Central Neutron Star Research Articles

Phase-dependent Spectral Shape Changes in the Ultraluminous X-Ray Pulsar NGC 5907 ULX1

The discovery of coherent pulsations from several ultraluminous X-ray pulsars (ULXPs) has provided direct evidence of a supercritical accretion flow. However, the geometrical structure of such an accretion flow onto the central neutron star remains poorly understood. NGC 5907 ULX1 is one of the most luminous ULXPs, with a luminosity exceeding 1041 erg s−1. Here we present a broadband X-ray study of this ULXP using the data from simultaneous observations with XMM-Newton and NuSTAR conducted in 2014 July. The phase-resolved spectra are well reproduced by a model consisting of a multicolor disk blackbody emission with a temperature gradient of p = 0.5 (T ∝ r −p ) and a power law with an exponential cutoff. The disk component is phase-invariant and has an innermost temperature of ∼ 0.3 keV. Its normalization suggests a relatively low inclination angle of the disk, in contrast to the previous claims in other literature. The power-law component, attributed to the emission from the accretion flow inside the magnetosphere of the neutron star, indicates phase-dependent spectral shape changes; the spectrum is slightly harder in the pre-peak phase than in the post-peak phase. This implies that the magnetosphere has an asymmetric geometry around the magnetic axis and that hotter regions close to the magnetic pole become visible before the pulse peak.

GRBs from Collapse of Thorne–Żytkow–like Objects as the Aftermath of WD-NS Coalescence

The Type II gamma-ray burst (GRB) 200826A challenges collapsar models by questioning how they can generate a genuinely short-duration event. The other Type I GRB 211211A confused us with a kilonova signature observed in the afterglow of a long burst. Here, we propose a comprehensive model in which both bursts are the results of the collapse of Thorne–Żytkow–like objects (TZlOs). The TZlO consists of a central neutron star (NS), with a dense white dwarf (WD) material envelope, which is formed as the aftermath of a WD-NS coalescence. We find that the characteristics of the resultant GRBs depend on whether the TZlO collapses immediately following the WD-NS merger or not. Additionally, the observational properties of the consequent GRBs manifest variations contingent upon whether the collapse of the TZlO results in a magnetar or a black hole. We also show that our model is consistent with the observations of GRB 211211A and GRB 200826A. Specifically, the optical excess in GRB 211211A can be attributed to an engine-fed kilonova, while the supernova bump in GRB 200826A is likely due to the collision between the ejecta and the disk wind shell.

Gamma-ray spectra of the Crab, Vela and Geminga pulsars fitted with SED of the emission from their current sheet

We show that the spectral energy distribution (SED) of the tightly focused radiation generated by the superluminally moving current sheet in the magnetosphere of a non-aligned neutron star fits the gamma-ray spectra of the Crab, Vela and Geminga pulsars over the entire range of photon energies so far detected by Fermi-LAT, MAGIC and HESS from them: over 102 MeV to 20 TeV. While emblematic of any emission that entails caustics, the SED introduced here radically differs from those of the disparate emission mechanisms currently invoked in the literature to fit the data in different sections of these spectra. We specify, moreover, the connection between the values of the fit parameters for the analysed spectra and the physical characteristics of the central neutron stars of the Crab, Vela and Geminga pulsars and their magnetospheres.

Radio spectra of pulsars fitted with the spectral distribution function of the emission from their current sheet

ABSTRACT In their catalogue of pulsars’ radio spectra, Swainston et al. distinguish between five different forms of these spectra: those that can be fitted with (i) a simple power law, (ii) a broken power law, (iii) a low-frequency turn-over, (iv) a high-frequency turn-over or (v) a double turn-over spectrum. Here, we choose two examples from each of these categories and fit them with the spectral distribution function of the caustics that are generated by the superluminally moving current sheet in the magnetosphere of a non-aligned neutron star. In contrast to the prevailing view that the curved features of pulsars’ radio spectra arise from the absorption of the observed radiation in high-density environments, our results imply that these features are intrinsic to the emission mechanism. We find that all observed features of pulsar spectra (including those that are normally fitted with simple or broken power laws) can be described by a single spectral distribution function and regarded as manifestations of a single emission mechanism. From the results of an earlier analysis of the emission from a pulsar’s current sheet and the values of the fit parameters for each spectrum, we also determine the physical characteristics of the central neutron star of each considered example and its magnetosphere.

