ALBERT

Description: We report the broad-band spectral properties of the X-ray pulsar Cep X-4 by using a Suzaku observation in 2014 July. The 0.8–70 keV spectrum was found to be well described by three continuum models – Negative and Positive power-law with Exponential cut-off (NPEX), high-energy cut-off power-law and CompTT models. Additional components such as a cyclotron line at ~28 keV and two Gaussian components for iron lines at 6.4 and 6.9 keV were required in the spectral fitting. Apart from these, an additional absorption feature at ~45 keV was clearly detected in residuals obtained from the spectral fitting. This additional feature at ~45 keV was clearly seen in phase-resolved spectra of the pulsar. We identified this feature as the first harmonic of the fundamental cyclotron line at ~28 keV. The ratio between the first harmonic and fundamental line energies (1.7) was found to be in disagreement with the conventional factor of 2, indicating that the heights of line-forming regions are different or viewed at larger angles. The phase-resolved spectroscopy of the fundamental and first harmonic cyclotron lines shows significant pulse-phase variation of the line parameters. This can be interpreted as the effect of viewing angle or the role of complicated magnetic field of the pulsar.

Description: 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 x 10 38 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 x 10 12  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.

Description: We present the results obtained from broad-band spectroscopy of the high-mass X-ray binary 4U 1700–37 using data from a Suzaku observation in 2006 September 13–14 covering 0.29–0.72 orbital phase range. The light curves showed significant and rapid variation in source flux during entire observation. We did not find any signature of pulsations in the light curves. However, a quasi-periodic oscillation at ~20 mHz was detected in the power density spectrum of the source. The 1–70 keV spectrum was fitted with various continuum models. However, we found that the partially absorbed high-energy cut-off power law and Negative and Positive power law with Exponential cut-off (NPEX) models described the source spectrum well. Iron emission lines at 6.4 and 7.1 keV were detected in the source spectrum. An absorption-like feature at ~39 keV was detected in the residuals while fitting the data with NPEX model. Considering the feature as cyclotron absorption line, the surface magnetic field of the neutron star was estimated to be ~3.4 10 12 G. To understand the cause of rapid variation in the source flux, time-resolved spectroscopy was carried out by dividing the observation into 20 narrow segments. The results obtained from the time-resolved spectroscopy are interpreted as the accretion of inhomogeneously distributed matter in the stellar wind of the supergiant companion star as the cause of observed flux variation in 4U 1700–37. A sharp increase in column density after ~0.63 orbital phase indicates the presence of an accretion wake that blocks the continuum and produces the eclipse like low-flux segment.

Description: 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.

Description: We report the detection of quasi-periodic oscillation (QPO) at ~41 mHz in the transient high-mass Be/X-ray binary pulsar 4U 0115+634 using data from the Rossi X-Ray Timing Explorer ( RXTE ) observatory. The observations used in the present work were carried out during X-ray outbursts in 1999 March–April, 2004 September–October and 2008 March–April. This frequency of the newly detected QPO was found to vary in 27–46 mHz range. This ~41 mHz QPO was detected in four of the 36 pointed RXTE Proportional Counter Array (PCA) observations during 1999 outburst where as during 2004 and 2008 outbursts, it was detected in four and three times out of 33 and 26 observations, respectively. Though QPOs at ~2 mHz and ~62 mHz were reported earlier, the ~41 mHz QPO and its first harmonic were detected for the first time in this pulsar. There are three RXTE /PCA observations where multiple QPOs were detected in the power-density spectrum of 4U 0115+634. Simultaneous presence of multiple QPOs is rarely seen in accretion-powered X-ray pulsars. Spectral analysis of all the pointed RXTE /PCA observations revealed that the 3–30 keV energy spectrum was well described by Negative and Positive power law with EXponential (NPEX) cut-off continuum model along with interstellar absorption and cyclotron absorption components. During the three X-ray outbursts, however, no systematic variation in any of the spectral parameters other than the earlier reported anti-correlation between cyclotron absorption energy and luminosity was seen. Presence of any systematic variation of QPO frequency and rms of QPO with source flux were also investigated yielding negative results.

