Discovery of a pulse-phase-transient cyclotron line in the X-ray pulsar Swift J1808.4-1754 and identification of an optical companion

Abstract

In this work, the temporal and spectral properties of the poorly studied X-ray pulsar Swift J1808.4$-$1754 were investigated in the 0.8-79 keV energy range based on the data from the NuSTAR and Swift observatories collected during the 2014 outburst. Strong pulsations with a period of $909.73\pm 0.03$ s were detected in the source light curve, with the pulsed fraction demonstrating a nonmonotonic dependence on the energy with a local minimum around 17-22 keV. Phase lags in one of the pulse profile components, reaching the maximal value approximately at the same energy, were discovered. The pulse phase-averaged spectrum of the source has a power-law shape with an exponential cutoff at high energies, which is typical of X-ray pulsars. Pulse phase-resolved spectroscopy revealed the presence of a pulse phase-transient cyclotron absorption line at $\sim$21 keV, allowing us to estimate the neutron star magnetic field of $2.4\times 10^{12}$ G. This makes Swift J1808.4$-$1754 a member of very small family of X-ray pulsars with a pulse-phase-transient cyclotron line in a narrow phase range. The data from the Nordic Optical Telescope allowed us to study the properties of the IR companion in the system and to conclude that most probably it is a Be-type star located at a distance of 5-8 kpc.

arXiv commentAccepted for publication in MNRAS. 9 pages, 10 figures, 3 tables

References in corpus (33)

• On the Absorption of X-rays in the Interstellar Medium
• The UKIRT Infrared Deep Sky Survey (UKIDSS)
• The Nuclear Spectroscopic Telescope Array (NuSTAR) Mission
• Estimating distances from parallaxes. V: Geometric and photogeometric distances to 1.47 billion stars in Gaia Early Data Release 3
• Methods and results of an automatic analysis of a complete sample of Swift-XRT observations of GRBs
• HI4PI: A full-sky HI survey based on EBHIS and GASS
• The UKIRT Infrared Deep Sky Survey ZYJHK Photometric System: Passbands and Synthetic Colours
• Optimal binning of X-ray spectra and response matrix design
• The WFCAM Science Archive
• Magnetic Fields of Accreting X-Ray Pulsars with the Rossi X-Ray Timing Explorer
• Cyclotron lines in highly magnetized neutron stars
• V0332+53 in the outburst of 2004--2005: luminosity dependence of the cyclotron line and pulse profile
• Probing X-ray Absorption and Optical Extinction in the Interstellar Medium Using Chandra Observations of Supernova Remnants
• Timing Characteristics of the Hard X-ray Emission from Bright X-ray Pulsars Based on INTEGRAL Data
• Completing the puzzle of the 2004-2005 outburst in V0332+53: the brightening phase included
• 4U 0115+63 from RXTE and INTEGRAL Data: Pulse Profile and Cyclotron Line Energy
• Study of the accreting pulsar 4U 0115+634 with a bulk and thermal Comptonization model
• On Timing and Spectral Characteristics of the X-ray Pulsar 4U 0115+63: Evolution of the Pulsation Period and the Cyclotron Line Energy
• Propeller effect in two brightest transient X-ray pulsars: 4U 0115+63 and V 0332+53
• The Nordic Optical Telescope
• Confirmation of Two Cyclotron Lines in Vela X-1
• Hard Spectra of X-Ray Pulsars from INTEGRAL Data
• The X-ray properties of Be/X-ray pulsars in quiescence
• 4U 0115+63: phase lags and cyclotron resonant scattering
• The 100-month Swift catalogue of supergiant fast X-ray transients I. BAT on-board and transient monitor flares
• Discovery of a pulse-phase-transient cyclotron line in the X-ray pulsar GROJ2058+42
• Formation of phase lags at the cyclotron energies in the pulse profiles of magnetized, accreting neutron stars
• NuSTAR observations of the supergiant X-ray pulsar IGR J18027-2016: accretion from the stellar wind and possible cyclotron absorption line
• Discovery of the 5 keV cyclotron line followed by three harmonics in SwiftJ1626.6-5156
• X-ray pulsar XTE J1858+034: discovery of the cyclotron line and the revised optical identification
• BeppoSAX observations of XTE J1946+274
• Properties of the transient X-ray pulsar Swift J1816.7--1613 and its optical companion
• IGR J17463-2854, a Possible Symbiotic Binary System in the Galactic Center Region

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