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

Abstract

Soft-γ-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are slowly rotating, isolated neutron stars that sporadically undergo episodes of long-term flux enhancement (outbursts) generally accompanied by the emission of short bursts of hard X-rays 1, 2 . This behaviour can be understood in the magnetar model [3][4][5] , according to which these sources are mainly powered by their own magnetic energy. This is supported by the fact that the magnetic fields inferred from several observed properties 6-8 of AXPs and SGRs are greater than -or at the high end of the range of -those of radio pulsars. In the peculiar case of SGR 0418+5729, a weak dipole magnetic moment is derived from its timing parameters 9 , whereas a strong field has been proposed to reside in the stellar interior 10,11 and in multipole components on the surface 12 . Here we show that the X-ray spectrum of SGR 0418+5729 has an absorption line, the properties of which depend strongly on the star's rotational phase. This line is interpreted as a proton cyclotron feature and its energy implies a magnetic field ranging from 2×10 14 gauss to more than 10 15 gauss.On 2009 June 5 two short bursts of hard X-rays, detected by Fermi and other satellites, revealed the previously unknown source SGR 0418+5729 13 . Subsequent observations with the Rossi X-ray Timing Explorer (RXTE), Swift, Chandra and X-ray Multi-mirror Mission (XMM) Newton satellites found the new SGR to be an X-ray pulsar with a period of ∼9.1 s and a luminosity of ∼1.6 × 10 34 erg s −1 (in the 0.5-10 keV band and for a distance of 2 kpc) 13,14 . During the three years after the onset of the outburst, the spectrum softened and the luminosity declined by three orders of magnitude, but remained still too high to be powered by rotational energy 9, 10, 14 . The measured spin-down rate of 4 × 10 −15 s s −1 translates (under the assumption of rotating magnetic dipole in vacuo) into a magnetic field B = 6 × 10 12 G at the magnetic equator 9 , a value well in the 1 arXiv:1308.4987v1 [astro-ph.HE] 22 Aug 2013 range of normal radio pulsars. However, the presence of high-order multipolar field components of 10 14 G close to the surface has been invoked to interpret the spectrum of the source in the framework of atmosphere models 12 . In any case, a strong crustal magnetic field (> 10 14 G) seems to be required to explain the overall properties of SGR 0418+5729 within the magnetar model 9, 11 .Hints of the presence of an absorption feature at 2 keV in the spectrum of SGR 0418+5729 were found in the phase-resolved analysis of data (with relatively low-count statistics) from the Swift X-ray Telescope (XRT) taken during 2009 July 12-16 14 . Thanks to the large collecting area and good spectral resolution of the European Photon Imaging Camera (EPIC), we were able to perform a more detailed investigation using data collected by XMM-Newton during a 67-ks long observation performed on 2009 August 12, when the source flux was still high (5 × 10 −12 erg cm −2 s −1 in the 2-10 keV band).To examine the spectral va...

NuSTAR OBSERVATIONS OF MAGNETAR 1E 1048.1–5937

“…We comment further that our analysis did not find any spectral lines or absorption features in the phase-averaged and phaseresolved spectra that could be interpreted as cyclotron lines (Tiengo et al 2013). We also searched for short bursts in the data, but in contrast to 1E 1048.1−5937, for which recently six bright X-ray bursts have been reported in NuSTAR data ), we did not observe a similar behavior for 1E 2259+586.…”

The outburst decay of the low magnetic field magnetar SWIFT J1822.3−1606: phase-resolved analysis and evidence for a variable cyclotron feature

“…10 and 11). This kind of analysis has proven useful to identify possible narrow spectral features that strongly vary with phase, like the absorption-like feature discovered in SGR 0418+5729 (Tiengo et al 2013; hereafter T13). A dark straight line, slightly inclined to the right, is visible at phase ∼0.5 in the RXTE, Chandra and XMM-Newton images (Figs.…”

Spectral formation in a radiative shock: application to anomalous X-ray pulsars and soft gamma-ray repeaters

“…The fallback disk model puts the super-strong magnetic field in a multipole component, while the classical magnetar model puts it in the dipole component. Yet, a recent interpretation (Tiengo et al 2013) of a variable absorption feature in the spectrum of the magnetar SGR 0418+5729 as a proton cyclotron feature invokes a multipole magnetic field. Thus, multipole components could in principle operate in both models.…”