Description: We report on the phase-coherent timing analysis of the accreting millisecond X-ray pulsar IGR J00291+5934, using Neutron Star Interior Composition Explorer (NICER) data taken during the outburst of the source between 2018 August 15 and 2018 October 17. We obtain an updated orbital solution of the binary system. We investigate the evolution of the neutron star spin frequency during the outburst, reporting a refined estimate of the spin frequency and the first estimate of the spin frequency derivative ($dot{ u }sim -7imes 10^{-14}$ Hz s−1), confirmed independently from the modelling of the fundamental frequency and its first harmonic. We further investigate the evolution of the X-ray pulse phases adopting a physical model that accounts for the accretion material torque as well as the magnetic threading of the accretion disc in regions where the Keplerian velocity is slower than the magnetosphere velocity. From this analysis we estimate the neutron star magnetic field Beq = 2.8(3) × 108 G. Finally, we investigate the pulse profile dependence on energy finding that the observed behaviour of the pulse fractional amplitude and lags as a function of energy are compatible with the down-scattering of hard X-ray photons in the disk or the neutron star surface.

Description: We report the discovery by the Nuclear Spectroscopic Telescope Array (NuSTAR) and the Neutron Star Interior Composition Explorer (NICER) of the accreting millisecond X-ray pulsar IGR J17591–2342. Coherent X-ray pulsations around 527.4 Hz (1.9 ms) with a clear Doppler modulation were detected. This implies an orbital period of ∼8.8 h and a projected semi-major axis of ∼1.23 lt-s. With the binary mass function, we estimate a minimum companion mass of 0.42 M⊙, obtained assuming a neutron star mass of 1.4 M⊙ and an inclination angle lower than 60°, as suggested by the absence of eclipses or dips in the light curve of the source. The broad-band energy spectrum, obtained by combining NuSTAR, swift and INTEGRAL observations, is dominated by Comptonisation of soft thermal seed photons with a temperature of ∼0.7 keV by electrons heated to 21 keV. We also detect black-body-like thermal direct emission that is compatible with an emission region of a few kilometers and a temperature compatible with the seed source of Comptonisation. A weak Gaussian line centred on the iron Kα complex can be interpreted as a signature of disc reflection. A similar spectrum characterises the NICER spectra, which was measured when the outburst faded.

Description: The object IGR J17503–2636 is a hard X-ray transient discovered by INTEGRAL on 2018 August 11. This was the first ever reported X-ray emission from this source. Following the discovery, follow-up observations were carried out with Swift, Chandra, NICER, and NuSTAR. Here we report on the analysis of all of these X-ray data and the results obtained. Based on the fast variability in the X-ray domain, the spectral energy distribution in the 0.5–80 keV energy range, and the reported association with a highly reddened OB supergiant at ∼10 kpc, we conclude that IGR J17503–2636 is most likely a relatively faint new member of the supergiant fast X-ray transients. Spectral analysis of the NuSTAR data revealed a broad feature in addition to the typical power-law with exponential roll-over at high energy. This can be modeled either in emission or as a cyclotron scattering feature in absorption. If confirmed by future observations, this feature would indicate that IGR J17503–2636 hosts a strongly magnetized neutron star with B ∼ 2 × 1012 G.

Description: In 2019 November, MAXI detected an X-ray outburst from the known Be X-ray binary system RX J0209.6−7427 located in the outer wing of the Small Magellanic Cloud. We followed the outburst of the system with NICER, which led to the discovery of X-ray pulsations with a period of 9.3 s. We analysed simultaneous X-ray data obtained with NuSTAR and NICER, allowing us to characterize the spectrum and provide an accurate estimate of its bolometric luminosity. During the outburst, the maximum broad-band X-ray luminosity of the system reached (1–2) × 1039 erg s−1, thus exceeding by about one order of magnitude the Eddington limit for a typical 1.4 M⊙ mass neutron star (NS). Monitoring observations with Fermi/GBM and NICER allowed us to study the spin evolution of the NS and compare it with standard accretion torque models. We found that the NS magnetic field should be of the order of 3 × 1012 G. We conclude that RX J0209.6−7427 exhibited one of the brightest outbursts observed from a Be X-ray binary pulsar in the Magellanic Clouds, reaching similar luminosity level to the 2016 outburst of SMC X-3. Despite the super-Eddington luminosity of RX J0209.6−7427, the NS appears to have only a moderate magnetic field strength.