We report broad-band spectral properties of the high-mass X-ray binary pulsar SMC X-2 by using three simultaneous Nuclear Spectroscopy Telescope Array and Swift/XRT observations during its 2015 ...outburst. The pulsar was significantly bright, reaching a luminosity up to as high as ∼5.5 × 1038 erg s−1 in 1–70 keV range. Spin period of the pulsar was estimated to be 2.37 s. Pulse profiles were found to be strongly luminosity dependent. The 1–70 keV energy spectrum of the pulsar was well described with three different continuum models such as (i) negative and positive power law with exponential cutoff, (ii) Fermi-Dirac cutoff power law and (iii) cutoff power-law models. Apart from the presence of an iron line at ∼6.4 keV, a model independent absorption like feature at ∼27 keV was detected in the pulsar spectrum. This feature was identified as a cyclotron absorption line and detected for the first time in this pulsar. Corresponding magnetic field of the neutron star was estimated to be ∼2.3 × 1012 G. The cyclotron line energy showed a marginal negative dependence on the luminosity. The cyclotron line parameters were found to be variable with pulse phase and interpreted as due to the effect of emission geometry or complicated structure of the pulsar magnetic field.
We report the timing and spectral properties of the Be/X-ray binary pulsar GX 304-1 using two Suzaku observations during its 2010 August and 2012 January X-ray outbursts. Pulsations at ∼275 s were ...clearly detected in the light curves from both observations. Pulse profiles were found to be strongly energy-dependent. During the 2010 observation, the prominent dips seen in soft X-ray (≤10 keV) pulse profiles were found to be absent at higher energies. However, during the 2012 observation, the pulse profiles were complex as a result of the presence of several dips. Significant changes in the shape of the pulse profiles were detected at high energies (>35 keV). A phase shift of ∼0.3 was detected while comparing the phase of the main dip in the pulse profiles below and above ∼35 keV. The broad-band energy spectrum of the pulsar was well described by a partially absorbed negative and positive power law with exponential cut-off (NPEX) model with 6.4-keV iron line and a cyclotron absorption feature. The energy of the cyclotron absorption line was found to be ∼53 and 50 keV for the 2010 and 2012 observations, respectively, indicating a marginal positive dependence on source luminosity. Based on the results obtained from phase-resolved spectroscopy, the absorption dips in the pulse profiles can be interpreted as due to the presence of additional matter at same phases. Observed positive correlation between the cyclotron line energy and luminosity, and the significant pulse-phase variation of cyclotron parameters are discussed from the perspective of theoretical models on the cyclotron absorption line in X-ray pulsars.
ABSTRACT
We report the results obtained from a detailed timing and spectral studies of Be/X-ray binary pulsar 2S 1417−624 using data from Swift and NuSTAR observatories. The observations were carried ...out at the peak of a giant outburst of the pulsar in 2018. X-ray pulsations at ∼17.475 s were detected in the source light curves up to 79 keV. The evolution of the pulse profiles with energy was found to be complex. A four-peaked profile at lower energies gradually evolved into a double-peak structure at higher energies. The pulsed fraction of the pulsar, calculated from the NuSTAR observation was found to follow an anticorrelation trend with luminosity as observed during previous giant X-ray outburst studies in 2009. The broad-band spectrum of the pulsar is well described by a composite model consisting of a cut-off power-law model modified with the interstellar absorption, a thermal blackbody component with a temperature of ≈1 keV, and a Gaussian function for the 6.4 keV iron emission line. Though the pulsar was observed at the peak of the giant outburst, there was no signature of presence of any cyclotron line feature in the spectrum. The radius of the blackbody emitting region was estimated to be ≈2 km, suggesting that the most probable site of its origin is the stellar surface of the neutron star. Physical models were also explored to understand the emission geometry of the pulsar and are discussed in the paper.
ABSTRACT
We present the results obtained from detailed spectral and timing studies of extragalactic black hole X-ray binaries LMC X–1 and LMC X–3, using simultaneous observations with Nuclear ...Spectroscopic Telescope Array (NuSTAR) and Neil Gehrels Swift observatories. The combined spectra in the 0.5–30 keV energy range, obtained between 2014 and 2019, are investigated for both sources. We do not find any noticeable variability in 0.5–30 keV light curves, with 0.1–10 Hz fractional rms estimated to be <2 per cent. No evidence of quasi-periodic oscillations is found in the power density spectra. The sources are found to be in the high soft state during the observations with disc temperature Tin ∼ 1 keV, photon index, Γ > 2.5 and thermal emission fraction, fdisc > 80 per cent. An Fe Kα emission line is detected in the spectra of LMC X–1, though no such feature is observed in the spectra of LMC X–3. From the spectral modelling, the spins of the black holes in LMC X–1 and LMC X–3 are estimated to be in the range of 0.92–0.95 and 0.19−0.29, respectively. The accretion efficiency is found to be, η ∼ 0.13 and η ∼ 0.04 for LMC X–1 and LMC X–3, respectively.
