Observed X-ray Light Curve Research Articles

The Impacts of Neutron Star Structure and Base Heating on Type I X-Ray Bursts and Code Comparison

Type I X-ray bursts are rapidly brightening phenomena triggered by thermonuclear burning on the accreting layers of a neutron star (NS). The light curves represent the physical properties of NSs and the nuclear reactions on the proton-rich nuclei. The numerical treatments of the accreting NS and physics of the NS interior are not established, which shows uncertainty in modeling for observed X-ray light curves. In this study, we investigate theoretical X-ray burst models compared with burst light curves with GS 1826-24 observations. We focus on the impacts of the NS mass and radius and base heating on the NS surface using the MESA code. We find a monotonic correlation between the NS mass and the parameters of the light curve. The higher the mass, the longer the recurrence time and the greater the peak luminosity. While the larger the radius, the longer the recurrence time, the peak luminosity remains nearly constant. In the case of increasing base heating, both the recurrence time and peak luminosity decrease. We also examine the above results with a different numerical code, HERES, based on general relativity and consider the central NS. We find that the burst rate, energy, and strength are almost the same in two X-ray burst codes by adjusting the base heat parameter in MESA (the relative errors ≲5%), while the duration and rise times are significantly different between (the relative error is possibly ∼50%). The peak luminosity and the e-folding time change irregularly between two codes for different accretion rates.

The Multipolar Magnetic Field of the Millisecond Pulsar PSR J0030+0451

Abstract Modeling of the NICER X-ray waveform of the pulsar PSR J0030+0451, aimed at constraining the neutron star mass and radius, has inferred surface hot spots (the magnetic polar caps) that imply significantly nondipolar magnetic fields. To this end, we investigate magnetic field configurations that comprise offset dipole-plus-quadrupole components using a static vacuum field and force-free global magnetosphere models. Taking into account the compactness and observer angle values provided by Miller et al. and Riley et al., we compute geodesics from the observer plane to the polar caps to compute the resulting X-ray light curve. We explore, through Markov Chain Monte Carlo techniques, the detailed magnetic field configurations that can reproduce the observed X-ray light curve and have discovered degeneracies, i.e., diverse field configurations, which can provide sufficient descriptions of the NICER X-ray waveforms. Having obtained the force-free field structures, we then compute the corresponding synchronous γ-ray light curves following Kalapotharakos et al.; these we compare to those obtained by Fermi-LAT, to provide models consistent with both the X-ray and the γ-ray data, thereby restricting further the multipole field parameters. An essential aspect of this approach is the proper computation of the relative phase between the synchronous X- and γ-ray light curves. We conclude with a discussion of the broader implications of our study.

CDF-S XT1 and XT2: White Dwarf Tidal Disruption Events by Intermediate-mass Black Holes?

Abstract Recently, two fast X-ray transients (XT1 and XT2) have been reported from the search in the Chandra Deep Field (CDF) data. Each transient shows an initial plateau lasting around hundreds to thousands of seconds followed by a rapid decay in the light curve. In particular, CDF-S XT2 is found to be associated with a galaxy at redshift z = 0.738 and was explained as a counterpart of a binary neutron-star merger event. In this Letter, motivated by the short duration and decay slopes of the two transients, we consider an alternative interpretation in which both events are accretion-driven flares from tidal disruption of white dwarfs by intermediate-mass black holes. We derive a theoretical model of the accretion rate history and find that it fits the observed X-ray light curves well. The extremely super-Eddington peak luminosity of XT2 can be explained by the beaming effect of the system, likely in the form of a jet.

