Low Magnetic Field Neutron Stars Research Articles

Optical and ultraviolet pulsed emission from an accreting millisecond pulsar

Millisecond spinning, low magnetic field neutron stars are believed to attain their fast rotation in a 0.1-1 Gyr-long phase during which they accrete matter endowed with angular momentum from a low-mass companion star. Despite extensive searches, coherent periodicities originating from accreting neutron star magnetospheres have been detected only at X-ray energies and in ~10% of the presently known systems. Here we report the detection of optical and ultraviolet coherent pulsations at the X-ray period of the transient low mass X-ray binary system SAX J1808.4-3658, during an accretion outburst that occurred in August 2019. At the time of the observations, the pulsar was surrounded by an accretion disc, displayed X-ray pulsations and its luminosity was consistent with magnetically funneled accretion onto the neutron star. Current accretion models fail to account for the luminosity of both optical and ultraviolet pulsations; these are instead more likely driven by synchro-curvature radiation in the pulsar magnetosphere or just outside of it. This interpretation would imply that particle acceleration can take place even when mass accretion is going on, and opens up new perspectives in the study of coherent optical/UV pulsations from fast spinning accreting neutron stars in low-mass X-ray binary systems.

What Powered the Optical Transient AT2017gfo Associated with GW170817?

Abstract The groundbreaking discovery of the optical transient AT2017gfo associated with GW170817 opens a unique opportunity to study the physics of double neutron star (NS) mergers. We argue that the standard interpretation of AT2017gfo as being powered by radioactive decay of r-process elements faces the challenge of simultaneously accounting for the peak luminosity and peak time of the event, as it is not easy to achieve the required high mass, and especially the low opacity of the ejecta required to fit the data. A plausible solution would be to invoke an additional energy source, which is probably provided by the merger product. We consider energy injection from two types of the merger products: (1) a post-merger black hole powered by fallback accretion; and (2) a long-lived NS remnant. The former case can only account for the early emission of AT2017gfo, with the late emission still powered by radioactive decay. In the latter case, both early- and late-emission components can be well interpreted as due to energy injection from a spinning-down NS, with the required mass and opacity of the ejecta components well consistent with known numerical simulation results. We suggest that there is a strong indication that the merger product of GW170817 is a long-lived (supramassive or even permanently stable), low magnetic field NS. The result provides a stringent constraint on the equations of state of NSs.

Demonstrating the likely neutron star nature of five M31 globular cluster sources withSwift-NuSTAR spectroscopy

We present the results of a joint Swift-NuSTAR spectroscopy campaign on M31. We focus on the five brightest globular cluster X-ray sources in our fields. Two of these had previously been argued to be black hole candidates on the basis of apparent hard-state spectra at luminosities above those for which neutron stars are in hard states. We show that these two sources are likely to be Z-sources (i.e. low magnetic field neutron stars accreting near their Eddington limits), or perhaps bright atoll sources (low magnetic field neutron stars which are just a bit fainter than this level) on the basis of simultaneous Swift and NuSTAR spectra which cover a broader range of energies. These new observations reveal spectral curvature above 6-8 keV that would be hard to detect without the broader energy coverage the NuSTAR data provide relative to Chandra and XMM-Newton. We show that the other three sources are also likely to be bright neutron star X-ray binaries, rather than black hole X-ray binaries. We discuss why it should already have been realized that it was unlikely that these objects were black holes on the basis of their being persistent sources, and we re-examine past work which suggested that tidal capture products would be persistently bright X-ray emitters. We discuss how this problem is likely due to neglecting disk winds in older work that predict which systems will be persistent and which will be transient.

