High-mass X-ray Binary 4U Research Articles

Timing analysis of Be/X-ray transient 4U 0115 + 63 during Type II outburst of 2023 using NuSTAR observations

In this paper, we present the timing analysis of high mass X-ray binary 4U 0115 + 63during its Type II outburst of March 2023 using two NuSTARobservations. The first observation was made near the peak of the outburst, while the second observation was made near the end of declining phase. During the first observation, 3.0–79.0 keV coherent pulsations at 3.61456(4) s (at MJD 60044.003) were detected. During the second observation, 3.0–68.0 keV coherent pulsations at 3.61309(1) s (at MJD 60060.732) were detected, thereby indicating a secular spin up of -1.02(3)×10-9ss-1. During these observations, the pulse profile was dual peaked with the relative intensity of minor peak reducing with energy. The main peak was narrower at low energies and it broadened as energy increased. The pulsed fraction showed a positive correlation with energy, along with local maxima close to the cyclotron absorption features. Quasi periodic oscillations were also detected up to 20.0 keV. Broad ∼12.4 mHz and ∼58.3 mHz QPOs were detected near the peak of the outburst and towards its end, QPOs centered at ∼2.3 mHz, ∼8.4 mHz and ∼66.5 mHz were detected. We discuss the implication of these results on our understanding of accretion mechanism during outbursts.

Timing and Spectral Analysis of HMXB 4U 1700-37 Observed with Insight-HXMT

We report timing and spectral studies of the high-mass X-ray binary 4U 1700-37 using Insight-HXMT observations carried out in 2020 during its out-of-eclipse state. We found significant variations in flux on a timescale of kilo-seconds, while the hardness (count rate ratio between 10–30 keV and 2–10 keV) remains relatively stable. No evident pulsations were found over a frequency range of 10−3–2000 Hz. During the spectral analysis, for the first time, we took the configuration of different Insight-HXMT detectors’ orientations into account, which allows us to obtain reliable results even if stable contamination exists in the field of view. We found that the spectrum could be well described by some phenomenological models that are commonly used in accreting pulsars (e.g., a power law with a high energy cutoff) in the energy range of 2–100 keV. We found hints of cyclotron absorption features around ∼16 keV or/and ∼50 keV.

Nustar observation of the binary system 4U 0114 + 65

ABSTRACT The high-mass X-ray binary system 4U 0114 + 65 was observed by Nustar in October 2019 and by XMM-Newton in August 2015. Here we performed spectral and timing analysis of the Nustar observation and carry out timing analysis on the XMM-Newton data. We measured the spin period of the neutron star from both observations and found a spin-up rate $\dot{p}$ = 1.54 ± 0.38 × 10−6$\mathrm{s\, s}^{-1}$ . During the Nustar observation, two flares occurred, one shortly after the start of the observation and the other near the end separated by a long period of low/quiescent state. The large and sudden flares mostly resulted from accretion of corotating interaction region (CIR) material. A common spectral model to high mass X-ray binaries (HMXBs), powerlaw with high energy cutoff and absorption at low energy, gave a good fit to both flaring and quiescent states. A fluorescent iron line was not required in fitting any of the states. On the other hand, very tentative evidence of Cyclotron Resonant Scattering Feature (CRSF) at ∼17 keV was found during fitting using cyclabs model; however, fitting improvement was not significant enough to confirm its detection, plus a very narrow width (<1 keV) was obtained for the line and its first harmonic. Visual inspection of the spectra showed a deficiency of emission near the expected first and second harmonic. Another important feature visually noticed in the spectra is the presence of hard tail above 50 keV. This could be explained by the shocked material bounding the CIR.

