Cyclotron lines in highly magnetized neutron stars

Abstract

Cyclotron lines, also called cyclotron resonant scattering features (CRSF) are spectral features, generally appearing in absorption, in the X-ray spectra of objects containing highly magnetized neutron stars, allowing the direct measurement of the magnetic field strength in these objects. Cyclotron features are thought to be due to resonant scattering of photons by electrons in the strong magnetic fields. The main content of this contribution focusses on electron cyclotron lines as found in accreting X-ray binary pulsars (XRBP) with magnetic fields on the order of several 10 12 Gauss. Also, possible proton cyclotron lines from single neutron stars with even stronger magnetic fields are briefly discussed. With regard to electron cyclotron lines, we present an updated list of XRBPs that show evidence of such absorption lines. The first such line was discovered in a 1976 balloon observation of the accreting binary pulsar Hercules X-1, it is considered to be the first direct measurement of the magnetic field of a neutron star. As of today (mid 2018), we list 36 XRBPs showing evidence of one ore more electron cyclotron absorption line(s). A few have been measured only once and must be confirmed (several more objects are listed as candidates). In addition to the Tables of objects, we summarize the evidence of variability of the cyclotron line as a function of various parameters (especially pulse phase, luminosity and time), and add a discussion of the different observed phenomena and associated attempts of theoretical modeling. We also discuss our understanding of the underlying physics of accretion onto highly magnetized neutron stars. For proton cyclotron lines, we present tables with seven neutron stars and discuss their nature and the physics in these objects.

Electron-positron pairs in hot plasma of accretion column in bright X-ray pulsars

“…Walter et al 2015). Typical magnetic field strength at the NS surface in XRPs is 10 12 G, which is confirmed independently by a number of different methods: detection of cyclotron lines (see Staubert et al 2019 for review), transitions into the "propeller" state (Tsygankov et al 2016;Lutovinov et al 2017), detection of spin-up and spin-down effects (Sugizaki et al 2017). Detected luminosities of XRPs cover a few orders of magnitude from 10 33 erg s −1 and up to 10 41 erg s −1 , where the brightest pulsars belong to the recently discovered class of pulsating ultraluminous X-ray sources (ULXs, Bachetti et al, 2014;Israel et al, 2017).…”

Evidence for the radiation-pressure dominated accretion disk in bursting pulsar GRO J1744−28 using timing analysis

“…Recently, D' Aì et al (2015) found a cyclotron absorption line at 4.7 keV in the spectrum obtained with EPIC/pn instrument on board of XMM-Newton, indicating the surface magnetic field strength of B = (5.27 ± 0.06) × 10 11 G. The same feature was independently discovered by Doroshenko et al (2015) in the older BeppoSAX data. Thus GRO J1744−28 has a magnetic field of intermediate strength when compared to other accreting magnetized NSs (see a review by Staubert et al 2019). This, together with the observed super-Eddington outbursts, implies that the magnetospheric radius and thus the inner radius of the accretion disk should be rather small, around 5×10 7 cm, which makes the Bursting Pulsar a unique object to study, particularly, in the context of super-Eddington accretion onto a NS as a mechanism to power ultraluminous X-ray sources.…”

Observations of a GX 301–2 Apastron Flare with the X-Calibur Hard X-Ray Polarimeter Supported by NICER, the Swift XRT and BAT, and Fermi GBM

“…Thus, Fürst et al (2018) interpret the two features as being fundamentals from distinct regions, in which case they possess higher fields, namely ∼ 3 × 10 12 G and ∼ 4.3 × 10 12 G (for z = 0), corresponding to cyclotron absorption radii differing by only around 12%. These fields are substantially above the values inferred from accretion torque models (see Table 1 of Staubert et al 2019), the converse of what is usually obtained when comparing these two field estimates for X-ray binary pulsars. Some CRSFs are observed to depend on pulse phase, time, and luminosity (Staubert et al 2019, and references therein).…”

“…These fields are substantially above the values inferred from accretion torque models (see Table 1 of Staubert et al 2019), the converse of what is usually obtained when comparing these two field estimates for X-ray binary pulsars. Some CRSFs are observed to depend on pulse phase, time, and luminosity (Staubert et al 2019, and references therein). These variations are sometimes attributed to a movement of the radiative shock along the accretion column, or by changes in the magnetic field geometry (e.g.…”