a low-mass (<=2.0 M⊙) unevolved star and that the eccentricity of the orbit is 0.94±0.04. We have evaluated the dynamical effects of the supernova explosion and we find it must have been asymmetric. On average, we find that a kick of ∼740 km s-1 is needed to account for the recently measured radial velocity of +430 km s-1 (Johnston, Fender & Wu) for this extreme system. The corresponding minimum kick velocity is ∼500 km s-1. This is the largest kick needed to explain the motion of any observed binary system. If Cir X-1 is associated with the supernova remnant G321.9-0.3 then we find a limiting minimum age of this remnant of ∼60 000 yr. Furthermore, we predict that the companion star has lost ∼10 per cent of its mass as a result of stripping and ablation from the impact of the supernova shell shortly after the explosion. [ABSTRACT FROM AUTHOR]