Coupling of radiation and magnetospheric accretion flow in ULX pulsars: radiation pressure and photon escape time

ABSTRACT The accretion flow within the magnetospheric radius of bright X-ray pulsars can form an optically thick envelope, concealing the central neutron star from the distant observer. Most photons are emitted at the surface of a neutron star and leave the system after multiple reflections by the accretion material covering the magnetosphere. Reflections cause momentum to be transferred between photons and the accretion flow, which contributes to the radiative force and should thus influence the dynamics of accretion. We employ Monte Carlo simulations and estimate the acceleration along magnetic field lines due to the radiative force as well as the radiation pressure across magnetic field lines. We demonstrate that the radiative acceleration can exceed gravitational acceleration along the field lines, and similarly, radiation pressure can exceed magnetic field pressure. Multiple reflections of X-ray photons back into the envelope tend to amplify both radiative force along the field lines and radiative pressure. We analyse the average photon escape time from the magnetosphere of a star and show that its absolute value is weakly dependent on the magnetic field strength of a star and roughly linearly dependent on the mass accretion rate being $\sim 0.1\, {\rm s}$ at $\dot{M}\sim 10^{20}\, {\rm g\, s^{-1}}$. At high mass accretion rates, the escape time can be longer than free-fall time from the inner disc radius.

The evolutionary route to form planetary nebulae with central neutron star–white dwarf binary systems

ABSTRACT We present a possible evolutionary pathway to form planetary nebulae (PNe) with close neutron star (NS)–white dwarf (WD) binary central stars. By employing the binary population synthesis technique, we find that the evolution involves two common envelope evolution (CEE) phases and a core collapse supernova explosion between them that forms the NS. Later the lower mass star engulfs the NS as it becomes a red giant, a process that leads to the second CEE phase and to the ejection of the envelope. This leaves a hot horizontal branch star that evolves to become a helium WD and an expanding nebula. Both the WD and the NS power the nebula. The NS in addition might power a pulsar wind nebula inside the expanding PN. From our simulations we find that the Galactic formation rate of NS–WD PNe is $1.8 \times 10^{-5}\, {\rm yr}^{-1}$ while the Galactic formation rate of all PNe is $0.42 \, {\rm yr}^{-1}$. There is a possibility that one of the observed Galactic PNe might be a NS–WD PN, and a few NS–WD PNe might exist in the Galaxy. The central binary systems might be sources for future gravitational wave detectors like LISA, and possibly of electromagnetic telescopes.

A Flat-spectrum Radio Transient at 122 Mpc Consistent with an Emerging Pulsar Wind Nebula

We report the discovery and follow-up observations of VT 1137–0337, an unusual radio transient found in our systematic search for extragalactic explosions in the Very Large Array Sky Survey. It is located in the brightest region of a dwarf starburst galaxy at a luminosity distance of 121.6 Mpc. Its 3 GHz luminosity is comparable to luminous radio supernovae associated with dense circumstellar interaction and relativistic outflows. However, its broadband radio spectrum—proportional to ν −0.35 over a range of ≳10× in frequency and fading at a rate of 5% yr–1—cannot be directly explained by the shock of a stellar explosion. Jets launched by various classes of accreting black holes also struggle to account for VT 1137–0337's combination of observational properties. Instead, we propose that VT 1137–0337 is a decades-old pulsar wind nebula that has recently emerged from within the free–free opacity of its surrounding supernova ejecta. If the nebula is powered by spin-down, the central neutron star should have a surface dipole field of ∼1013–1014 G and a present-day spin period of ∼10–100 ms. Alternatively, the nebula may be powered by the release of magnetic energy from a magnetar. Magnetar nebulae have been proposed to explain the persistent radio sources associated with the repeating fast radio bursts FRB 121102 and FRB 190520B. These FRB persistent sources have not previously been observed as transients but do bear a striking resemblance to VT 1137–0337 in their radio luminosity, spectral index, and host galaxy properties.