A high-mass X-ray binary pulsar 4U 1907+09 with multiple absorption-line features in the spectrum

ABSTRACT We report X-ray observations of the high mass X-ray binary (HMXB) pulsar 4U 1907+09. Spectral and timing analysis of the source has been performed using Nuclear Spectroscopic Telescope Array (NuSTAR) observation. Timing analysis of the photon events revealed the coherent X-ray pulsation of the source with a pulse period of 442.92 ± 0.03 s. It is observed that the source is spinning down at a rate of 0.1971(4) s yr−1. The pulse profile is characterized by a decaying amplitude of the secondary peak and relative growth in the amplitude of the primary peak with an increase in energy. The broad-band spectral coverage of NuSTAR has been used to observe multiple absorption features in the X-ray continuum of the source. We confirm the presence of two prominent cyclotron absorption features at ∼17 and ∼38 keV, respectively. In addition, we have detected an absorption-line feature at ∼8 keV, with an equivalent width of ∼1.3 keV. The variation of the spectral parameters with pulse phase has been observed using phase-resolved spectroscopy and the relevant variabilities of the parameters have been discussed with the underlying physical implications. The continuum evolution and variations in spectral parameters have also been studied by time-resolved spectroscopy.

The origin of the high-mass X-ray binary 4U 2206+54/BD+532790

ABSTRACT Based on the Gaia EDR3 astrometric parameters and our new systemic radial velocity of the high-mass X-ray binary 4U 2206+54/BD+532790, we studied the trace back motion of the system and propose that it originated in the subgroup of the Cepheus OB1 association (Age ∼4–10 Myr) with its brightest star, BD+532820 (B0V; ${\cal L}$∼104.7${\cal L}$⊙). The kinematic age of 4U 2206+54 is about 2.8 ± 0.4 Myr, it is at a distance of 3.1–3.3 kpc and has a space velocity of 75–100 km s−1 with respect to this member star (BD+532820) of the Cep OB1 association. This runaway velocity indicates that the progenitor of the neutron star hosted by 4U 2206+54 lost about 4–9 M⊙ during the supernova explosion and the latter one received a kick velocity of at least 200–350 km s−1 . Since the high-mass X-ray binary 4U 2206+54/BD+532790 was born as a member of a subgroup of Cep OB1, the initially most massive star in the system terminated its evolution within $\lesssim 7-9$ Myr, corresponding to an initial mass $\gtrsim 32$ M⊙.

Spectroscopic modelling of two high-mass X-ray binaries, Cyg X–3 and 4U 1538–522

ABSTRACT We report a detailed modelling of soft X-ray emission lines from two stellar wind-fed Galactic high-mass X-ray binary (HMXB) systems, Cyg X-3 and 4U 1538-522, and estimate physical parameters, e.g. hydrogen density, radiation field, chemical abundances, wind velocity, etc. The spectral synthesis code cloudy is utilized for this modelling. We model highly ionized X-ray spectral lines, such as Fe XXV (6.700 keV) and Fe XXVI (6.966 keV), and reproduce the observed line flux values. We find that for Cyg X–3 and 4U 1538-522, the inner radius of the ionized gas is at a distance of 1012.25 cm and 1010.43 cm, respectively, from the primary star, which is the main source of ionization. The densities of the ionized gas for Cyg X–3 and 4U 1538–522 are found to be ∼1011.35 cm−3 and 1011.99 cm−3, respectively. The corresponding wind velocities are 2000 km s−1 and 1500 km s−1. The respective predicted hydrogen column densities for Cyg X–3 and 4U 1538–522 are 1023.2 cm−2 and 1022.25 cm−2. In addition, we find that magnetic field affects the strength of the spectral lines through cyclotron cooling. Hence, we perform separate model comparisons including magnetic field for both the sources. Most of the parameters, except the hydrogen column density, have similar values with and without magnetic field. We estimate that the most probable strength of the magnetic field for Cyg X–3 and 4U 1538–522, where the Fe XXV and Fe XXVI lines originate, is ∼102.5 G.

