X-ray Pulsar GRO Research Articles

X-ray pulsar GRO J1008−57 as an orthogonal rotator

X-ray polarimetry is a unique way to probe the geometrical configuration of highly magnetized accreting neutron stars (X-ray pulsars). GRO J1008−57 is the first transient X-ray pulsar observed at two different flux levels by the Imaging X-ray Polarimetry Explorer (IXPE) during its outburst in November 2022. We find the polarization properties of GRO J1008−57 to be independent of its luminosity, with the polarization degree varying between nondetection and about 15% over the pulse phase. Fitting the phase-resolved spectro-polarimetric data with the rotating vector model allowed us to estimate the pulsar inclination (130°, which is in good agreement with the orbital inclination), the position angle (75°) of the pulsar spin axis, and the magnetic obliquity (∼74°). This makes GRO J1008−57 the first confidently identified nearly orthogonal rotator among X-ray pulsars. We discuss our results in the context of the neutron star atmosphere models and theories of the axis alignment of accreting pulsars.

X-ray polarimetry of X-ray pulsar X Persei: another orthogonal rotator?

ABSTRACT X Persei is a persistent low-luminosity X-ray pulsar of period of ≈ 835 s in a Be binary system. The field strength at the neutron star surface is not known precisely, but indirect signs indicate a magnetic field above 1013 G, which makes the object one of the most magnetized known X-ray pulsars. Here we present the results of observations X Persei performed with the Imaging X-ray Polarimetry Explorer (IXPE). The X-ray polarization signal was found to be strongly dependent on the spin phase of the pulsar. The energy-averaged polarization degree in 3–8 keV band varied from several to ∼20 per cent over the pulse with a phase dependence resembling the pulse profile. The polarization angle shows significant variation and makes two complete revolutions during the pulse period, resulting in nearly nil pulse-phase averaged polarization. Applying the rotating vector model to the IXPE data we obtain the estimates for the rotation axis inclination and its position angle on the sky, as well as for the magnetic obliquity. The derived inclination is close to the orbital inclination, reported earlier for X Persei. The polarimetric data imply a large angle between the rotation and magnetic dipole axes, which is similar to the result reported recently for the X-ray pulsar GRO J1008−57. After eliminating the effect of polarization angle rotation over the pulsar phase using the best-fitting rotating vector model, the strong dependence of the polarization degree with energy was discovered, with its value increasing from 0 at ∼2 keV to 30per cent at 8 keV.

Discovery of cyclotron and narrow Fe Kα lines in HMXB GRO J1750–27

ABSTRACT We report on the timing and spectral analysis of transient Be X-ray pulsar GRO J1750–27 using the Nuclear Spectroscopic Telescope Array(NuSTAR) observation from 2021 September. This is the fourth outburst of the system since 1995. The NuSTAR observation was performed during the rising phase of the outburst. Pulsations at a period of 4.450710(1) s were observed in the 3–60 keV energy range. The average pulse profile comprised of a broad peak with a weak secondary peak, which evolved with energy. We did not find any appreciable variation in the X-ray emission during this observation. The broad-band phase-averaged spectrum is described by a blackbody, a power law, or Comptonization component. We report the discovery of Fe Kα line at 6.4 keV, along with the presence of two cyclotron resonant scattering features of around 36 and 42 keV. These lines indicate a magnetic field with the strength of $3.7_{-0.3}^{+0.1} \times 10^{12}$ and 4.4 ± 0.10 × 1012 G for the neutron star. We have estimated a source distance of ∼13.6–16.4 kpc based on the accretion-disc torque models.

