Neutron Star LMXBs Research Articles

The X-Ray Binary Population in the Galactic Center Revealed through Multi-decade Observations

Abstract We present an investigation of the quiescent and transient X-ray binaries (XRBs) of the Galactic Center (GC). We extended our Chandra analysis of the non-thermal X-ray sources, located in the central parsec, from Hailey et al. (2018), using an additional 4.6 Msec of ACIS-S data obtained in 2012–2018. The individual Chandra spectra of the 12 sources fit to an absorbed power-law model with a mean photon index Γ ≈ 2 and show no Fe emission lines. Long-term variability was detected from nine of them, confirming that a majority are quiescent XRBs. Frequent X-ray monitoring of the GC revealed that the 12 non-thermal X-ray sources, as well as four X-ray transients have shown at most a single outburst over the last two decades. They are distinct from the six known neutron star LMXBs in the GC, which have all undergone multiple outbursts with ≲ 5 year recurrence time on average. Based on the outburst history data of the broader population of X-ray transients, we conclude that the 16 sources represent a population of ∼240–630 tightly bound BH-LMXBs with ∼4−12 hr orbital periods, consistent with the stellar/binary dynamics modeling in the vicinity of Sgr A*. The distribution of the 16 BH-LMXB candidates is disk-like (at 87% CL) and aligned with the nuclear star cluster. Our results have implications for XRB formation and the rate of gravitational wave events in other galactic nuclei.

Thermal Emission in the Quiescent Neutron Star SAX J1810.8-2609

Abstract We have observed the neutron star low-mass X-ray binary SAX J1810.8−2609 in quiescence with XMM-Newton. SAX J1810.8−2609 is one of the faintest non-pulsing neutron star low-mass X-ray binaries in quiescence and previously only had upper limits on its quiescent thermal emission. We found SAX J1810.8−2609 at the same 0.5–10 keV, unabsorbed luminosity as the previous quiescent observation in 2003, erg s−1. We show that the spectrum requires both thermal and nonthermal components, each contributing approximately half the total emission. The low neutron star luminosity suggests a time-averaged outburst accretion rate of M ⊙ yr−1, in conflict with its observed outburst activity corresponding to a mass accretion rate that is an order of magnitude larger ( M ⊙ yr−1). Our observation designates SAX J1810.8−2609 more firmly as a member of a population of faint quiescent neutron star LMXBs whose quiescent thermal luminosity is not aligned with standard cooling models.

M/R estimates for two neutron stars in LMXBs with possible r-mode frequencies detected

The puzzling existence of a number of neutron stars that appear to be in the r-mode instability window supports further investigations of the r-mode frequencies, damping times and saturation amplitudes, especially now in light of the much anticipated and exciting results from LIGO and NICER. It has been suggested by Mahmoodifar and Strohmayer that coherent frequencies found in the RXTE data of the accreting millisecond pulsar XTE J1751-305 and during the 2001 superburst of 4U 1636-536 (one of the accreting sources to be observed by NICER) could, in fact, be r-modes. Based on these observations, we expand here the results of a previous work on relativistic, rotational and differential rotation corrections to the r-mode frequency in order to provide more accurate estimates of the $M-R$ relation for these neutron stars. Finally, we compare our results with predictions from a few realistic tabulated equations of state, providing further constraints that favor more compact models. We find that, if the observed frequencies indeed correspond to r-modes, then the masses of XTE J1751-305 and 4U 1636-536 should lie in the approximate ranges $1.48 - 1.56 M_{\odot}$ and $1.60 - 1.68 M_{\odot}$, respectively.

