Discovery of the 5 keV Cyclotron Line Followed by Three Harmonics in Swift J1626.6-5156

Abstract We report on observations of the Be/X-ray binary system Swift J1626.6–5156 performed with the Nuclear Spectroscopic Telescope ARray (NuSTAR) during a short outburst in 2021 March, following its detection by the MAXI monitor and Spektrum–Roentgen–Gamma (SRG) observatory. Our analysis of the...

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Bibliographic Details
Published in:	Astrophysical journal. Letters 2021-07, Vol.915 (2), p.L27
Main Authors:	Molkov, S., Doroshenko, V., Lutovinov, A., Tsygankov, S., Santangelo, A., Mereminskiy, I., Semena, A.
Format:	Article
Language:	eng
Subjects:	Absorption Binary pulsars Broadband Cyclotron resonance Deposition Energy resolution Harmonics Magnetic fields Neutron stars Polarimetry Pulsars Resonance scattering Soft x rays Spectroscopic telescopes X ray astronomy X ray binaries X ray imagery X ray spectra X ray stars X-rays
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Summary:	Abstract We report on observations of the Be/X-ray binary system Swift J1626.6–5156 performed with the Nuclear Spectroscopic Telescope ARray (NuSTAR) during a short outburst in 2021 March, following its detection by the MAXI monitor and Spektrum–Roentgen–Gamma (SRG) observatory. Our analysis of the broadband X-ray spectrum of the source confirms the presence of two absorption-like features at energies E ∼ 9 and E ∼ 17 keV. These had been previously reported in the literature and interpreted as the fundamental cyclotron resonance scattering feature (CRSF) and its first harmonic (based on Rossi X-ray Timing Explorer (RXTE) data). The better sensitivity and energy resolution of NuSTAR, combined with the low-energy coverage of Neutron star Interior Composition Explorer (NICER), allowed us to detect two additional absorption-like features at E ∼ 4.9 keV and E ∼ 13 keV. Therefore, we conclude that, in total, four cyclotron lines are observed in the spectrum of Swift J1626.6–5156: the fundamental CRSF at E ∼ 4.9 keV and three higher spaced harmonics. This discovery makes Swift J1626.6–5156 the second accreting pulsar, after 4U 0115+63, whose spectrum is characterized by more than three lines of a cyclotronic origin, and implies that the source has the weakest confirmed magnetic field among all X-ray pulsars, B ∼ 4 × 10 11 G. This discovery makes Swift J1626.6–5156 one of the prime targets for the upcoming X-ray polarimetry missions covering the soft X-ray band, such as Imaging X-ray Polarimetry Explorer (IXPE) and enhanced X-ray Timing and Polarimetry mission (eXTP).
ISSN:	2041-8205 2041-8213