Electromagnetic emission from newly born magnetar spin-down by gravitational-wave and magnetic dipole radiations

A newly-born magnetar is thought to be central engine of some long gamma-ray bursts (GRBs). We investigate the evolution of the electromagnetic (EM) emission from the magnetic dipole (MD) radiation wind injected by spin-down of a newly-born magnetar via both quadrupole gravitational-wave (GW) and MD radiations. We show that the EM luminosity evolves as $L_{\rm em}\propto (1+t/\tau_c)^{\alpha}$, and $\alpha$ is $-1$ and $-2$ in the GW and MD radiation dominated scenarios, respectively. Transition from the GW to MD radiation dominated epoch may show up as a smooth break with slope changing from $-1$ to $-2$. If the magnetar collapses to a black hole before $\tau_c$, the MD radiation should be shut down, then the EM light curve should be a plateau followed by a sharp drop. The expected generic light curve in this paradigm is consistent with the canonical X-ray light curve of {\em Swift} long GRBs. The X-ray emission of several long GRBs are identified and interpreted as magnetar spin-down via GW or MD, as well as constrain the physical parameters of magnetar. The combination of MD emission and GRB afterglows may make the diversity of the observed X-ray light curves. This may interpret the observed chromatic behaviors of the X-ray and optical afterglow light curves and the extremely low detection rate of a jet-like break in the X-ray afterglow light curves of long GRBs.

Understanding X-ray irradiation in low-mass X-ray binaries directly from their light-curves

The X-ray light-curves of the recurring outbursts observed in low-mass X-ray binaries provide strong test beds for constraining (still) poorly understood disc-accretion processes. These light-curves act as a powerful diagnostic to probe the physics behind the mechanisms driving mass inflow and outflow in these binary systems. We have thus developed an innovative methodology, combining a foundation of Bayesian statistics, observed X-ray light-curves, and accretion disc theory. With this methodology, we characterize the angular-momentum (and mass) transport processes in an accretion disc, as well as the properties of the X-ray irradiation-heating that regulates the decay from outburst maximum in low-mass X-ray transients. We recently applied our methodology to the Galactic black-hole low-mass X-ray binary population, deriving from their lightcurves the first-ever quantitative measurements of the $\alpha$-viscosity parameter in these systems \citep{tetarenko2018}. In this paper, we continue the study of these binaries, using Bayesian methods to investigate the X-ray irradiation of their discs during outbursts of strong accretion. We find that the predictions of the disc-instability model, assuming a source of X-ray irradiation proportional to the central accretion rate throughout outburst, do not adequately describe the later stages of BH-LMXB outburst light-curves. We postulate that the complex and varied light-curve morphology observed across the population is evidence for irradiation that varies in time and space within the disc, throughout individual transient outbursts. Lastly, we demonstrate the robustness of our methodology, by accurately reproducing the synthetic model light-curves computed from numerical codes built to simulate accretion flows in binary systems.

X-Ray Study of Variable Gamma-Ray Pulsar PSR J2021+4026

Abstract PSR J2021+4026 showed a sudden decrease in the gamma-ray emission at the glitch that occurred around 2011 October 16, and a relaxation of the flux to the pre-glitch state at around 2014 December. We report X-ray analysis results of the data observed by XMM-Newton on 2015 December 20 in the post-relaxation state. To examine any change in the X-ray emission, we compare the properties of the pulse profiles and spectra at the low gamma-ray flux state and at the post-relaxation state. The phase-averaged spectra for both states can be well described by a power-law component plus a blackbody component. The former is dominated by unpulsed emission and probably originated from the pulsar wind nebula as reported by Hui et al. The emission property of the blackbody component is consistent with the emission from the polar cap heated by the back-flow bombardment of the high-energy electrons or positrons that were accelerated in the magnetosphere. We found no significant change in the X-ray emission properties between two states. We suggest that the change of the X-ray luminosity is at an order of ∼4%, which is difficult to measure with the current observations. We model the observed X-ray light curve with the heated polar cap emission, and we speculate that the observed large pulsed fraction is owing to asymmetric magnetospheric structure.

The supergiant O + O binary system HD 166734: a new study

AbstractWe present here a modern study of the radial velocity curve and of the photometric light curve of the very interesting supergiant O7.5If + O9I(f) binary system HD 166734. The physical parameters of the stars and the orbital parameters are carefully determined. We also perform the analysis of the observed X-ray light curve of this colliding-wind binary.