Congruity of the Crab Pulsar’s γ-ray spectrum with the spectral distribution of tightly focused caustics

The spectrum derived here for the most tightly focused component of the radiation generated by the super-luminally moving current sheet in the magnetosphere of a non-aligned neutron star has a distribution function that fits the entire γ-ray spectrum of the Crab Pulsar on its own. This is the first time that the undivided breadth of this spectrum, from 102 to 106 MeV, is not only described by a single distribution function but is also explained by means of a single emission mechanism. To illustrate that the derived function describes the spectral distribution of the high-energy emission from any non-aligned neutron star, we analyse, in addition, the spectra of two other γ-ray pulsars for which sufficiently large datasets are available: PSR J0101−6422 and PSR J1709−4429. From the connection between the parameters of the fitted spectra and the physical characteristics of their sources, we moreover infer certain attributes of the magnetospheres of the analysed pulsars: the angle between the magnetic and spin axes of their central neutron stars, the scale factor of the electric current density that is associated with their current sheet, and the privileged latitudinal direction (relative to the spin axis) in which they are observed.

Analysis of the Parameters of Radio Pulsars with Giant Pulses Using the Principal Component Analysis

Clustering of radio pulsars with observable giant pulses (GPs) has been carried out using the principal component analysis. Five parameters were used (period, its derivative, observed luminosity, kinematic age, and the angle between the rotation axis and the magnetic moment of the central neutron star). It was shown that the set of all known GP pulsars is divided into two clusters in the phase space of the principal components. One of the clusters contains four pulsars with short periods and high luminosity; the second cluster contains nine long-period and fainter sources. A separate object, not included in these two clusters, is the pulsar in the Crab Nebula. Possible models that could explain the observed difference between GP pulsars were considered.

On the Evolution of Angles between the Magnetic Moment and the Axis of Rotation in Radio Pulsars

Three methods were used to calculate the angles β between the magnetic moment and the axis of rotation of central neutron stars for 307 known radio pulsars. There is no explicit statistical dependence of the calculated angles β on the kinematic age of pulsars. It follows from this that either the indicated angle does not change noticeably with the age of the pulsar, or pulsars are born with an arbitrary inclination angle.

Circular Polarization of Fast Radio Bursts in the Curvature Radiation Scenario

The curvature radiation is applied to the explanation of the circular polarization of fast radio bursts (FRBs). Significant circular polarization is reported in both apparently non-repeating and repeating FRBs. Curvature radiation can produce significant circular polarization at the wing of the radiation beam. In the curvature radiation scenario, in order to see significant circular polarization in FRBs, (1) more energetic bursts, (2) bursts with electrons having higher Lorentz factor, and (3) a slowly rotating neutron star at the center are required. Different rotational period of the central neutron star may explain why some FRBs have high circular polarization, while others do not. Considering possible difference in refractive index for the parallel and perpendicular components of electric field, the position angle may change rapidly over the narrow pulse window of the radiation beam. The position angle swing in FRBs may also be explained by this non-geometric origin, besides that of the rotating vector model.