Abstract
We present a comprehensive timing and spectral studies of Be/X-ray binary pulsar EXO 2030+375 using extensive Rossi X-ray Timing Explorer observations from 1995 till 2011, covering numerous ...Type I and 2006 Type II outbursts. Pulse profiles of the pulsar were found to be strongly luminosity dependent. At low luminosity, the pulse profile consisted of a main peak and a minor peak that evolved into a broad structure at high luminosity with a significant phase shift. A narrow and sharp absorption dip, also dependent on energy and luminosity, was detected in the pulse profile. Comparison of pulse profiles showed that the features at a particular luminosity are independent of type of X-ray outbursts. This indicates that the emission geometry is solely a function of mass accretion rate. The broad-band energy spectrum was described with a partial covering high energy cutoff model as well as a physical model based on thermal and bulk Comptonization in accretion column. We did not find any signature of cyclotron resonance scattering feature in the spectra obtained from all the observations. A detailed analysis of spectral parameters showed that, depending on source luminosity, the power-law photon index was distributed in three distinct regions. It suggests the phases of spectral transition from sub-critical to super-critical regimes in the pulsar as proposed theoretically. A region with constant photon index was also observed in ∼(2–4) × 1037 erg s−1 range, indicating critical luminosity regime in EXO 2030+375.
ABSTRACT
We present the results obtained from broad-band X-ray timing and spectral analysis of black hole (BH) candidate MAXI J1803–298 using an AstroSat observation on 2021 May 11–12. Four periodic ...absorption dips with a periodicity of 7.02 ± 0.18 h are detected in the light curve. AstroSat observe the source when it was undergoing a transition from hard-intermediate state to soft-intermediate state. Our timing analysis reveals the presence of a sharp Type-C quasi-periodic oscillation (QPO) in the power-density spectra (PDS) with an evolving QPO frequency ranging from 5.31 ± 0.02 to 7.61 ± 0.09 Hz. We investigate the energy dependence of the QPO and do not find this feature in the PDS above 30 keV. The combined 0.7–80 keV SXT and LAXPC spectra are fitted with a model consisting of thermal multi-colour blackbody emission and Comptonized emission components. We perform time-resolved spectroscopy by extracting spectra during the dip and non-dip phases of the observation. A neutral absorber is detected during the dip and non-dip phases though a signature of an ionized absorber is also present in the dip phases. The spectral and temporal parameters are found to evolve during our observation. We estimate the mass function of the system as f(M) = 2.1–7.2 M⊙ and the mass of the BH candidate in the range of MBH ∼ 3.5–12.5 M⊙.
Swift J0243.6+6124 is a newly discovered Galactic Be/X-ray binary, revealed in late 2017 September in a giant outburst with a peak luminosity of 2 × 1039(d/7 kpc)2 erg s−1 (0.1-10 keV), with no ...formerly reported activity. At this luminosity, Swift J0243.6+6124 is the first known galactic ultraluminous X-ray pulsar. We describe Neutron star Interior Composition Explorer (NICER) and Fermi Gamma-ray Burst Monitor (GBM) timing and spectral analyses for this source. A new orbital ephemeris is obtained for the binary system using spin frequencies measured with GBM and 15-50 keV fluxes measured with the Neil Gehrels Swift Observatory Burst Alert Telescope to model the system's intrinsic spin-up. Power spectra measured with NICER show considerable evolution with luminosity, including a quasi-periodic oscillation near 50 mHz that is omnipresent at low luminosity and has an evolving central frequency. Pulse profiles measured over the combined 0.2-100 keV range show complex evolution that is both luminosity and energy dependent. Near the critical luminosity of L ∼ 1038 erg s−1, the pulse profiles transition from single peaked to double peaked, the pulsed fraction reaches a minimum in all energy bands, and the hardness ratios in both NICER and GBM show a turnover to softening as the intensity increases. This behavior repeats as the outburst rises and fades, indicating two distinct accretion regimes. These two regimes are suggestive of the accretion structure on the neutron star surface transitioning from a Coulomb collisional stopping mechanism at lower luminosities to a radiation-dominated stopping mechanism at higher luminosities. This is the highest observed (to date) value of the critical luminosity, suggesting a magnetic field of B ∼ 1013 G.