Discovery of decaHz flaring in SAX J1808.4-3658

We report on the discovery of strong decaHz flaring in the early decay of two out of five outbursts of the accreting millisecond X-ray pulsar SAX J1808.4-3658. The decaHz flaring switches on and, after ~3 days, off again, on a time scale of 1-2 hours. When the flaring is present, the total 0.05-10 Hz variability has a fractional rms amplitude of 20 to 30 percent, well in excess of the 8 to 12 percent rms broad-band noise usually seen in power spectra of SAX J1808 in this frequency range. Coherent 401 Hz pulsations are seen throughout the observations in which the decaHz flaring is detected. We find that the absolute amplitude of the pulsations varies with the flux modulation of the decaHz flaring, indicating that the flaring is caused by an accretion rate modulation already present in the accretion flow prior to matter entering the accretion funnel. We suggest that the decaHz flaring is the result of the Spruit-Taam instability [1]. This instability arises when the inner accretion disk approaches co-rotation. The rotation of the stellar magnetosphere then acts as a propeller, suppressing accretion onto the neutron star. A matter reservoir forms in the inner accretion disk, which episodically empties onto the neutron star, causing flares at a decaHz timescale. A similar explanation was proposed earlier for 1 Hz flaring occurring late in three of five outbursts, mutually exclusive with the decaHz flaring. The 1 Hz flaring was observed at luminosities a factor 5 to 10 below where we see the decaHz flaring. That a different branch of the Spruit-Taam instability could also act at the much higher luminosity levels of the decaHz flaring had recently been predicted by D’Angelo & Spruit [2, 3]. We discuss these findings in the context of the parameters of the Spruit-Taam-d’Angelo model of the instability. If confirmed, after millisecond pulsations, 1 Hz and decaHz flaring would be another diagnostic of the presence of a magnetosphere in accreting low-magnetic field neutron stars.

MAGNETIC INTERACTIONS IN COALESCING NEUTRON STAR BINARIES

It is expected on both evolutionary and empirical grounds that many merging neutron star (NS) binaries are composed of a highly magnetized NS in orbit with a relatively low magnetic field NS. I study the magnetic interactions of these binaries using the framework of a unipolar inductor model. The e.m.f. generated across the non-magnetic NS as it moves through the magnetosphere sets up a circuit connecting the two stars. The exact features of this circuit depend on the uncertain resistance in the space between the stars R_space. Nevertheless, I show that there are interesting observational and/or dynamical effects irrespective of its exact value. When R_space is large, electric dissipation as great as ~10^{46} erg/s (for magnetar-strength fields) occurs in the magnetosphere, which would exhibit itself as a hard X-ray precursor in the seconds leading up to merger. With less certainty, there may also be an associated radio transient, but this would be observed well past merger (~hrs) because of interstellar dispersion. When R_space is small, electric dissipation largely occurs in the surface layers of the magnetic NS. This can reach ~10^{49} erg/s during the final ~1 sec before merger, similar to the energetics and timescales of short gamma-ray bursts. In addition, for dipole fields greater than ~10^{12} G and a small R_space, magnetic torques spin up the magnetized NS. This drains angular momentum from the binary and accelerates the inspiral. A faster coalescence results in less orbits occurring before merger, which would impact matched-filtering gravitational-wave searches by ground-based laser interferometers and could create difficulties for studying alternative theories of gravity with compact inspirals.

SYSTEMATIC UNCERTAINTIES IN THE SPECTROSCOPIC MEASUREMENTS OF NEUTRON-STAR MASSES AND RADII FROM THERMONUCLEAR X-RAY BURSTS. I. APPARENT RADII

The masses and radii of low-magnetic field neutron stars can be measured by combining different observable quantities obtained from their X-ray spectra during thermonuclear X-ray bursts. One of these quantities is the apparent radius of each neutron star as inferred from the X-ray flux and spectral temperature measured during the cooling tails of bursts, when the thermonuclear flash is believed to have engulfed the entire star. In this paper, we analyze 13,095 X-ray spectra of 446 X-ray bursts observed from 12 sources in order to assess possible systematic effects in the measurements of the apparent radii of neutron stars. We first show that the vast majority of the observed X-ray spectra are consistent with blackbody functions to within a few percent. We find that most X-ray bursts follow a very well determined correlation between X-ray flux and temperature, which is consistent with the whole neutron-star surface emitting uniformly during the cooling tails. We develop a Bayesian Gaussian mixture algorithm to measure the apparent radii of the neutron stars in these sources, while detecting and excluding a small number of X-ray bursts that show irregular cooling behavior. This algorithm also provides us with a quantitative measure of the systematic uncertainties in the measurements. We find that those errors in the spectroscopic determination of neutron-star radii that are introduced by systematic effects in the cooling tails of X-ray bursts are in the range $\simeq 3-8$%. Such small errors are adequate to distinguish between different equations of state provided that uncertainties in the distance to each source and the absolute calibration of X-ray detectors do not dominate the error budget.

Are the magnetic fields of millisecond pulsars ∼108 G?