On the origin of runaway binaries: the case of the HMXB 4U 2206+54/BD +53 2790

We present most probable place and time of the origin of the runaway high-mass X-ray binary 4U 2206+54 based on its Gaia EDR3 astrometric parameters and our new systemic radial velocity. We studied the trace back motion of the system and propose that it originated in the subgroup of the Cepheus OB1 association (Age∼4-10 Myr) with its brightest star BD+53 2820 (B0V; L∼104.7L⊙). The kinematic age of 4U 2206+54 is about 2.8 ± 0.4 Myr, it is at a distance of 3.1-3.3 kpc and has a space velocity of 75-100 km/s with respect to this member star (BD+53 2820) of the Cep OB1 association. This runaway velocity indicates that the progenitor of the neutron star hosted by 4U 2206+54 lost about 4-9M⊙ during the supernova explosion and the latter one received a kick velocity of at least 200-350 km/s. The high-mass X-ray binary 4U 2206+54/BD+53 2790 was born as a member of a subgroup of the Cep OB1 association, the initially most massive star in the system terminated its evolution within ≲ 7 − 9 Myr, corresponding to an initial mass ≳ 32 M⊙.

Superorbital Modulation in the High-mass X-Ray Binary 4U 1538–52 and Possible Modulation in IGR J16393–4643

Abstract Hard X-ray observations with the Neil Gehrels Swift Observatory Burst Alert Telescope (BAT) reveal superorbital modulation in the wind-accreting supergiant high-mass X-ray binary (HMXB) 4U 1538−52 at a period of 14.9130 ± 0.0026 days that is consistent with four times the 3.73 day orbital period. These periods agree with a previously suggested correlation between superorbital and orbital periods in similar HMXBs. During the ∼14 yr of observations the superorbital modulation changes amplitude, and since ∼MJD 57,650 it was no longer detected in the power spectrum, although a peak near the second harmonic of this was present for some time. Measurements of the spin period of the neutron star in the system with the Fermi Gamma-ray Burst Monitor show a long-term spin-down trend, which halted toward the end of the light curve, suggesting a connection between and superorbital modulation, as proposed for 2S 0114+650. However, an earlier torque reversal from INTEGRAL observations was not associated with superorbital modulation changes. B- and V-band photometry from the Las Cumbres Observatory reveals orbital ellipsoidal photometric variability, but no superorbital optical modulation. However the photometry was obtained when the 14.9130 day period was no longer detected in the BAT power spectrum. We revisit possible superorbital modulation in BAT observations of IGR J16393−4643 but cannot conclusively determine whether this is present, although it is not persistent. We consider superorbital modulation mechanisms, and suggest that the corotating interaction region model, with small deviations from orbital synchronization, appears promising.

High resolution X-ray spectroscopy of Supergiant HMXB 4U1700−37 during the compact object eclipse.

Abstract We present an analysis of the first observation of the iconic High Mass X-ray Binary 4U 1700−37 with the Chandra High Energy Transmission Gratings during an X-ray eclipse. The goal of the observation was to study the structure/physical conditions in the clumpy stellar wind through high resolution spectroscopy. We find that: a) emission line brightness from K shell transitions, corresponding to near neutral species, directly correlates with continuum illumination. However, these lines do not greatly diminish during eclipse. This is readily explained if fluorescence Kα emission comes from the bulk of the wind. b) The highly ionised Fe xxv and Fe xxvi Lyα diminish during eclipse. Thus, they must be produced in the vicinity of the compact object where log ξ > 3. c) to describe the emission line spectrum, the sum of two self consistent photo ionisation models with low ionisation (log ξ ∼ −1) and high ionisation (log ξ ∼ 2.4) is required. From their emission measures, the clump-to-interclump density ratio can be estimated to be nc/ni ∼ 300. To fit the complex He-like Si xiii profile, the plasma requires a broadening with vbulk ∼ 840 km s−1. Reproducing the observed r ≈ f line fluxes requires the addition of a third collisionally ionised plasma. d) Emission lines widths appear unresolved at the hetg gratings resolution with exception of Silicon. There is no clear radial segregation between (quasi)neutral and ionised species, consistent with cold wind clumps interspersed in a hot rarefied interclump medium.