Diffuse X-ray emission around an ultraluminous X-ray pulsar

Ultraluminous X-ray sources (ULXs) are extragalactic X-ray emitters located off-centre of their host galaxy and with a luminosity in excess of a few 1039 erg s−1, if emitted isotropically1,2. The discovery of periodic modulation revealed that in some ULXs the accreting compact object is a neutron star3–7, indicating luminosities substantially above their Eddington limit. The most extreme object in this respect is NGC 5907 ULX-1 (ULX1), with a peak luminosity that is 500 times its Eddington limit. During a Chandra observation to probe a low state of ULX1, we detected diffuse X-ray emission at the position of ULX1. Its diameter is 2.7 ± 1.0 arcsec and contains 25 photons, none below 0.8 keV. We interpret this extended structure as an expanding nebula powered by the wind of ULX1. Its diameter of about 200 pc, characteristic energy of ~1.9 keV and luminosity of ~2 × 1038 erg s−1 imply a mechanical power of 1.3 × 1041 erg s−1 and an age of ~7 × 104 yr. This interpretation suggests that a genuinely super-Eddington regime can be sustained for timescales much longer than the spin-up time of the neutron star powering the system. As the mechanical power from a single ULX nebula can rival the injection rate of cosmic rays of an entire galaxy8, ULX nebulae could be important cosmic ray accelerators9. Diffuse X-ray emission is detected around an ultraluminous X-ray source, interpreted as a wind-powered expanding nebula. Its energetics suggests that a super-Eddington regime can be longer than the spin-up time of the central neutron star.

A Potential Cyclotron Resonant Scattering Feature in the Ultraluminous X-Ray Source Pulsar NGC 300 ULX1 Seen by NuSTAR and XMM-Newton

Abstract Based on phase-resolved broadband spectroscopy using XMM-Newton and NuSTAR, we report on a potential cyclotron resonant scattering feature (CRSF) at E ∼ 13 keV in the pulsed spectrum of the recently discovered ultraluminous X-ray source (ULX) pulsar NGC 300 ULX1. If this interpretation is correct, the implied magnetic field of the central neutron star is B ∼ 1012 G (assuming scattering by electrons), similar to that estimated from the observed spin-up of the star, and also similar to known Galactic X-ray pulsars. We discuss the implications of this result for the connection between NGC 300 ULX1 and the other known ULX pulsars, particularly in light of the recent discovery of a likely proton cyclotron line in another ULX, M51 ULX-8.

Ultraluminous X-ray sources: new distance indicators?

Aims. In this paper we fit the NuSTAR and XMM-Newton data of three sources: NGC 7793 P13, NGC5907 ULX1, and Circinus ULX5. Methods. Our single model contains emission from a non-spherical system: a neutron star plus an accretion disk directed towards the observer. Results. We obtained a very good fit with the reduced χ2 per degree of freedom equal to 1.08 for P13, 1.01 for ULX1, and 1.14 for ULX5. The normalization of our model constrains the distance to the source. The resulting distances are D = 3.41−0.10+0.11, 6.55−0.81+0.69, and 2.60−0.03+0.05 Mpc for P13, ULX1, and ULX5 respectively. The distances to P13 and ULX5 are in perfect agreement with previous distance measurements to their host galaxies. Conclusions. Our results confirm that P13, ULX1, and ULX5 may contain central hot neutron stars. When the outgoing emission is computed by integration over the emitting surface and successfully fitted to the data, then the resulting model normalization is the direct distance indicator.

Pulse phase-resolved analysis of SMC X-3 during its 2016–2017 super-Eddington outburst