Changes in the Nature of the Spectral Continuum and Stability of the Cyclotron Line in the X-ray Pulsar GRO J2058+42

Changes in the Nature of the Spectral Continuum and Stability of the Cyclotron Line in the X-ray Pulsar GRO J2058+42

SRG/ART-XC and NuSTAR Observations of the X-Ray pulsar GRO J1008–57 in the Lowest Luminosity State

Abstract We report results of the first broadband observation of the transient X-ray pulsar GRO J1008−57 performed in the quiescent state. Observations were conducted quasi-simultaneously with NuSTAR and the Mikhail Pavlinsky ART-XC telescope on board SRG right before the beginning of a Type I outburst. GRO J1008−57 was detected in the state with the lowest observed luminosity around several ×1034 erg s−1 and consequently accreting from the cold disk. Timing analysis allowed us to detect pulsations during this state for the first time. The observed pulsed fraction of about 20% is, however, almost three times lower than in brighter states when the accretion proceeds through the standard disk. We traced the evolution of the broadband spectrum of the source on a scale of three orders of magnitude in luminosity and found that at the lowest luminosities the spectrum transforms into the double-hump structure similarly to other X-ray pulsars accreting at low luminosities (X Persei, GX 304–1, and A 0535+262) reinforcing the conclusion that this spectral shape is typical for these objects.

Evidence for the radiation-pressure dominated accretion disk in bursting pulsar GRO J1744−28 using timing analysis

The X-ray pulsar GRO J1744−28 is a unique source that shows both pulsations and type-II X-ray bursts, allowing studies of the interaction of the accretion disk with the magnetosphere at huge mass-accretion rates exceeding 1019 g s−1 during its super-Eddington outbursts. The magnetic field strength in the source, B ≈ 5 × 1011 G, is known from the cyclotron absorption feature discovered in the energy spectrum around 4.5 keV. Here, we have explored the flux variability of the source in context of interaction of its magnetosphere with the radiation-pressure dominated accretion disk. Specifically, we present the results of the analysis of noise power density spectra (PDS) using the observations of the source in 1996–1997 by the Rossi X-ray Timing Explorer (RXTE). Accreting compact objects commonly exhibit a broken power-law PDS shape with a break corresponding to the Keplerian orbital frequency of matter at the innermost disk radius. The observed frequency of the break can thus be used to estimate the size of the magnetosphere. We find, however, that the observed PDS of GRO J1744−28 differs dramatically from the canonical shape. The observed break frequency appears to be significantly higher than expected based on the magnetic field estimated from the cyclotron line energy. We argue that these observational facts can be attributed to the existence of the radiation-pressure dominated region in the accretion disk at luminosities above ∼2 × 1037 erg s−1. We discuss a qualitative model for the PDS formation in such disks, and show that its predictions are consistent with our observational findings. The presence of the radiation-pressure dominated region can also explain the observed weak luminosity dependence of the inner radius, and we argue that the small inner radius can be explained by a quadrupole component dominating the magnetic field of the neutron star.

Stable accretion from a cold disc in highly magnetized neutron stars

The aim of this paper is to investigate the transition of a strongly magnetized neutron star into the accretion regime with very low accretion rate. For this purpose we monitored the Be-transient X-ray pulsar GRO J1008-57 throughout a full orbital cycle. The current observational campaign was performed with the Swift/XRT telescope in the soft X-ray band (0.5-10 keV) between two subsequent Type I outbursts in January and September 2016. The expected transition to the propeller regime was not observed. However, the transitions between different regimes of accretion were detected. In particular, after an outburst the source entered a stable accretion state characterised by the accretion rate of ~10^14-10^15 g/s. We associate this state with accretion from a cold (low-ionised) disc of temperature below ~6500 K. We argue that a transition to such accretion regime should be observed in all X-ray pulsars with certain combination of the rotation frequency and magnetic field strength. The proposed model of accretion from a cold disc is able to explain several puzzling observational properties of X-ray pulsars.