Abstract We present the results obtained from detailed X-ray timing and spectral studies of X-ray pulsar Swift J0243.6+6124 during its giant and normal X-ray outbursts between 2017 and 2023 observed ...by the Neutron star Interior Composition Explorer (NICER). We focused on a timing analysis of the normal outbursts. A distinct break is found in the power density spectra of the source. The corresponding break frequency and slopes of the power laws around the break vary with luminosity, indicating a change in the accretion dynamics with the mass accretion rate. Interestingly, we detected quasiperiodic oscillations within a specific luminosity range, providing further insights into the underlying physical processes. We also studied the neutron star spin period evolution and a luminosity variation in the pulse profile during the recent 2023 outburst. The spectral analysis was conducted comprehensively for the giant and all other normal outbursts. We identified a double transition at luminosities of ≈7.5 × 10 37 and 2.1 × 10 38 erg s −1 in the evolution of continuum parameters like the photon index and cutoff energy with luminosity. This indicates three distinct accretion modes experienced by the source, mainly during the giant X-ray outburst. A soft blackbody component with a temperature of 0.08–0.7 keV is also detected in the spectra. The observed temperature undergoes a discontinuous transition when the pulsar evolves from a sub- to super-Eddington state. Notably, in addition to an evolving 6–7 keV iron line complex, a 1 keV emission line was observed during the super-Eddington state of the source, implying X-ray reflection from the accretion disk or outflow material.
ABSTRACT
We report results obtained from the optical and X-ray studies of the Be/X-ray binary 1A 0535+262/HD 245770 during the 2020 October giant X-ray outburst, using the 1.2-m telescope at Mount ...Abu Infrared observatory and AstroSat, respectively. The peak flux of the outburst was recorded to be ∼11 Crab in the 15–50 keV range, the highest ever observed from the pulsar. We performed optical observations in the 6000–7200 Å band before, during, and after the outburst to investigate the evolution of the circumstellar disc of the Be star between 2020 February and 2022 February. Our optical spectra exhibit prominent emission lines at 6563 Å (H i), 6678 Å (He i), and 7065 Å (He i). We found a significantly variable H α line in the spectra. The single-peaked line profile appeared asymmetric with broad red- and blue-wings in the data before and during the outburst. The post-outburst observations, however, resulted in a double-peaked profile with asymmetry in the blue-wing. Our observations before the outburst confirmed a larger Be disc that decreased in size as the outburst progressed. Furthermore, the observed variabilities in the H α line profile and parameters suggest the presence of a highly misaligned, precessing, and warped Be disc. AstroSat observation of the pulsar detected pulsations at ∼103.55 s in the light curve up to 110 keV. We found strongly energy-dependent pulse profiles with increasing contribution of the pulsing component in hard X-rays. The broad-band spectral fitting in the 0.7–90.0 keV range confirmed the presence of the known cyclotron resonance scattering feature at ∼46.3 keV.
ABSTRACT
We study the nature of the changing-look active galactic nucleus NGC 1566 during its 2018 June outburst. During the outburst, the X-ray intensity of the source rises up to ∼25–30 times ...compared to its quiescent state intensity. We perform timing and spectral analysis of the source during pre-outburst, outburst, and post-outburst epochs using semisimultaneous observations with the XMM–Newton, Nuclear Spectroscopic Telescope Array(NuSTAR), and Neil Gehrels Swift Observatories. We calculate variance, normalized variance, and fractional rms amplitude in different energy bands to study the variability. The broad-band 0.5–70 keV spectra are fitted with phenomenological models, as well as physical models. A strong soft X-ray excess is detected in the spectra during the outburst. The soft-excess emission is found to be complex and could originate in the warm Comptonizing region in the inner accretion disc. We find that the increase in the accretion rate is responsible for the sudden rise in luminosity. This is supported by the ‘q’-shape of the hardness-intensity diagram that is generally found in outbursting black hole X-ray binaries. From our analysis, we find that NGC 1566 most likely harbours a low-spinning black hole with the spin parameter a* ∼ 0.2. We also discuss a scenario where the central core of NGC 1566 could be a merging supermassive black hole.
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