A quantitative explanation of the cyclotronline variation in accreting magnetic neutron stars of supercritical luminosity
Abstract
Magnetic neutron stars (NSs) often exhibit a cyclotron resonant scattering feature (CRSF) in their Xray spectra. Cyclotron lines are believed to form in the radiative shock in the accretion column. Highluminosity NSs show a smooth anticorrelation between the cyclotronline centroid ($E_{CRSF}$) and Xray luminosity ($L_X$). The observed $E_{CRSF}L_X$ smooth anticorrelation has been in tension with the theoretically predicted one by the radiative shock model. Since there is no other candidate site for the cyclotronline formation, we reexamine the predicted rate of change of the cyclotronline energy with luminosity at the radiative shock, taking a closer look at the Physics involved. We developed a purely analytical model describing the overall dependence of the observed cyclotron energy centroid on the shock front's height, including the relativistic boosting effect due to the mildly relativistic motion of the accreting plasma upstream with respect to the shock's reference frame and the gravitational redshift. We find that the CRSF energy varies with a) the shock height due to the dipolar magnetic field, b) the Doppler boosting between the shock and bulkmotion frames, and c) the gravitational redshift. We show that the relativistic effects noticeably weaken the predicted $E_{CRSF}L_X$ anticorrelation. We use our model to fit the data of the Xray source V0332+53 and demonstrate that the model fits the data impressively well, alleviating the tension between observations and theory. The reported $E_{CRSF}L_X$ weak anticorrelation in the supercritical accretion regime may be explained by the combination of the variation of the magneticfield strength along the accretion column, the effect of Doppler boosting, and the gravitational redshift. Thus, the actual magnetic field on the NS surface may be a factor of $\sim 2$ larger than the naively inferred value from the observed CRSF.
 Publication:

arXiv eprints
 Pub Date:
 June 2024
 DOI:
 10.48550/arXiv.2406.09511
 arXiv:
 arXiv:2406.09511
 Bibcode:
 2024arXiv240609511L
 Keywords:

 Astrophysics  High Energy Astrophysical Phenomena
 EPrint:
 9 pages, 2 figures, 1 table. Accepted for publication in A&