Study of the accretion torque during the 2014 outburst of the X-ray pulsar GRO J1744−28

We present the spectral and timing analysis of the X-ray pulsar GRO J1744-28 during its 2014 outburst using data collected with the X-ray satellites Swift, INTEGRAL, Chandra, and XMM-Newton. We derived, by phase-connected timing analysis of the observed pulses, an updated set of the source ephemeris. We were also able to investigate the spin-up of the X-ray pulsar as a consequence of the accretion torque during the outburst. Relating the spin-up rate and the mass accretion rate as $\dot{\nu}\propto\dot{M}^{\beta}$, we fitted the pulse phase delays obtaining a value of $\beta=0.96(3)$. Combining the results from the source spin-up frequency derivative and the flux estimation, we constrained the source distance to be between 3.4-4.1 kpc, assuming a disc viscous parameter $\alpha$ to be in the range 0.1-1. Finally, we investigated the presence of a possible spin-down torque by adding a quadratic component to the pulse phase delay model. The marginal statistical improvement of the updated model does not allow us to firmly confirm the presence of this component.

Broad band X-ray properties of GRO J1948+32/KS 1947+300

We present here results obtained from three BeppoSAX observations of the transient X-ray pulsar GRO J1948+32 carried out during the declining phase of its 2000 November–2001 June outburst. Timing analysis of the data clearly shows a 18.7 s pulsation in the X-ray light curves in 0.1–100 keV energy band. The pulse profile of GRO 1948+32 is characterized by a broad peak with a sharp rise followed by a narrow dip. The dip in the pulse profile shows very strong energy dependence. Phase-averaged spectroscopy obtained with three of the BeppoSAX instruments shows that the 0.1–100 keV energy spectrum is described by a Comptonized component, a weak blackbody component (∼7% of the total emission) for soft X-rays, a narrow and weak iron emission line at 6.7 keV and low column density of material in the line of sight. The results obtained from the analysis are discussed in this paper.

Observation of hard X-ray emission from GRO J1744-28 in the 20–200 keV band during declining phase

We report spectral measurements of the bursting X-ray pulsar GRO J1744-28, made during a balloon flight experiment using high sensitivity scintillation counter telescope operating in the 20–200 keV energy band. The observations were made on March 30th, 1997 near to fading away of the source during its second outburst in December 1996 to April 1997. The X-ray flux in the 25–100 keV energy range was a factor of 10 lower than the averaged flux during the burst. The continuum spectrum was found to be extremely hard and is best fitted with a power law of the form dN dE = KE −α with spectral index α = 2.05 ± 0.1 and is very different compared to the averaged burst spectrum. The form of the spectrum compares with the transient X-ray pulsating source 4U0535+26 during its quiescent phase. At a distance of 8 kpc, the observed source luminosity in hard X-rays is ∼ 10 37 erg s −1. The power law nature of the X-ray emission, close to fading of the source during the declining phase suggests that unpulsed emission arises in the accretion disk which gradually disappears.

The ASCA observation of GRO J1744-28

The ASCA results of the bursting X-ray pulsar GRO J1744-28 are reported. The observations were made twice, in February 1996 and March 1997. We detected 12 and 17 Type II bursts during the two observations, with mean bursting intervals of about 27 min and 37 min. Each burst is followed by an intensity dip with the depleted flux depending on the burst fluence. The energy spectra are approximated by an absorbed power law with additional structure around 6–7 keV, which is most probably due to iron and maybe reproduced by the disk line model with additional broadening mechanism. The absorption column is constant ((5 − 6) × 10 22 cm −2), independent of the observation dates and emission phase (persistent, burst and dip). This means the source may be actually located near the galactic center (8.5kpc).

Disk Instability in Wide Neutron Star Low‐Mass X‐Ray Binaries

Conditions for disk instability in low-mass X-ray binaries (LMXBs) have been recently considered with X-ray irradiation of the disk included. Conservative evolution was generally assumed, which, however, fails in wide LMXBs with a neutron star primary (NLMXBs). If the orbital period of an LMXB is long enough, the mass transfer rate becomes super-Eddington, and the evolution is likely to be nonconservative. Adopting the Eddington limited accretion hypothesis, we have calculated the evolution of NLMXBs driven by nuclear expansion of the secondary, and obtained a modified boundary between persistent and transient behavior in NLMXBs. We briefly discuss the applications to low-mass binary radio pulsars (LMBPs) and suggest a new scenario for the progenitor system of the bursting X-ray pulsar GRO J1744-28.