The M Supergiant High-mass X-Ray Binary 4U 1954+31

Abstract The X-ray binary 4U 1954+31 has been classified as a low-mass X-ray binary containing an M giant and a neutron star (NS). It has also been included in the rare class of X-ray symbiotic binaries. The Gaia parallax, infrared colors, spectral type, abundances, and orbital properties of the M star demonstrate that the cool star in this system is not a low-mass giant but a high-mass M supergiant. Thus, 4U 1954+31 is a high-mass X-ray binary (HMXB) containing a late-type supergiant. It is the only known binary system of this type. The mass of the M I is M ⊙ giving an age of this system in the range 12–50 Myr with the NS no more than 43 Myr old. The spin period of the NS is one of the longest known, 5 hr. The existence of M I plus NS binary systems is in accord with stellar evolution theory, with this system a more evolved member of the HMXB population.

Revisiting the spectral and timing properties of 4U 1909+07 with NuSTAR and Astrosat

ABSTRACT We present the results obtained from the analysis of high-mass X-ray binary pulsar 4U 1909+07 using NuSTAR and Astrosat observations in July 2015 and 2017, respectively. X-ray pulsations at ≈604 s are clearly detected in our study. Based on the long-term spin-frequency evolution, the source is found to spun-up in the last 17 yr. We observed a strongly energy-dependent pulse profile that evolved from a complex broad structure in soft X-rays into a profile with a narrow emission peak followed by a plateau in energy ranges above 20 keV. This behaviour ensured a positive correlation between the energy and pulse fraction. The pulse profile morphology and its energy evolution are almost similar during both the observations, suggesting a persistent emission geometry of the pulsar over time. The broad-band energy spectrum of the pulsar is approximated by an absorbed high-energy exponential cut-off power-law model with iron emission lines. In contrast to the previous report, we found no statistical evidence for the presence of cyclotron absorption features in the X-ray spectra. We performed phase-resolved spectroscopy using data from the NuSTAR observation. Our results showed a clear signature of absorbing material at certain pulse phases of the pulsar. These findings are discussed in terms of stellar wind distribution and its effect on the beam geometry of this wind-fed accreting neutron star. We also reviewed the subsonic quasi-spherical accretion theory and its implication on the magnetic field of 4U 1909+07 depending on the global spin-up rate.

Possible detection of a new cyclotron feature in 4U 1700–37

ABSTRACT We present a spectral and timing study of the high-mass X-ray binary transient source 4U 1700–37 using NuSTAR and ASTROSAT/LAXPC. The source is observed in two different flux states. A combined spectral analysis of NuSTAR’s focal plane modules A and B shows the possible hint of a cyclotron line feature at ∼16 keV. The line feature is consistently present in different continuum models with at least 3σ confidence level. We do not detect the presence of a previously reported 39 keV cyclotron line in the combined spectra. A ∼16 keV cyclotron feature would suggest that the compact object is a neutron star with a magnetic field strength ∼2.1 × 1012 G in the emission region. We also find the presence of a rare Ni Kα emission line around 7.6 keV in the NuSTAR spectrum. We searched the NuSTAR and ASTROSAT data for coherent or quasi-periodic oscillation signals but found no evidence in the frequency range 0.1 mHz to 103 Hz.

The giant outburst of 4U 0115+634 in 2011 with Suzaku and RXTE

We present an analysis of X-ray spectra of the high-mass X-ray binary 4U 0115+634 as observed with Suzaku and RXTE in 2011 July, during the fading phase of a giant X-ray outburst. We used a continuum model consisting of an absorbed cutoff power law and an ad hoc Gaussian emission feature centered around 8.5 keV, which we attribute to cyclotron emission. Our results are consistent with a fundamental cyclotron absorption line centered at ∼10.2 keV for all observed flux ranges. At the same time we rule out significant influence of the 8.5 kev Gaussian on the parameters of the cyclotron resonant scattering feature, which are not consistent with the cyclotron line energies or the depths of previously reported flux-dependent descriptions. We also show that some continuum models can lead to artificial line-like residuals in the analyzed spectra, which are then misinterpreted as unphysically strong cyclotron lines. Specifically, our results do not support the existence of a previously claimed additional cyclotron feature at ∼15 keV. Apart from these features, we find for the first time evidence for a He-like Fe XXV emission line at ∼6.7 keV and weak H-like Fe XXVI emission close to ∼7.0 keV.