The Be X-ray pulsar SMC X-3 underwent an extra long and ultraluminous giant outburst from 2016 August to 2017 March. The peak X-ray luminosity is up to $\sim 10^{39}$ erg/s, suggesting a mildly super-Eddington accretion onto the strongly magnetized neutron star. It therefore bridges the gap between the Galactic Be/X-ray binaries ($L_{\rm X}^{\rm peak} \leq 10^{38}$ erg/s) and the ultraluminous X-ray pulsars ($L_{\rm X}^{\rm peak} \geq 10^{40}$ erg/s) found in nearby galaxies. A number of observations were carried out to observe the outburst. In this paper, we perform a comprehensive phase-resolved analysis on the high quality data obtained with the Nustar and XMM-Newton, which were observed at a high and intermediate luminosity levels. In order to get a better understanding on the evolution of the whole extreme burst, we take the Swift results at the low luminosity state into account as well. At the early stage of outburst, the source shows a double-peak pulse profile, the second main peak approaches the first one and merges into the single peak at the low luminosity. The second main peak vanishes beyond 20 keV, and its radiation becomes much softer than that of the first main peak. The line widths of fluorescent iron line vary dramatically with phases, indicating a complicated geometry of accretion flows. In contrast to the case at low luminosity, the pulse fraction increases with the photon energy. The significant small pulse fraction detected below 1 keV can be interpreted as the existence of an additional thermal component located at far away from the central neutron star.

Super-Eddington accretion on to the neutron star NGC 7793 P13: Broad-band X-ray spectroscopy and ultraluminous X-ray sources

We present a detailed, broadband X-ray spectral analysis of the ULX pulsar NGC 7793 P13, a known super-Eddington source, utilizing data from the $XMM$-$Newton$, $NuSTAR$ and $Chandra$ observatories. The broadband $XMM$-$Newton+NuSTAR$ spectrum of P13 is qualitatively similar to the rest of the ULX sample with broadband coverage, suggesting that additional ULXs in the known population may host neutron star accretors. Through time-averaged, phase-resolved and multi-epoch studies, we find that two non-pulsed thermal blackbody components with temperatures $\sim$0.5 and $\sim$1.5 keV are required to fit the data below 10 keV, in addition to a third continuum component which extends to higher energies and is associated with the pulsed emission from the accretion column. The characteristic radii of the thermal components appear to be similar, and are too large to be associated with the neutron star itself, so the need for two components likely indicates the accretion flow outside the magnetosphere is complex. We suggest a scenario in which the thick inner disc expected for super-Eddington accretion begins to form, but is terminated by the neutron star's magnetic field soon after its onset, implying a limit of $B \lesssim 6 \times 10^{12}$ G for the dipolar component of the central neutron star's magnetic field. Evidence of similar termination of the disc in other sources may offer a further means of identifying additional neutron star ULXs. Finally, we examine the spectrum exhibited by P13 during one of its unusual 'off' states. These data require both a hard powerlaw component, suggesting residual accretion onto the neutron star, and emission from a thermal plasma, which we argue is likely associated with the P13 system.

Probing the possibility of hotspots on the central neutron star in HESS J1731−347

The X-ray spectra of the neutron stars located in the centers of supernova remnants Cas A and HESS J1731-347 are well fit with carbon atmosphere models. These fits yield plausible neutron star sizes for the known or estimated distances to these supernova remnants. The evidence in favor of the presence of a pure carbon envelope at the neutron star surface is rather indirect and is based on the assumption that the emission is generated uniformly by the entire stellar surface. Although this assumption is supported by the absence of pulsations, the observational upper limit on the pulsed fraction is not very stringent. In an attempt to quantify this evidence, we investigate the possibility that the observed spectrum of the neutron star in HESS J1731-347 is a combination of the spectra produced in a hydrogen atmosphere of the hotspots and of the cooler remaining part of the neutron star surface. The lack of pulsations in this case has to be explained either by a sufficiently small angle between the neutron star spin axis and the line of sight, or by a sufficiently small angular distance between the hotspots and the neutron star rotation poles. As the observed flux from a non-uniformly emitting neutron star depends on the angular distribution of the radiation emerging from the atmosphere, we have computed two new grids of pure carbon and pure hydrogen atmosphere model spectra accounting for Compton scattering. Using new hydrogen models, we have evaluated the probability of a geometry that leads to a pulsed fraction below the observed upper limit to be about 8.2 %. Such a geometry thus seems to be rather improbable but cannot be excluded at this stage.

