Proton Cyclotron Lines in

1
Proton Cyclotron Lines in Thermal Magnetar
Spectra S. Zane, R. Turolla, L. Stella and A.
Treves Mullard Space Science Laboratory UCL ,
University of Padova, Roma Observatory,
University of Milano
Broadening Effects
The assumption of constant B is reasonable for
polar cap emission, but breaks down if radiation
comes from the entire NS surface. For a dipolar
field, the change of B in both magnitude and
direction produces a broadening of the cyclotron
lines. Moreover, in magnetized NSs a meridional
temperature variation is expected. We have
estimated the broadening due to both these effect
within a simple, approximated model in diffusion
approximation. Results are reported in the table
and in the figure below.
In the last few years increasing observational
evidence gathered in favour of the existence of
ultra-magnetized neutron stars (NSs), with
surface field gt1014 G. Magnetars were first
hypothesized by Thompson and Duncan (1993), who
realized that strong convective motions during
the core collapse can strongly amplify the seed
magnetic field. In magnetars magneto-dipolar
radiation will cause rapid spin-down at a rate
10-11(B/1014 G)2/P ss-1, and it has been the
detection of a secular spin down of the same
order in two Soft ?-repeaters (SGRs) that for
the first time suggested the association of these
sources with ultra-magnetized NSs. Besides
their bursting activity, SGRs show also
persistent X-ray emission with L1034-1036 erg/s
and the possible presence of a thermal component
at KT0.5 keV. In the magnetar model, this is
believed to originate from the star surface which
is kept hot by the dissipation of the B-field.

Artist
impression of a magnetar Chandra and XMM-Newton
can already provide the required energy
resolution to allow for a detailed comparison
with theoretical models and to probe the
existence of such huge fields. Detailed radiative
transfer calculations are therefore needed.
Table 2. Line Broadening
Spectral Models
Following Zane, Turolla and Treves (2000), we
modelled thermal emission from the NS surface,
exploring the ranges 1013GltBlt1015G and 1034
erg/sltLlt1036 erg/s, believed to be typical of
magnetars in quiescent SGRs and AXPs. The NS
atmosphere has been treated assuming
plane-parallel symmetry and a constant field
parallel to the vertical axis. Emerging spectra
are shown below. They are nearly planckian in
shape and show a small hardening with respect to
the blackbody at star effective temperature..
Note D dipolar field constant B field. E
c is the line centroid energies have not been
corrected for gravitational redshift.
Magnetars have been also invoked to explain
another enigmatic class of galactic high energy
sources, the Anomalous X-ray pulsars (AXPs).
AXPs have periods in a very narrow range (P6-12
s) luminosities similar to SGRs and show no
evidence of a massive binary companion. They
show a stable spin period evolution with a long
term spin down trend. The emission of AXPs has a
thermal component at 0.5 keV and, like SGRs,
some of them are associated with a supernova
remnant.
As expected, the proton cyclotron line turns out
to be broader when emission comes from the
entire star surface, typically by 10-20. Also,
the change of the field strength produces a shift
of the line centroid toward lower energies of
20-30. Both these effects are quite independent
on the values of B and L.
Table 1. Model Parameters
Our calculations confirm the existence of a
strong absorption feature at the proton
cyclotron energy in the thermal spectrum of
magnetars, as first suggested by Thompson
(2000). The line equivalent width is ?0.1 keV
and, for B1014-1015 G, the line centre is
located at 0.5-5 keV. The detection of the
main cyclotron line is well within the range of
both Chandra and XMM-Newton grating
spectrometers. Its actual observation in the soft
X-ray spectra of AXPs and SGRs may therefore not
only give a definite confirmation of their
magnetar nature, but also an independent
measure of the magnetic field. HETGSACIS-S
observations of SGR 190014 and AXP 1E1048-59
have already been scheduled.
Note that the value of the line energy is not
corrected for the gravitational redshift. The
value observed at Earth is a factor yg 0.8
lower.
The most prominent spectral signature is the
absorption feature at the proton cyclotron
resonance, Ecp yG0.63(B/1014 G), which falls in
the soft-medium X-rays for such high fields (yG
0.8 accounts for gravitational redshift). The
line equivalent width, EW, and the inverse of the
required resolving power for detection, ?E/E,
are reported in table 1.
The position of two SGRs. Data from Cosmic
Background Explorer.
The many similarities between AXPs and SGRs
strengthen the idea that the two classes of
sources are powered by the same mechanism,
dissipation of a super-strong B-field in a
magnetar.
Two main effects contribute to this feature the
intrinsic resonance in the magnetic absorption
coefficients that essentially gives Fraunhofer
absorption lines and the mode crossing at the
mode collapse point, the latter being amplified
when collapse points introduced by vacuum effects
fall near the line energy and in the photosferic
region (as in model A4).