It is generally assumed that the magnetic fields of millisecond pulsars (MSPs) are ∼108 G. We argue that this may not be true and the fields may be appreciably greater. We present six evidences for this: (1) The ∼108G field estimate is based on magnetic dipole emission losses which is shown to be questionable; (2) The MSPs in low mass X-ray binaries (LMXBs) are claimed to have <1011 G on the basis of a Rayleygh-Taylor instability accretion argument. We show that the accretion argument is questionable and the upper limit 1011 G may be much higher; (3) Low magnetic field neutron stars have difficulty being produced in LMXBs; (4) MSPs may still be accreting indicating a much higher magnetic field; (5) The data that predict ∼108 G for MSPs also predict ages on the order of, and greater than, ten billion years, which is much greater than normal pulsars. If the predicted ages are wrong, most likely the predicted ∼108 G fields of MSPs are wrong; (6) When magnetic fields are measured directly with cyclotron lines in X-ray binaries, fields ≫108 G are indicated. Other scenarios should be investigated. One such scenario is the following. Over 85% of MSPs are confirmed members of a binary. It is possible that all MSPs are in large separation binaries having magnetic fields >108 G with their magnetic dipole emission being balanced by low level accretion from their companions.

The variable radio counterpart and possible large-scale jet of the new Z source XTE J1701−462

Abstract We report radio observations, made with the Australia Telescope Compact Array, of the X-ray transient XTE J1701−462. This system has been classified as a new ‘Z’ source, displaying characteristic patterns of behaviour probably associated with accretion on to a low magnetic field neutron star at close to the Eddington limit. The radio counterpart is highly variable, and was detected in six of 16 observations over the period 2006 January–April. The coupling of radio emission to X-ray state, despite limited sampling, appears to be similar to that of other ‘Z’ sources, in that there is no radio emission on the flaring branch. The mean radio and X-ray luminosities are consistent with the other Z sources for a distance of 5–15 kpc. The radio spectrum is unusually flat, or even inverted, in contrast to the related sources, Sco X-1 and Cir X-1, which usually display an optically thin radio spectrum. Deep wide-field observations indicate an extended structure 3 arcmin to the south which is aligned with the X-ray binary. This seems to represent a significant overdensity of radio sources for the field and so, although a background source remains a strong possibility, we consider it plausible that this is a large-scale jet associated with XTE J1701−462.

Discovery of Twin kHz QPOs in the Peculiar X‐Ray Binary Circinus X‐1

We report the discovery with the RXTE PCA of twin kHz QPOs in the peculiar X-ray binary Circinus X-1. Eleven cases of simultaneous double QPOs occurred, with significances of up to 6.3 and 5.5 σ and centroid frequencies ranging between approximately 56 and 225 and 230 and 500 Hz for the two QPO peaks, respectively, i.e., for the most part at frequencies well below those of other sources. The QPO properties clearly indicate that these double peaks are the kHz QPOs known from low magnetic field neutron stars, and not black hole high-frequency QPOs, confirming that Cir X-1 is a neutron star, as suspected since the detection of type I X-ray bursts from the field of the source 20 years ago. The kHz QPO peak separation varies over a wide range, ~175-340 Hz, and increases as a function of kHz QPO frequency. This is contrary to what has been observed in other sources but in good agreement with predictions of the relativistic precession model and Alfven wave models; beat-frequency models require modification to accommodate this. In other observations, single kHz QPOs can be seen down to frequencies as low as ~12 Hz, as well as a strong low-frequency quasi-periodic oscillation (LF QPO) between 1 and 30 Hz. The relations between the kHz QPOs and the LF QPO are in good agreement with those found previously in Z sources, confirming that Cir X-1 may be a peculiar Z source. We suggest that the low-frequency range over which the kHz QPOs occur in Cir X-1 and to a lesser extent in (other) Z sources, might be due to a stronger radial accretion flow relative to the disk flow than in other kHz QPO sources, possibly related to the nature of the companion star.

Overview of QPOs in neutron-star low-mass X-ray binaries

Some aspects of the rapid X-ray variability of low magnetic-field neutron stars in low-mass X-ray binaries are briefly summarized.

Comparing Black Hole and Neutron Star Variability

There are remarkable similarities between the rapid X-ray variability of low-magnetic field neutron stars in low mass X-ray binaries, and that of black holes. In particular at frequencies <100 Hz, their power spectra can be strikingly similar. The highest frequency phenomena (kilohertz QPOs, black hole high-frequency QPOs and neutron star hectohertz QPOs) are the ones that showmost differences, perhaps because they originate closest to the compact object. Most variability components vary in frequency in correlation with one another, and the correlations once again are very similar across neutron stars and black holes — some extend even to white dwarfs. Although this does not strictly require that all phenomena whose frequencies are involved are caused by the same physics in all three source types, this does indicate that basic properties of the accretion flow which are the same in all three source types play an important role in generating at least some of the frequencies.

