A variable absorption feature in the X-ray spectrum of a magnetar

Area of Science:

• * Astrophysics
• * Neutron Star Physics
• * High-Energy Astronomy

Background:

• * Soft-gamma-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are isolated neutron stars exhibiting outbursts and hard X-ray bursts.
• * The magnetar model proposes these phenomena are powered by immense magnetic energy, with inferred fields exceeding those of typical radio pulsars.
• * SGR 0418+5729 presents a unique case with a weak dipole magnetic moment but a hypothesized strong internal/surface magnetic field.

Purpose of the Study:

• * To investigate the magnetic field strength of the peculiar neutron star SGR 0418+5729.
• * To analyze the X-ray spectrum of SGR 0418+5729 for evidence of its proposed strong magnetic field.

Main Methods:

• * Observation and analysis of the X-ray spectrum of SGR 0418+5729.
• * Identification and interpretation of spectral features, specifically absorption lines.
• * Correlation of spectral feature properties with the star's rotational phase.

Main Results:

• * An absorption line was detected in the X-ray spectrum of SGR 0418+5729.
• * The properties of this absorption line exhibit a strong dependence on the neutron star's rotational phase.
• * The line is identified as a proton cyclotron feature, with its energy implying a magnetic field between 2 × 10^14 and over 10^15 gauss.

Conclusions:

• * The observed proton cyclotron line strongly supports the presence of an extremely intense magnetic field in SGR 0418+5729.
• * These findings bolster the magnetar model for SGRs and AXPs, particularly for sources with complex field geometries.
• * The rotational phase dependence of the absorption line provides new insights into the structure and dynamics of magnetar magnetospheres.