Swift/X-ray Telescope monitoring of the candidate supergiant fast X-ray transient IGR J16418−4532

We report on the Swift monitoring of the candidate supergiant fast X-ray transient (SFXT) IGR J16418-4532, for which both orbital and spin periods are known (~3.7d and ~1250s, respectively). Our observations, for a total of ~43ks, span over three orbital periods and represent the most intense and complete sampling of the light curve of this source with a sensitive X-ray instrument. With this unique set of observations we can address the nature of this transient. By applying the clumpy wind model for blue supergiants to the observed X-ray light curve, and assuming a circular orbit, the X-ray emission from this source can be explained in terms of the accretion from a spherically symmetric clumpy wind, composed of clumps with different masses, ranging from ~5E16 g to 1E21g. Our data suggest, based on the X-ray behaviour, that this is an intermediate SFXT.

Modelling the 2010 blast wave of the symbiotic-like nova V407 Cygni

(Abridged) The symbiotic-like binary Mira and nova V407 Cyg was observed in outburst on March 2010 and monitored in several wavelength bands. Here we report on multi-dimensional hydrodynamic simulations describing the 2010 outburst of V407 Cyg, exploring the first 60 days of evolution. The model takes into account thermal conduction and radiative cooling; the pre-explosion system conditions included the companion star and a circumbinary density enhancement. The simulations showed that the blast and the ejecta distribution are both aspherical due to the inhomogeneous circumstellar medium in which they expand; in particular they are significantly collimated in polar directions (producing a bipolar shock morphology) if the circumstellar envelope is characterized by an equatorial density enhancement. The blast is partially shielded by the Mira companion, producing a wake with dense and hot post-shock plasma on the rear side of the companion star; most of the X-ray emission produced during the evolution of the blast arises from this plasma structure. The observed X-ray lightcurve can be reproduced, assuming values of outburst energy and ejected mass similar to those of RS Oph and U Sco, if a circumbinary gas density enhancement is included in the model. In this case, the 2010 blast propagated through a circumbinary gas density enhancement with radius of the order of 40 AU and gas density \approx 10^6 cm^{-3} and the mass of ejecta in the outburst was M_{ej} \approx 2\times 10^{-7} M_{\odot} with an explosion energy E_{0} \approx 2\times 10^{44} erg. Alternatively, the model can produce a similar X-ray lightcurve without the need of a circumbinary gas density enhancement only if the outburst energy and ejected mass were similar to those at the upper end of ranges for classical novae, namely M_{ej} \approx 5\times 10^{-5} M_{\odot} and E_{0} \approx 5\times 10^{46} erg.

CONSTRAINING GAMMA-RAY BURST INITIAL LORENTZ FACTOR WITH THE AFTERGLOW ONSET FEATURE AND DISCOVERY OF A TIGHT Γ0-Eγ,isoCORRELATION

The onset of GRB afterglow is characterized by a smooth bump in the early afterglow lightcurve. We make an extensive search for such a feature. Twenty optically selected GRBs and 12 X-ray selected GRBs are found, among which 17 optically selected GRBs and 2 X-ray-selected GRBs have redshift measurements. We fit the lightcurves with a smooth broken power-law and measure the temporal characteristic timescales of the bumps at FWHM. Strong mutual correlations among these timescales are found, and a dimmer and broader bump tends to peak at a later peak time. The ratio of rising to decaying timescales is almost universal among bursts, but the ratio of the rising time to the peak time varies from 0.3~1. The E_iso is tightly correlated with the peak luminosity and the peak time of the bump in the burst frame. Assuming that the bumps signal the deceleration of the GRB fireballs in a constant density medium, we calculate the initial Lorentz factor (Gamma_0) and the deceleration radius (R_dec) of the GRBs in the optical-selected sample. It is found that Gamma_0 are typically a few hundreds, and the typical deceleration radius is R_dec~10^{17} cm. More intriguingly, a tight correlation between the Gamma_0 and E_iso is found, namely Gamma_0 ~ 195 E_iso, 52}^{0.27} (satisfied for both the optical and X-ray z-known samples). It is helpful to understand GRB physics, and may serve as an indicator of Gamma_0. We find that the early bright X-rays are usually dominated by a different component from the external shock emission, but occasionally (for one case) an achromatic deceleration feature is observed. Components in X-rays would contribute to the diversity of the observed X-ray lightcurves (abridge).