Correlation between radio luminosity and X-ray timing frequencies in neutron star and black hole X-ray binaries

We report on correlations between radio luminosity and X-ray timing features in X-ray binary systems containing low magnetic field neutron stars and black holes. The sample of neutron star systems consists of 4U 1728–34, 4U 1820–34, Ser X-1, MXB 1730-335, GX 13+1, the millisecond X-ray pulsars SAX J1808.4-3658 and IGR J00291+5934, and these are compared with the black hole system GX 339–4. The analysis has been performed using data from pointed observations of the Rossi X-ray Timing Explorer coordinated with radio observations. In the neutron star systems the radio luminosity LR is correlated with the characteristic frequency of the Lh Lorentzian component detected contemporaneously in the power spectrum, and anticorrelated with its strength. Similarly, in the black hole system GX 339–4 LR is correlated with the frequency of the Lℓ component in the power spectrum and anticorrelated with its strength. The index of a power-law fit to the correlation is similar in both cases, LR∝ν∼1.4 and LR∝ (rms)−2.3. At lower timing frequencies, the radio luminosity is further found to be correlated with the characteristic (break) frequency of the Lb component of the power spectra in the neutron stars and, marginally, with the equivalent break frequency in GX 339–4. We briefly discuss the coupling between the innermost regions of the accretion disc and the production of the jet and, from the behaviour of the millisecond accreting X-ray pulsars, the possible role of the neutron star magnetic field.

Hysteresis in the light curves of soft X-ray transients

Using Proportional Counter Array (PCA) data from the Rossi X-Ray Timing Explorer (RXTE), we track the spectral states of the neutron star transient system Aql X-1 through a complete outburst cycle. We find a hard-to-soft state transition during the very early, rising phase of the outburst, and show that there is a hysteresis effect such that the transition back to the hard state occurs at a luminosity ∼5 times lower than the hard-to-soft transition. This hysteresis effect rules out the propeller mechanism as the sole cause of state transitions in Aql X-1. Assuming the propeller mechanism only operates at a luminosity equal to or below that of the observed soft-to-hard transition requires that the magnetic field of Aql X-1 be less than 7 × 107 G, the lowest neutron star field known to date. To compare the state transition behaviour of Aql X-1 with that found in transient black hole systems, we use RXTE All-Sky Monitor (ASM) data to compute hardness—intensity diagrams for four black hole candidate transients where the ASM data should also give us state information throughout much of the outburst cycles. In all four systems, we find evidence for a hard-to-soft state transition during the rising outburst phase and for the source staying in a soft state down to much lower luminosities during the declining phase, i.e. a hysteresis effect. This similarity suggests a common origin for state transitions in low magnetic field neutron star and black hole systems, and the hysteresis effect rules out the ‘strong ADAF (advection-dominated accretion flow) principle’ for determining the state of an accretion disc. We discuss the general implications of these observations for current models of state transitions. We note the contrast to previous observations of the non-transient systems Cygnus X-1 and X-3, which do not show a hysteresis effect.

Kilohertz quasi-periodic oscillations in low-mass X-ray binaries

I review the current observational status of the new kilohertz quasi-periodic oscillations in accreting lowmagnetic field neutron stars, and critically discuss some of the proposed models.

Low-Mass X-ray Binaries—Recent Developments

Recent developments in the field of low-mass X-ray binaries are briefly reviewed, with particular emphasis on a comparison between the systems that contain accreting low magnetic-field neutron stars and those that contain black-hole candidates. The possibility that inclination effects play a role in black-hole candidate phenomenology is explored.

Quasi-periodic Oscillations and Noise in Accreting Black Holes and Low-magnetic Field Neutron Stars

The X-ray spectrum and the rapid X-ray variability of accreting compact objects have a common origin, and their properties can therefore be expected to be coupled. Indeed, it turns out that the X-ray spectrum and the power spectrum show correlated variations, which occur as a function of variations in the mass fluxx Ṁ.Stellar mass black holes and neutron stars have similar mass and size, and therefore their accretion phenomena may be expected to show similarities. Indeed, similarities exist that indicate that a unified description may be possible. If a particular phenomenon is seen in both neutron star and black-hole candidate systems this shows immediately that it cannot be due to any property that is unique to either neutron stars or black holes, such as the presence or absence of a surface, or of a strong non-aligned magnetic field. The quest for characteristics that are unique to black holes continues - I shall mention a few candidates below.