Discovery and Orbital Determination of the Transient X‐Ray Pulsar GRO J1750−27

We report on the discovery and hard X-ray (20-70 keV) observations of the 4.45 s period transient X-ray pulsar GRO J1750-27 with the BATSE all-sky monitor on board CGRO. A relatively faint outburst (<30 mcrab peak) lasting at least 60 days was observed during which the spin-up rate peaked at 38 pHz s^(-1) and was correlated with the pulsed intensity. An orbit with a period of 29.8 days was found. The large spin-up rate, spin period, and orbital period together suggest that accretion is occurring from a disk and that the outburst is a giant outburst typical of a Be/X-ray transient system. No optical counterpart has yet been reported.

Thermonuclear Burning on the Accreting X‐Ray Pulsar GRO J1744−28

We investigate the thermal stability of nuclear burning on the accreting X-ray pulsar GRO J1744-28. The neutron star's dipolar magnetic field is 3 × 1011 G if persistent spin-up implies that the magnetospheric radius is less than the corotation radius. Bildsten earlier noted that magnetic fields this weak might not quench the vigorous convection sometimes associated with thermonuclear instabilities, so that a convective burning front can propagate around the star in a few seconds and rapidly release the accumulated nuclear energy. After inferring the properties of the neutron star, we study the thermal stability of hydrogen/helium burning and show that thermonuclear instabilities are unlikely causes of the hourly bursts seen at very high accretion rates. Then we discuss how the stability of the thermonuclear burning depends on both the global accretion rate and the neutron star's magnetic field strength. If the accreted matter spreads over the star prior to ignition, then the burning will become unstable when the accretion rate is below 6 × 10-9 M☉ yr-1. We emphasize that the appearance of the instability (i.e., whether it looks like a type I X-ray burst or a flare lasting a few minutes) will yield crucial information on the neutron star's surface magnetic field and the role of magnetic fields in convection. We suggest that a thermal instability in the accretion disk is the origin of the long (~300 days) outburst and that the recurrence time of these outbursts is more than 50 yr. We also discuss the nature of the binary and point out that a velocity measurement of the stellar companion (most likely a Roche lobe filling giant with mK 17) will constrain the neutron star mass. This is of interest in this binary, where the inferred low mass of the companion implies that 0.5 M☉ of transfer occurred.

The Accretion Curtain and Pulse Phase Variations of the Bursting X-Ray Pulsar GRO J1744−28

The X-ray pulsar GRO J1744-28 accretes so rapidly that the gas flow onto its magnetic poles should be decelerated by radiation forces rather than gas pressure gradients. The vertical scale of the deceleration region, where the bulk of the flow's kinetic energy is transformed to radiation, is large compared to the size of the accretion flow's footprint on the stellar surface, and radiation escapes in a fan beam through the sides of the decelerating gas column. If one assumes that the X-ray bursts observed from GRO J1744-28 are triggered in the accretion disk, this picture of the accretion column geometry and a consideration of the interaction between the disk and the stellar magnetic field at the magnetospheric boundary lead to a natural explanation for the phase lags of the X-ray pulses observed during bursts: The accretion column is not round but flattened, so its footprint forms an arc on the stellar surface close to the magnetic pole. The broad side of the arc faces the magnetic pole. When the accretion rate rises, the gas flow shifts to a different set of field lines, and the accretion footprint pivots through a moderate angle around the magnetic pole. Although the footprint has remained close to the magnetic pole, the direction it faces has changed. The time of pulse maximum, when an observer sees the broad side of the accretion column most face-on, changes accordingly, and a phase-lag results.