The Likely Fermi Detection of the Be X-Ray Binary GRO J1008-57

Abstract In our search for γ-ray emission from Be X-ray binaries from the analysis of the data obtained with the Large Area Telescope onboard the Fermi Gamma-Ray Space Telescope, we find likely detection of GRO J1008−57. The binary has an orbital period of 249.48 days, and it is only significantly detected in its orbital phase 0.8–0.9 (>4σ). Further light-curve analysis indicates that the detection is probably largely due to an emitting event in one orbital cycle around year 2012–2013, following a giant X-ray outburst of the source. This property of having occasional γ-ray emitting events is similar to that seen in another high-mass X-ray binary (HMXB) 4U 1036−56. However, models considering possible γ-ray emission from an accreting neutron star have difficulty in explaining the observed ∼1034 erg s−1 luminosity of the source, unless the distance was largely overestimated. Further observational studies are required, in order to more clearly establish the high-energy emission properties of GRO J1008−57 or similar HMXBs and find clues for understanding how γ-ray emission is produced from them.

Pulse Phase Variation of the Cyclotron Line in HMXB 4U 1907+09 with AstroSat LAXPC

We present timing and spectral analysis of data from an observation of the High Mass X-ray Binary pulsar 4U 1907+09 with the LAXPC instrument onboard AstroSat. The light curve consisted of a flare at the beginning of the observation, followed by persistent emission. The pulsar continues to spin down, and the pulse profile is found to be double-peaked up to 16 keV with the peaks separated by a phase of $\sim0.45$. Significant energy dependence of the pulse profile is seen with diminishing amplitude of the secondary peak above 16 keV, and increasing amplitude of the main peak upto 40 keV and a sharp decline after that. We confirm earlier detections of the Cyclotron Resonance Scattering Feature (CRSF) in 4U 1907+09 at $\sim 18.5\pm 0.2$ keV in the phase-averaged spectrum with a high detection significance. An intensity resolved spectral analysis of the initial flare in the light curve shows that the CRSF parameters do not change with a change in luminosity by a factor of 2.6. We also performed pulse phase-resolved spectral analysis with ten independent phase bins. The energy and the strength of the cyclotron line show pulse phase dependence that is in agreement with previous measurements. Two features from the current observation: different energy dependence of the two pulse peaks and a strong CRSF only around the secondary peak, both indicate a deviation from a dipole geometry of the magnetic field of the neutron star, or complex beaming pattern from the two poles.

The First NuSTAR Observation of 4U 1538–522: Updated Orbital Ephemeris and a Strengthened Case for an Evolving Cyclotron Line Energy

Abstract We have performed a comprehensive spectral and timing analysis of the first NuSTAR observation of the high-mass X-ray binary 4U 1538−522. The observation covers the X-ray eclipse of the source, plus the eclipse ingress and egress. We use the new measurement of the mid-eclipse time to update the orbital parameters of the system and find marginally significant evolution in the orbital period, with . The cyclotron line energy is found to be approximately 1.2 keV higher than RXTE measurements from 1997 to 2003, in line with the increased energy observed by Suzaku in 2012 and strengthening the case for secular evolution of 4U 1538−522’s CRSF. We additionally characterize the behavior of the iron fluorescence and emission lines and line-of-sight absorption as the source moves into and out of eclipse.

Probing the Cyclotron line characteristics of 4U 1538–522 using AstroSat-LAXPC

ABSTRACT We report the first report on cyclotron line studies with the LAXPC instrument onboard AstroSat of the high-mass X-ray binary pulsar 4U 1538–52. During the observation of source that spanned about 1 d with a net exposure of 50 ks, the source X-ray flux remained constant. Pulse profile is double peaked in low-energy range and has a single peak in high-energy range, the transition taking place around the cyclotron line energy of the source. Cyclotron scattering feature (CRSF) is detected at ∼22 keV with a very high significance in phase-averaged spectrum. It is one of the highest signal to noise ratio detections of CRSF for this source. We performed detailed pulse-phase-resolved spectral analysis with 10 independent phase bins. We report the results of pulse-phase-resolved spectroscopy of the continuum and CRSF parameters. The cyclotron line parameters show pulse phase dependence over the entire phase with a CRSF energy variation of ${\sim }13{{\ \rm per\ cent}}$, which is in agreement with previous studies. We also confirm the increase in the centroid energy of the CRSF observed between the 1996–2004 (RXTE) and the 2012 (Suzaku) observations, reinforcing that the increase was a long-term change.