Similarities in neutron star and black hole accretion

view Abstract Citations (190) References (41) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Similarities in Neutron Star and Black Hole Accretion van der Klis, M. Abstract In this paper, I compare the X-ray properties of accreting low magnetic field neutron stars and accreting stellar mass black hole candidates. Rather than looking for signatures that are unique to black holes, I use the similarities in the observed properties of accreting neutron stars and black hole candidates in order to see whether it is possible to find a common pattern of phenomenology across the two groups. Following this purely phenomenological approach, I propose as a working hypothesis that three different states are common to the accretion process onto low magnetic field neutron stars and black holes. In a given source, the instantaneous accretion rate determines the state; between sources, other parameters, such as compact object mass and magnetic field strength, may also play a role. Publication: The Astrophysical Journal Supplement Series Pub Date: June 1994 DOI: 10.1086/192006 Bibcode: 1994ApJS...92..511V Keywords: Black Holes (Astronomy); Neutron Stars; Stellar Mass Accretion; X Ray Astronomy; X Ray Binaries; X Ray Spectra; Color-Color Diagram; Light Curve; Power Spectra; Stellar Oscillations; Astronomy; ACCRETION; ACCRETION DISKS; BLACK HOLE PHYSICS; STARS: NEUTRON full text sources ADS | data products SIMBAD (10)

Monte Carlo Simulations of Radio Pulsars and Their Progenitors

The formation of neutron stars in binary systems is often used to explain the nature of specific radio pulsars and characteristics of the pulsar population as a whole. We have investigated the extent to which such scenarios provide a self-consistent description of the pulsar population. Using a computer simulation, we modeled the evolution of the main sequence stellar population and compared the predicted neutron star population to the observed radio pulsar population, focusing our attention on the pulsar velocity distribution and the incidence of binary pulsars. These characteristics relate very directly to the binary nature of pulsar progenitors, and are not strongly dependent on models of pulsar magentic field and luminosity evolution.The need to reproduce both the high velocities typical of pulsars and the low incidence of binary pulsars strongly constrains the formation of pulsars in binary systems. Unless one assumes that virtually all pulsars originate in close binary systems, the observed velocity distribution cannot result from the disruption of binary systems by symmetric supernova explosions; some additional acceleration process (e.g. asymmetric supernova mass ejection or asymmetries in pulsar radiation) must act during or soon after a pulsar's formation. It is possible to reproduce the velocity distribution by assuming that all pulsars are born in binary systems with initial orbital periods less than about 30 years. However, the predicted incidence of binaries is then too large by more than an order of magnitude, unless one also assumes that the process of mass transfer from the primary to the secondary is almost always non-conservative, or that the minimum mass necessary for a stripped helium core to explode as a supernova is larger (over 4 M⊙) than currently believed. Further analyses of the radio pulsar population, the X-ray binary population and the abundances of elements ejected in supernovae should help determine which of these alternatives is most reasonble. Additional studies of the main sequence stellar population, accounting more accurately for evolutionary and observational selection effects, will reduce the uncertainties in modeling the formation of the neutron star population.It has also been suggested that the observed correlation between pulsar velocities and magnetic moments (see Cordes, these Proceedings) is induced by the differing evolutionary paths by which stars in binary systems form radio pulsars. Our simulation does not reproduce this correlation, and we do not find any paths likely to produce low velocity, low magnetic field neutron stars not in binary systems.We are submitting a full description of our model and results to The Astrophysical Journal.

A new class of radio pulsars

Sufficiently low magnetic field neutron stars which accrete for long times from a surrounding keplerian disk can be spun up to millisecond periods. After accretion ceases, such stars could become isolated or binary pulsars, depending on whether their companions have subsequently disrupted the binary by either their tidal break ups or their supernova explosions. Such pulsars constitute a new class, with short periods, long apparent ages (P/Ṗ ≳ 108 yr) and pulsed optical, X-ray and γ-ray fluxes significantly below those expected for canonical pulsars with similar periods. Because of their long subsequent spin-down lifetimes, such pulsars would probably be observable if their birth rate exceeded even 10−4 that of the canonical ones. We propose here that the recently discovered millisecond pulsar 4C21.53 belongs to this new class, together with the binaries PSRs 1913+16, 0820+02 and 0655+64, as well as possibly several isolated pulsars such as PSRs 1952+29 and 1804−08. The possibility of such a class of fast accretion-spun-up pulsars has also been noted by Backus et al.1.