The X-ray eclipse of the dwarf nova HT Cassiopeiae observed by theXMM-Newton satellite: spectral and timing analysis

A cataclysmic variable is a binary system consisting of a white dwarf that accretes material from a secondary object via the Roche-lobe mechanism. In the case of long enough observation, a detailed temporal analysis can be performed, allowing the physical properties of the binary system to be determined. We present an XMM-Newton observation of the dwarf nova HT Cas acquired to resolve the binary system eclipses and constrain the origin of the X-rays observed. We also compare our results with previous ROSAT and ASCA data. After the spectral analysis of the three EPIC camera signals, the observed X-ray light curve was studied with well known techniques and the eclipse contact points obtained. The X-ray spectrum can be described by thermal bremsstrahlung of temperature $kT_1=6.89 \pm 0.23$ keV plus a black-body component (upper limit) with temperature $kT_2=30_{-6}^{+8}$ eV. Neglecting the black-body, the bolometric absorption corrected flux is $F^{\rm{Bol}}=(6.5\pm 0.1)\times10^{-12}$ erg s$^{-1}$ cm$^{-2}$, which, for a distance of HT Cas of 131 pc, corresponds to a bolometric luminosity of $(1.33\pm 0.02)\times10^{31}$ erg s$^{-1}$. The study of the eclipse in the EPIC light curve permits us to constrain the size and location of the X-ray emitting region, which turns out to be close to the white dwarf radius. We measure an X-ray eclipse somewhat smaller (but only at a level of $\simeq 1.5 \sigma$) than the corresponding optical one. If this is the case, we have possibly identified the signature of either high latitude emission or a layer of X-ray emitting material partially obscured by an accretion disk.

Light Echoes in Kerr Geometry: A Source of High‐Frequency QPOs from Random X‐Ray Bursts

We propose that high-frequency quasi-periodic oscillations (HFQPOs) can be produced from randomly formed X-ray bursts (flashes) by plasma interior to the ergosphere of a rapidly rotating black hole. We show by direct computation of their orbits that the photons comprising the observed X-ray light curves, if due to a multitude of such flashes, are significantly affected by the black hole's dragging of inertial frames; the photons of each such burst arrive to an observer at infinity in multiple (double or triple), distinct "bunches" separated by a roughly constant time lag of Δ tlag/M ≃ 14, regardless of the bursts' azimuthal position. We argue that every other such "bunch" represents photons that follow trajectories with an additional orbit around the black hole at the photon circular orbit radius (a photon "echo"). The presence of this constant lag in the response function of the system leads to a QPO feature in its power density spectra, even though the corresponding light curve consists of a totally stochastic signal. This effect is by and large due to the black hole spin and is shown to gradually diminish as the spin parameter a decreases or the radial position of the burst moves outside the static limit surface (ergosphere). Our calculations indicate that for a black hole with Kerr parameter of a/M = 0.99 and mass of M = 10 M☉, the QPO is expected at a frequency of νQPO ∼ 1.3–1.4 kHz. We discuss the plausibility and observational implications of our model results, as well as its limitations.

X-ray flares in Orion low-mass stars

Context. X-ray flares are common phenomena in pre-main sequence stars. Their analysis gives insights into the physics at work in young stellar coronae. The Orion Nebula Cluster offers a unique opportunity to study large samples of young low mass stars. This work is part of the Chandra Orion Ultradeep project (COUP), an ~10 day long X-ray observation of the Orion Nebula Cluster (ONC). Aims. Our main goal is to statistically characterize the flare-like variability of 165 low mass (0.1-0.3 M_sun) ONC members in order to test and constrain the physical scenario in which flares explain all the observed emission. Methods. We adopt a maximum likelihood piece-wise representation of the observed X-ray light curves and detect flares by taking into account both the amplitude and time derivative of the count-rate. We then derive the frequency and energy distribution of the flares. Results. The high energy tail of the energy distribution of flares is well described by a power-law with index 2.2. We test the hypothesis that light curves are built entirely by overlapping flares with a single power law energy distribution. We constrain the parameters of this simple model for every single light curve. The analysis of synthetic light curves obtained from the model indicates a good agreement with the observed data. Comparing low mass stars with stars in the mass interval (0.9-1.2M_sun), we establish that, at ~1 Myr, low mass and solar mass stars of similar X-ray luminosity have very similar flare frequencies. Conclusions. Our observational results are consistent with the following model/scenario: the light curves are entirely built by over- lapping flares with a power-law intensity distribution; the intense flares are individually detected, while the weak ones merge and form a pseudo-quiescent level, which we indicate as the characteristic level.