How to make a mature accreting magnetar

Several candidates for accreting magnetars have been proposed recently by different authors. Existence of such systems contradicts the standard magnetic field decay scenario where a large magnetic field of a neutron star reaches ≲a few ×1013 G at ages ≳1 Myr. Among other sources, the high-mass X-ray binary 4U 0114+65 seems to have a strong magnetic field around 1014 G. We develop a new Bayesian estimate for the kinematic age and demonstrate that 4U 0114+65 has kinematic age 2.4–5 Myr (95 per cent credential interval) since the formation of the neutron star. We discuss which conditions are necessary to explain the potential existence of magnetars in accreting high-mass binaries with ages about few Myr and larger. Three necessary ingredients are: the Hall attractor to prevent rapid decay of dipolar field, relatively rapid cooling of the crust in order to avoid Ohmic decay due to phonons, and finally, low values of the parameter Q to obtain long Ohmic time-scale due to impurities. If age and magnetic field estimates for proposed accreting magnetars are correct, then these systems set the strongest limit on the crust impurity for a selected sample of neutron stars and provide evidence in favour of the Hall attractor.

Orbital evolution and search for eccentricity and apsidal motion in the eclipsing HMXB 4U 1700−37

In the absence of detectable pulsations in the eclipsing High Mass X-ray binary 4U 1700-37, the orbital period decay is necessarily determined from the eclipse timing measurements. We have used the earlier reported mid-eclipse time measurements of 4U 1700-37 together with the new measurements from long term light curves obtained with the all sky monitors RXTE-ASM, Swift-BAT and MAXI-GSC, as well as observations with RXTE-PCA, to measure the long term orbital evolution of the binary. The orbital period decay rate of the system is estimated to be ${\dot{P}}/P = -(4.7 \pm 1.9) \times 10^{-7}$ yr$^{-1}$, smaller compared to its previous estimates. We have also used the mid-eclipse times and the eclipse duration measurements obtained from 10 years long X-ray light-curve with Swift-BAT to separately put constraints on the eccentricity of the binary system and attempted to measure any apsidal motion. For an apsidal motion rate greater than 5 degrees per year, the eccentricity is found to be less than 0.008, which limits our ability to determine the apsidal motion rate from the current data. We discuss the discrepancy of the current limit of eccentricity with the earlier reported values from radial velocity measurements of the companion star.

Evidence for an evolving cyclotron line energy in 4U 1538−522

We have performed a full time- and luminosity-resolved spectral analysis of the high-mass X-ray binary 4U 1538-522 using the available RXTE, INTEGRAL, and Suzaku data, examining both phase-averaged and pulse-phase-constrained datasets and focusing on the behavior of the cyclotron resonance scattering feature (CRSF). No statistically significant trend between the energy of the CRSF and luminosity is observed in the combined dataset. However, the CRSF energy appears to have increased by ~1.5 keV in the ~8.5 years between the RXTE and Suzaku measurements, with Monte Carlo simulations finding the Suzaku measurement 4.6$\sigma$ above the RXTE points. Interestingly, the increased Suzaku CRSF energy is much more significant and robust in the pulse-phase-constrained spectra from the peak of the main pulse, suggesting a change that is limited to a single magnetic pole. The 7 years of RXTE measurements do not show any strongly-significant evolution with time on their own. We discuss the significance of the CRSF's behavior with respect to luminosity and time in the context of historical observations of this source as well as recent observational and theoretical work concerning the neutron star accretion column, and suggest some mechanisms by which the observed change over time could occur.