MONTE CARLO NEUTRINO TRANSPORT THROUGH REMNANT DISKS FROM NEUTRON STAR MERGERS

We present Sedonu, a new open source, steady-state, special relativistic Monte Carlo (MC) neutrino transport code, available at bitbucket.org/srichers/sedonu. The code calculates the energy- and angle-dependent neutrino distribution function on fluid backgrounds of any number of spatial dimensions, calculates the rates of change of fluid internal energy and electron fraction, and solves for the equilibrium fluid temperature and electron fraction. We apply this method to snapshots from two-dimensional simulations of accretion disks left behind by binary neutron star mergers, varying the input physics and comparing to the results obtained with a leakage scheme for the case of a central black hole and a central hypermassive neutron star. Neutrinos are guided away from the densest regions of the disk and escape preferentially around 45 degrees from the equatorial plane. Neutrino heating is strengthened by MC transport a few scale heights above the disk midplane near the innermost stable circular orbit, potentially leading to a stronger neutrino-driven wind. Neutrino cooling in the dense midplane of the disk is stronger when using MC transport, leading to a globally higher cooling rate by a factor of a few and a larger leptonization rate by an order of magnitude. We calculate neutrino pair annihilation rates and estimate that an energy of 2.8e46 erg is deposited within 45 degrees of the symmetry axis over 300 ms when a central BH is present. Similarly, 1.9e48 erg is deposited over 3 s when an HMNS sits at the center, but neither estimate is likely to be sufficient to drive a GRB jet.

NEUTRINO-DRIVEN WINDS IN THE AFTERMATH OF A NEUTRON STAR MERGER: NUCLEOSYNTHESIS AND ELECTROMAGNETIC TRANSIENTS

We present a comprehensive nucleosynthesis study of the neutrino-driven wind in the aftermath of a binary neutron star merger. Our focus is the initial remnant phase when a massive central neutron star is present. Using tracers from a recent hydrodynamical simulation, we determine total masses and integrated abundances to characterize the composition of unbound matter. We find that the nucleosynthetic yields depend sensitively on both the life time of the massive neutron star and the polar angle. Matter in excess of up to $9 \cdot 10^{-3} M_\odot$ becomes unbound until $\sim 200~{\rm ms}$. Due to electron fractions of $Y_{\rm e} \approx 0.2 - 0.4$ mainly nuclei with mass numbers $A < 130$ are synthesized, complementing the yields from the earlier dynamic ejecta. Mixing scenarios with these two types of ejecta can explain the abundance pattern in r-process enriched metal-poor stars. Additionally, we calculate heating rates for the decay of the freshly produced radioactive isotopes. The resulting light curve peaks in the blue band after about $4~{\rm h}$. Furthermore, high opacities due to heavy r-process nuclei in the dynamic ejecta lead to a second peak in the infrared after $3-4~{\rm d}$.

CHANDRAOBSERVATIONS OF SNR RCW 103

We analyze three Chandra observations, with a combined exposure time of 99 ks, of the Galactic supernova remnant RCW 103, a young supernova remnant, previously with no clear detection of metal-rich ejecta. Based on our imaging and spectral analyses of these deep Chandra data, we find evidence for metal-rich ejecta emission scattered throughout the remnant. X-ray emission from the shocked ejecta is generally weak, and the shocked circumstellar medium (CSM) is a largely dominant component across the entire remnant. The CSM component shows abundances of ~0.5 solar, while Ne, Mg, Si, S, and Fe abundances of the ejecta are up to a few times solar. Comparison of these ejecta abundances with yields from supernova nucleosynthesis models suggests, together with the existence of a central neutron star, a progenitor mass of ~18-20 M$_\odot$, though the Fe/Si ratios are larger than predicted. The shocked CSM emission suggests a progenitor with high mass-loss rate and subsolar metallicity.