Understanding the spectral and timing behavior of a newly discovered transient X-ray pulsar Swift J0243.6+6124

We present the results obtained from timing and spectral studies of the newly discovered accreting X-ray binary pulsar Swift J0243.6+6124 using a NuSTAR observation in 2017 October at a flux level of ~280 mCrab. Pulsations at 9.85423(5) s were detected in the X-ray light curves of the pulsar. Pulse profiles of the pulsar were found to be strongly energy dependent. A broad profile at lower energies was found to evolve into a double peaked profile in $\ge$30keV. The 3-79 keV continuum spectrum of the pulsar was well described with a negative and positive exponential cutoff or high energy cutoff power law models modified with a hot blackbody at $\sim$3 keV. An iron emission line was also detected at 6.4 keV in the source spectrum. We did not find any signature of cyclotron absorption line in our study. Results obtained from phase-resolved and time-resolved spectroscopy are discussed in the paper.Comment: 7 pages, 6 figures, Accepted for publication in Monthly Notices of the Royal Astronomical Society Journa

A Soft X-Ray Spectral Episode for the Clocked Burster, GS 1826-24 as Measured by Swift and NuSTAR

We report on NuSTAR and Swift observations of a soft state of the neutron star low-mass X-ray binary GS 1826-24, commonly known as the "clocked" burster. The transition to the soft state was recorded in 2014 June through an increase of the 2-20 keV source intensity measured by MAXI, simultaneous with a decrease of the 15-50 keV intensity measured by Swift/BAT. The episode lasted approximately two months, after which the source returned to its usual hard state. We analyze the broad-band spectrum measured by Swift/XRT and NuSTAR, and estimate the accretion rate during the soft episode to be about 13% of Eddington, within the range of previous observations. However, the best fit spectral model, adopting the double Comptonization used previously, exhibits significantly softer components. We detect seven type-I X-ray bursts, all significantly weaker (and with shorter rise and decay times) than observed previously. The burst profiles and recurrence times vary significantly, ruling out the regular bursts that are typical for this source. One burst exhibited photospheric radius expansion, and we estimate the source distance at about (5.7 / xi_b^1/2) kpc, where xi_b parameterizes the possible anisotropy of the burst emission. Interpreting the soft state as a transition from an optically thin inner flow to an optically thick flow passing through a boundary layer, as is commonly observed in similar systems, is contradicted by the lower optical depth measured for the double Comptonization model we find for this soft state. The effect of a change in disk geometry on the burst behavior remains unclear.Comment: 40 pages (single-column, doubled spaced format), 9 figures, 3 tables; submitted to Ap