State transitions in ultracompact neutron star LMXBs: towards the low-luminosity limit

Luminosity of X-ray spectral state transitions in black hole and neutron star X-ray binaries can put constraint on the critical mass accretion rate between accretion regimes. Previous studies indicate that the hard-to-soft spectral state transitions in some ultracompact neutron star LMXBs have the lowest luminosity. With X-ray monitoring observations in the past decade, we were able to identify state transitions towards the lowest luminosity limit in 4U 0614+091, 2S 0918-549 and 4U 1246-588. By analysing corresponding X-ray pointed observations with the Swift/XRT and the RXTE/PCA, we found no hysteresis of state transitions in these sources, and determined the critical mass accretion rate in the range of 0.002 - 0.04 $\dot{\rm M}_{\rm Edd}$ and 0.003 - 0.05 $\dot{\rm M}_{\rm Edd}$ for the hard-to-soft and the soft-to-hard transition, respectively, by assuming a neutron star mass of 1.4 solar masses. This range is comparable to the lowest transition luminosity measured in black hole X-ray binaries, indicating the critical mass accretion rate is not affected by the nature of the surface of the compact stars. Our result does not support the Advection-Dominated Accretion Flow (ADAF) model which predicts that the critical mass accretion rate in neutron star systems is an order of magnitude lower if same viscosity parameters are taken. The low transition luminosity and insignificant hysteresis in these ultracompact X-ray binaries provide further evidence that the transition luminosity is likely related to the mass in the disc.

Probing the accretion disc structure by the twin kHz QPOs and spins of neutron stars in LMXBs

We analyze the relation between the emission radii of twin kilohertz quasi-periodic oscillations (kHz QPOs) and the co-rotation radii of the 12 neutron star low mass X-ray binaries (NS-LMXBs) which are simultaneously detected with the twin kHz QPOs and NS spins. We find that the average co-rotation radius of these sources is r_co about 32 km, and all the emission positions of twin kHz QPOs lie inside the corotation radii, indicating that the twin kHz QPOs are formed in the spin-up process. It is noticed that the upper frequency of twin kHz QPOs is higher than NS spin frequency by > 10%, which may account for a critical velocity difference between the Keplerian motion of accretion matter and NS spin that is corresponding to the production of twin kHz QPOs. In addition, we also find that about 83% of twin kHz QPOs cluster around the radius range of 15-20 km, which may be affected by the hard surface or the local strong magnetic field of NS. As a special case, SAX J1808.4-3658 shows the larger emission radii of twin kHz QPOs of r about 21-24 km, which may be due to its low accretion rate or small measured NS mass (< 1.4 solar mass).

MILLIHERTZ QUASI-PERIODIC OSCILLATIONS IN 4U 1636–536: PUTTING POSSIBLE CONSTRAINTS ON THE NEUTRON STAR SIZE

ABSTRACT Based on previous studies of quasi-periodic oscillations (QPOs) in neutron star (NS) low-mass X-ray binaries, mHz QPOs are believed to be related to “marginally stable” burning on the NS surface. Our study of phase-resolved energy spectra of these oscillations in 4U 1636–53 shows that the oscillations are not caused by variations in the blackbody temperature of the NS, but reveals a correlation between the change of the count rate during the mHz QPO pulse and the spatial extent of a region emitting blackbody emission. The maximum size of the emission area, km2, provides direct evidence that the oscillations originate from a variable surface area constrained on the NS and are therefore not related to instabilities in the accretion disk. The obtained lower limit on the size of the NS (11.0 km) rules out equations of state that prefer small NS radii. Observations of mHz QPOs in NS LMXBs with NICER and eXTP will reduce the statistical uncertainty in the lower limit on the NS radius, which together with better estimates of the hardening factor and distance, will allow for improved discrimination between different equations of state and compact star models. Furthermore, future missions will allow us to measure the peak blackbody emission area for a single mHz QPO pulse, which will push the lower limit to larger radii.

R-mode instability of strange stars and observations of neutron stars in LMXBs**Supported by the National Natural Science Foundation of China.

Using a realistic equation of state (EOS) of strange quark matter, namely, the modified bag model, and considering the constraints on the parameters of EOS by the observational mass limit of neutron stars, we investigate the r-mode instability window of strange stars, and find the same result as in the brief study of Haskell, Degenaar and Ho in 2012 that these instability windows are not consistent with the spin frequency and temperature observations of neutron stars in low mass X-ray binaries.