On the Nature of the Bursting X-Ray Pulsar GRO J1744−28

The unusual properties of the bursting X-ray pulsar GRO J1744-28 are explained in terms of a low-mass X-ray binary system consisting of an evolved stellar companion transferring mass through Roche-lobe overflow onto a neutron star, implying that the inclination of the system is ≲18°. Interpretation of the QPO at frequency νQPO = 40 Hz using the beat-frequency model of Alpar & Shaham and the measured period derivative with the Ghosh & Lamb accretion-torque model implies that the persistent X-ray luminosity of the source is approximately equal to the Eddington luminosity and that the neutron star has a surface equatorial magnetic field Beq ≅ 2 × 1010 (νQPO/40 Hz)-1 G for standard neutron star parameters. This implies a distance to GRO J1744-28 of ≅5 (νQPO/40 Hz)1/6b1/2 kpc, where b < 1 is a correction factor that depends on the orientation of the neutron star.

Hard X-Ray Spectroscopy and Pulsar Phase Analysis of the Bursting X-Ray Pulsar GRO J1744−28 with OSSE

We report OSSE observations of the bursting X-ray pulsar GRO J1744-28 made in 1995 December and 1996 January at hard X-ray energies >35 keV. The pulse profile of the persistent (i.e., nonbursting) pulsar emission is fitted with a sinusoid in the energy range 35-90 keV. Residuals reveal a second harmonic amplitude of 3.0% ± 0.5% of the fundamental. The distribution of time intervals between bursts measured in January is characterized by a broad flat-topped function with width ≃35 minutes and mean ≃33 minutes. The burst profile averaged over an ensemble of 104 bursts in the 35-60 keV energy range has FWHM width of 3.6 ± 0.3 s and displays a factor of 2 faster rise time than decay and a pronounced dip in persistent emission after the burst. The phase of the sinusoidal pulse profile during bursts lags the phase prior to bursts by ≃90 ms (≃1.2 radians), and a 29 ± 6 ms (0.39 ± 0.08 radians) lag persists following the burst. There are no statistically significant spectral differences between the hard X-ray spectra of the bursting and persistent emission in the OSSE energy range, nor is there any evidence of annihilation or neutron capture line emission or cyclotron absorption.

Discovery of the 18.7 Second Accreting X-Ray Pulsar GRO J1948+32

view Abstract Citations (34) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Discovery of the 18.7 Second Accreting X-Ray Pulsar GRO J1948+32 Chakrabarty, Deepto ; Koh, Towsian ; Bildsten, Lars ; Prince, Thomas A. ; Finger, Mark H. ; Wilson, Robert B. ; Pendleton, Geoffrey N. ; Rubin, Bradley C. Abstract We have detected an 18.7 s accreting X-ray pulsar in the Cygnus region, using the BATSE large-area detectors on the Compton Gamma Ray Observatory. GRO J1948+32 has been localized to within 10 deg2 using a method we developed for positioning weak pulsed sources with BATSE. During the 33 day outburst, the phase-averaged 20-75 keV pulsed flux rose from 25 mCrab to 50 mCrab over 10 days and then decayed below our detection j-threshold over nearly 25 days. A photon spectral index of γ = 2.65±0.15 (assuming photon flux density dN/dE ∝ E-γ) was measured during a bright interval. The observed modulation of the neutron star's pulse frequency is suggestive of orbital variation over less than one orbit cycle. Assuming a constant spin frequency derivative over the outburst, we can place the following individual 95% confidence limits on each of the pulsar parameters: orbital period 35d < Porb < 70d; orbital radius 75 lt-sec < αx sin i < 300 lt-sec, eccentricity e < 0.25, spin frequency derivative 5 × 10-13 Hz s-1 < vdot < 2.5 × 10-11 Hz s-1, X-ray mass function 0.5 Msun < fx(M) < 5 Msun. As the stellar type of the mass-providing companion is still not known for this source, we briefly speculate on the nature of mass transfer in this system. Publication: The Astrophysical Journal Pub Date: June 1995 DOI: 10.1086/175839 Bibcode: 1995ApJ...446..826C Keywords: X-RAYS: STARS; STARS: PULSARS: INDIVIDUAL ALPHANUMERIC: GRO J1948+32; STARS: PULSARS: INDIVIDUAL ALPHANUMERIC: GRO J2014+34 full text sources ADS | data products SIMBAD (2) HEASARC (1)