Lognormal X-Ray Flux Variations in an Extreme Narrow-Line Seyfert 1 Galaxy

We show that the large variations in the X-ray flux of the extreme narrow-line Seyfert 1 galaxy IRAS 13224-3809, measured during a 10 day ASCA observation, have a two-parameter lognormal distribution of multiplicative standard deviation σmult = 2.7 and that the amplitude of variability at any given moment is proportional to the flux level. We find similar behavior in earlier ROSAT soft X-ray monitoring. There is no evidence of a nonvariable component. The flux-dependent behavior of the variability rules out linear shot-noise models. Although at first glance the variations of the ASCA light curve for IRAS 13224-3809 appear to exhibit nonstationary behavior with quiescent low states and more active flaring high states, our results show that the multiplicative variance is constant. Monte Carlo simulations of constant σmult give excellent matches to the observed X-ray light curve without the need to invoke special low and high states. This supports a picture in which the long-term variability is fundamental both in active galactic nuclei and in X-ray binaries. The lognormal flux distribution and the constancy of σmult are incompatible with the power-law distribution of flaring amplitudes expected from self-organized critical behavior. We discuss mechanisms that can generate lognormal flux distributions.

X-Ray Detection of Presupernova Evolution for the SN 1993J Progenitor

We report on the first detection of presupernova evolution in the X-ray regime. The results are based on ROSAT observations of SN 1993J ranging from 6 days to 5 years after the outburst. The X-ray observations are used to probe the SN shell interaction with the ambient circumstellar matter (CSM). After exploring various scenarios that might explain the observed X-ray light curve with a t-0.27 rate of decline, we present a coherent picture in terms of the interaction of the SN shock front with the CSM deposited by the progenitor's stellar wind. During the observed period, the SN shell reaches a radius of 3 × 1017 cm from the site of the explosion, corresponding to ~104 yr in the progenitor's stellar wind history. Our analysis shows that the mass-loss rate of the progenitor has decreased constantly from = 4 × 10-4 to 4 × 10-5 M☉ yr-1 (vw/10 km s-1) during the late stage of the evolution. Assuming a spherically symmetric expansion, the circumstellar matter density profile is found to be significantly flatter (ρcsm r-1.63) than expected for a constant mass-loss rate and constant wind velocity profile (r-2). The observed evolution reflects a decrease in the mass-loss rate, an increase in the wind speed, or a combination of both, indicating that the progenitor likely was making a transition from the red to the blue supergiant phase during the late stage of its evolution.

Modeling the X‐Ray–Ultraviolet Correlations in NGC 7469

We model the correlated X-ray-UV observations of NGC 7469, for which well-sampled data in both these bands have recently been obtained in a multiwavelength monitoring campaign. To this end, we derive the transfer function in wavelength λ and time lag τ for reprocessing hard (X-ray) photons from a point source to softer ones (UV-optical) by an infinite plane (representing a cool, thin accretion disk) located at a given distance below the X-ray source, under the assumption that the X-ray flux is absorbed and emitted locally by the disk as a blackbody of temperature appropriate to the incident flux. Using the observed X-ray light curve as input, we have computed the expected continuum UV emission as a function of time at several wavelengths (1315, 6962, 15000, and 30000 A), assuming that the X-ray source is located one Schwarzschild radius above the disk plane, with the mass of the black hole M and the latitude angle θ of the observer relative to the disk plane as free parameters. We have searched the parameter space of black hole masses and observer azimuthal angles, but we were unable to reproduce UV light curves that would resemble, even remotely, those observed. We also explored whether particular combinations of the values of these parameters could lead to light curves whose statistical properties (i.e., the autocorrelation and cross-correlation functions) would match those corresponding to the observed UV light curve at 1315 A. Even though we considered black hole masses as large as 109 M☉, no such match was possible. Our results indicate that some of the fundamental assumptions of this model will have to be modified to obtain even approximate agreement between the observed and model X-ray-UV light curves.