Variability of winds in X-ray binaries

AbstractWe discuss the variability of winds in two low-mass X-ray binaries, GX 13+1 and 4U 1630-47. XMM-Newton observations of these sources show that strong, photoionised winds with column densities well above 1022 cm−2 can be present in both neutron star and black hole LMXBs. Such winds can fade significantly due to changes in the flux or spectral hardness of the continuum. In particular, a decrease of column density and/or an increase of ionisation are measured when the flux increases or the spectrum hardens. We show how variability studies can help us to understand what triggers changes in the wind and discuss the limitations of current instruments for such studies.

Relativistic Iron Line Emission from the Neutron Star Low‐Mass X‐Ray Binary 4U 1636‐536

We present an analysis of XMM-Newton and RXTE data from three observations of the neutron star LMXB 4U 1636-536. The X-ray spectra show clear evidence of a broad, asymmetric iron emission line extending over the energy range 4-9 keV. The line profile is consistent with relativistically broadened Fe K-alpha emission from the inner accretion disk. The Fe K-alpha line in 4U 1636-536 is considerably broader than the asymmetric iron lines recently found in other neutron star LMXBs, which indicates a high disk inclination. We find evidence that the broad iron line feature is a combination of several K-alpha lines from iron in different ionization states.

The correlations between the spin frequencies and kHz QPOs of neutron stars in LMXBs

Aims.We studied the correlations between spin frequencies and kilohertz quasi-periodic oscillations (kHz QPOs) in neutron star low-mass X-ray binaries.

Average hard X-ray emission from NS LMXBs: observational evidence of different spectral states in NS LMXBs

Aims. We studied and compared the long-term average hard X-ray (>20 keV) spectra of a sample of twelve bright low-mass X-ray binaries hosting a neutron star (NS). Our sample comprises the six well studied Galactic Z sources and six Atoll sources, four of which are bright (“GX”) bulge sources while two are weaker ones in the 2–10 keV range (H 1750–440 and H 1608–55). Methods. For all the sources of our sample, we analysed available public data and extracted average spectra from the IBIS/ISGRI detector on board INTEGRAL . Results. We can describe all the spectral states in terms of the bulk motion Comptonisation scenario. We find evidence that bulk motion is always present, its strength is related to the accretion rate and it is suppressed only in the presence of high local luminosity. The two low-dim Atoll source spectra are dominated by photons up-scattered presumably due to dynamical and thermal Comptonisation in an optically thin, hot plasma. For the first time, we extend the detection of H 1750–440 up to 150 keV. The Z and bright “GX” Atoll source spectra are very similar and are dominated by Comptonised blackbody radiation of seed photons, presumably coming from the accretion disc and NS surface, in an optically thick cloud with plasma temperature in the range of 2.5–3 keV. Six sources show a hard tail in their average spectrum: Cyg X-2 (Z), GX 340+0 (Z), GX 17+2 (Z), GX 5–1 (Z), Sco X–1 (Z) and GX 13+1 (Atoll). This is the first detection of a hard tail in the X-ray spectrum of the peculiar GX 13+1. Using radio data from the literature we find, in all Z sources and bright “GX” Atolls, a systematic positive correlation between the X-ray hard tail (40–100 keV) and the radio luminosity. This suggests that hard tails and energetic electrons causing the radio emission may have the same origin, most likely the Compton cloud located inside the NS magnetosphere.