Highly-Eccentric Be/X-ray Binaries: Evolution, Wind Rose Effect, Accretor-Propeller Luminosity Gap

AbstractThe influence of the spatial distribution of stellar wind velocities (Wind Rose effect) on the X-ray light curve of highly eccentric binaries is examined using the properties of Be disk-fed outflow. It is shown that a phase shift of the maximum X-ray luminosity is always present in the X-ray light curves. The observed X-ray light curves of some transient binaries are analysed using the developed model. We show that the Be/X-ray transients A0538–66, X0331+53 are likely to undergo transitions from the accreting neutron star regime to the propelling one. The example of evolutionary track which can lead to a formation of the Be/X-ray binary is presented. We conclude that synchronization is a very important process for Be star evolution in binary systems and calculate the critical orbital period for existence of a Be/X-ray binary.

Colliding winds in Wolf-Rayet binaries: further developments within a complicated story

We present large scale 3D simulations of colliding winds in the WR binary γ2 Velorum (WR 11). The O-star wind is confined by cold, high density shells and forms a spirally shaped region within the WR-wind. As a consequence of the elliptic orbit, the opening angle and the curvature of the spiral as well as the ratio of the volumes occupied by the WR- and the O-wind are functions of the orbital phase. Our model qualitatively reproduces the observed X-ray light-curve. The impact of magnetic fields and heat conduction on the physics of colliding winds is briefly discussed and some remarks on the important question of stability are made.

Soft X-ray transient light curves as standard candles: exponential versus linear decays

A recent paper by King &38; Ritter proposed that the light curves of soft X-ray transients (SXTs) are dominated by the effect of irradiation of the accretion disc by the central X-rays. This prevents the onset of the cooling wave which would otherwise return the disc to the quiescent state, and so prolongs the outbursts beyond those in dwarf nova discs. KR show that the decay of the resulting X-ray light curve should be exponential or linear depending on whether or not the observed peak X-ray luminosity is sufficient to ionize the outer edge of the accretion disc. Here we examine the observed X-ray decays, and show that they are exponential or linear according to whether the peak luminosity is greater or smaller than the critical value defined by KR, strongly suggesting that the light curves are indeed irradiation dominated. We show further that the occurrence of an exponential or linear decay tends to favour the same type of decay in subsequent outbursts, so that systems usually show only one or the other type. We use the equations of KR and the observed X-ray light curve to determine the size Rh of the hot disc at the peak of the outburst. For exponential decays, Rh is found to be comparable to the circularization radius, as expected, since the disc consists entirely of material transferred from the secondary since the previous outburst. Further, Rh is directly proportional to the time at which one sees the secondary maximum (ts), as expected, if ts is the viscous time-scale of the irradiated disc. This implies that the orders of magnitude of the viscosity parameter α and disc aspect ratio H/R are such that α(H/R) ∼ 0.01. Observation of a secondary maximum calibrates the peak luminosity and gives the distance (Dkpc) to the source as where Fp is the peak flux, τd is the e-folding time of the decay in days, η is the radiation efficiency parameter and f is the ratio of the disc mass at the start of the outburst to the maximum possible mass.

High-eccentric X-ray binary A0538-66: Evolution status, wind rose effect, and accretor-propeller luminosity GAP

Abstract Using the parameters appropriate for the binary X-ray transient A0538-66, the wind rose effect on the X-ray light curve is examined using the properties of circurnstellar disks derived from IR observations (Waters et al., 1988). The effects of the wind geometry, velocities and densities in the stellar wind of the Be star, and the orbital motion of the neutron star on the expected X-ray luminosity are investigated. It is shown that the shapes of the X-ray light curves depend strongly on the velocity of the gas outflow of the Be star. It is proposed that in some cases an accretion disk may be temporarily formed around the neutron star. The observed X-ray light curves of A0538-66 are analysed using the developed model. We show that the X-ray transient A0538-66 is likely to undergo transitions from the accreting neutron star regime to the propelling stage. The evolutionary scenario that can lead to the formation of the binary system A0538–66 is presented. For the first time, the evolutionary track includes both the orbital period changes and the neutron star spin period history.