Boundary layer emission and Z-track in the color–color diagram of luminous LMXBs

Aims. We explore the accretion disk and boundary layer emission in bright neutron star LMXBs and their dependence on the mass accretion rate. Methods. We used Fourier-frequency resolved spectroscopy of the archival RXTE data. Results. We demonstrate that Fourier-frequency resolved spectra of atoll and Z-sources are identical, despite significant differences in their average spectra and luminosity (by a factor of ∼10−20). This result fits in the picture we suggested earlier, namely that the f > 1 Hz variability in luminous LMXBs is primarily due to variations in the boundary layer luminosity. In this picture the frequency-resolved spectrum equals the boundary layer spectrum, which therefore can be straightforwardly determined from the data. This boundary layer spectrum is approximated well by the saturated Comptonization model, and its high energy cut-off follows a kT ≈ 2.4 keV blackbody. Its independence from the global mass-accretion rate lends support to a theoretical suggestion, that the boundary layer is supported by radiation pressure. With this assumption we constrain the gravity on the neutron star surface, its mass and radius. Equipped with the knowledge of the boundary layer spectrum, we attempt to relate the motion along the Z-track to changes of physically meaningful parameters. Our results suggest that the contribution of the boundary layer to the observed emission decreases along the Z-track from conventional ∼50% on the horizontal branch to a rather small number on the normal branch. This decrease can be caused, for example, by obscuration of the boundary layer by the geometrically thick accretion disk at u M ∼ u MEdd. Alternatively, this can indicate a significant change in the structure of the accretion flow at u M ∼ u MEdd and the disappearance of the boundary layer as a distinct region of the significant energy release associated with the neutron star surface.

The population of faint transients in the Galactic Centre

BeppoSAX has detected a population of faint transient X-ray sources in the Galactic Centre. I show that a simple irradiated-disc picture gives a consistent fit to the properties of this population, and that it probably consists of low-mass X-ray binaries which have evolved beyond their minimum periods ~80 min. Since all post-minimum systems are transient, and neutron-star LMXBs are more common than black-hole LMXBs in the Galaxy, the majority of these systems should contain neutron stars, as observed. This picture predicts that the Galactic Centre transients should have orbital periods in the range \~80-120 minutes, and that most of them should repeat in the next few years. In this case, the total number of post-minimum transients in the Galaxy would be considerably smaller than the usual estimates of its total LMXB population. I discuss possible reasons for this.

Kilohertz QPOs and general-relativistic orbital motion—a maximum likelihood test

AbstractUsing the maximum likelihood method we show that the observed distribution of maximum frequencies of kilohertz quasi periodic oscillations (QPOs) in LMXBs agrees with the frequencies of orbital motion in the marginally stable orbit around compact objects with masses chosen randomly from some interval, which we find to be (1.55M⊙, 2.40M⊙) at the 99% confidence level. As this remarkable agreement with the expected mass range of neutron stars in LMXBs demonstrates, the assumption that the maximum-frequency QPOs occur in the marginally stable orbit is consistent with the current data set.

Magnetic fields of neutron stars in low mass X-ray binaries

Abstract We report detailed radiation transport calculations which show that the shape of the 1–30 keV continuum spectrum of a weakly magnetic accreting neutron star depends on the strength of its magnetic field. We use this dependence to estimate the magnetic fields of several accreting neutron stars in LMXBs. We demonstrate that the magnetic field strengths estimated in this way correlate strongly with the properties of the 15–60 Hz magnetospheric QPOs and the kilohertz QPOs observed in many Z and atoll sources. We also find that sources currently accreting at high rates have higher magnetic field strengths than sources currently accreting at lower rates. This is an important constraint on models of the evolution of the magnetic field strengths of accreting neutron stars.

Evidence for a 2-h optical modulation in GS 1826-24

We report the discovery of a 2.1-h optical modulation in the transient source GS 1826 – 24, based on two independent, high-time-resolution photometric observing runs. There is additional irregular variability on shorter time-scales. The source also exhibited an optical burst during each observation, with peak fluxes consistent with those of the three X-ray bursts so far detected by BeppoSAX. We compare the low-amplitude variation (∼ 0.06 mag) to that seen on the orbital periods of the short-period X-ray bursters, X1636−536 and X1735−444, as well as the similarity in their non-periodic fluctuations. Other transient neutron star LMXBs possess short periods in the range 3.8–7.1 h. However, if confirmed as the orbital, a 2.1-h modulation would make GS 1826 – 24 unique and therefore of great interest within the context of LMXB formation and evolution.

Populations of low-mass black hole binaries

The evolution of a population of massive binaries with low-mass secondaries is considered by contrasting the fate of black hole-producing systems with those leaving neutron stars. It is found that the interacting black hole binaries may be even more common than the familiar neutron star LMXBs. Certain unusual expected systems are described, and a comparison with present observations is made to determine the accuracy of predictions of this scenario.