Timing%20Analysis%20of%20the%20Isolated%20Neutron%20Star%20RXJ0720.4

1
Timing Analysis of the Isolated Neutron Star
RXJ0720.43125
Silvia Zane, Frank Haberl, Mark Cropper,
Vyacheslav Zavlin, David Lumb, Steve Sembay
Christian Motch. (MSSL/UCL, University of
Strasbourg, MPE Garching, ESTEC, Univ. of
Leicester)
BTiming Analysis
A Introduction
We derive two pairs of values P, dP/dt which
cannot be further discriminated between on
statistical grounds (Table 2). Both fits have
dP/dt ?3-6 ?10-14 s/s. This is the most accurate
spin-down measure presented so far for a dim NSs
and, for the first time, it allows a
discrimination between models.

• RXJ0720.43125 is a nearby, isolated Neutron Star
  detected by ROSAT during a Galactic plane survey
  (Haberl et al., 1997) and recently re-observed
  with XMM-Newton on 2000 May 13 (Paerels et al.
  2001, Cropper et al., 2001) and 2001 November 21.
  
• The source shows all the common characteristics
  of the other 6 ROSAT Neutron Stars candidates
  (dim NSs, see Treves et al., 2000). In
  particular
• high X-ray to optical flux ratio Lx/Loptgt1000
• soft X-ray thermal spectrum (Tbb 86 eV)
• low column density (NH 6 x1019 cm2 ).
• The source is also pulsating with P 8.4 s.

Table 1 shows the different observations used in
our analysis. The major datasets are from the two
XMM observations, and from the 1996 Nov. 3 Rosat
pointing.
Our data originate from instrumentation with
widely different sensitivities typical count
rates vary from ?0.3 ct/s for Rosat HRI to ? 6
ct/s for XMM/PN. However, none of these count
rates is sufficiently high for a normal
distribution of counts to be expected, thus
standard discrete Fourier Transforms are not
directly applicable. For sparse data and event
list data, we use instead Rayleigh Transforms
(i.e. de Jager 1991, Mardia 1972). It is also
crucial for us to define precisely the confidence
intervals for the derived quantities, in
particular the period P. We do this by
constructing MLP (maximum likelihood
periodogrammes), which make no assumptions on
data distribution, and using the ?C-statistics
(Cash 1979). The uncertainty in the period and
the ?2 can be read directly from the y-axis of
the MLP (see e.g. Fig.1).
Until a few years ago two mechanisms were
proposed for dim NSs accretion from the
interstellar medium onto an old NS or cooling of
a younger object. More recently, based on the
similarity of the periods, it has been suggested
a possible evolutionary link between dim NSs,
Anomalous X-ray pulsars (AXPs), and soft
gamma-ray repeaters (SGRs). Two kind of
unified' scenarios have been then proposed. In
the first one, the 3 classes are powered by
dissipation of a decaying, super-strong magnetic
field (B? 1014 -1015 G). In this case, dim NSs
are the descendants of SGRs and AXPs and
RXJ0720.4-3125 may be the closest old-magnetar.
Alternatively, the 3 classes may contain standard
NSs (B ? 1012 G) endowed by a fossil disk (Alpar,
2001). In this case, dim NSs in the propellor
phase would be the progenitors of AXPs and SGRs,
the latter having entered an accretion phase.
We begin performing an MLP assuming dP/dt0 on
each of the longer pointing R93, R96b, X00a,
X00b (see Fig.1). There is no ambiguity in the
period determinations and a linear least square
fit using the 68 formal errors in the MLP gives
P0 8.39113 0.00011 s, dP/dt 0.0 5.5
?10-13 s/s (P0 is referenced to the start of the
R93 run).
C Discussion
The refined value of dP/dt reported here is
consistent with the measure of Haberl et al.
(1997), but two orders of magnitude lower. The
first implication is that RXJ0720.4-3125 is
hardly to be spinning down due to a propeller
torque. Accretion from a fossil disk implies 2
?10-11d2100lt dP/dt lt 2 ?10-9d2100 s/s, where d100
is the sources distance normalized at 100 pc
the value reported here is well below this range.
On the other hand, the measured spin-down is
considerable and, if interpreted as due to
magneto-dipolar losses, it gives a magnetic field
as high ?2?1013 G. The corresponding spin-down
age is P/(2dP/dt) ? 3?106 yrs, which is higher
but, given the numerous uncertainties, not too
far from the cooling age of 5?105 yrs.
Recently, Paerels et al. (2001) presented XMM
spectra of RXJ 0720.4-3125. The absence of
electron or proton cyclotron resonance in the RGS
range excluded magnetic fields of ?(0.32.0)?1011
and (0.52.0)?1014 G (see Zane at al.,
2001). Based on the same XMM observation, Cropper
et al. (2001) presented the pulse-shape analysis.
They derived an upper limit on the polar cap
size, showing that an emitting region larger than
?60-65 can be rejected at a confidence level of
90. Whatever the mechanism, the X-ray emitting
region is therefore confined to a relatively
small fraction of the star surface. They also
found that the hardness ratio is softest around
the flux maximum. The same has been later
discovered by Perna et al. (2001) in some AXPs.
Cropper et al. (2001) suggested two possible
explanations either radiation beaming (as in
their best-fitting model) or the presence of a
spatially variable absorbing matter, co-rotating
in the magnetosphere. The latter may be the case
if the star is propelling matter outward (Alpar,
2001).

This upper limit in dP/dt0 permits an
unambiguous determination of the peaks in the
Ch00 and R98 power spectra. Adding these to the
linear square fit gives P0 8.39107 0.00005
s, dP/dt 2.7 ?10-13 2.5 ?10-13 s/s. The 68,
90 and 99 confidence level are shown in Fig.2,
as well as the 68 and 90 intervals derived from
X00a. With the improved (P0, dP/dt) values, we
perform an MLP on the combined ROSAT 1993 and
1996 datasets. As a result, the confidence
contour break up into small region (aliases) in
the (P0, dP/dt) plane (see zoom in Fig.2). With
this further restriction, we finally do the MLP
on all the data.
Further information about this puzzling source
can be obtained by the spin history. Magnetars
will spin-down at a rate dP\dt ?10-11(B/10 14
G)2/P ss-1 , due to magneto-dipolar losses. A
measure of dP/dt for RXJ0720.4-3125 is therefore
crucial, as well an accurate tracking of its spin
history. Here we present a combined timing
analysis of XMM, Chandra and Rosat data, spanning
a period of 7 years.
It is now fundamental to assess the field
evolution and to understand if the source came
through an history of B-decay or if the magnetic
field has been almost constant over its lifetime.
We take for simplicity three different models for
the field decay Hall cascade and ambipolar
diffusion in the solenoidal or irrotational mode.
The laws are taken as in Colpi et al. (2000). As
we see, fast decaying processes as Hall cascade
predict a very low age for the source, which is
difficult to reconcile with its present
luminosity and with the relatively large number
of detected close-by objects of this class. It
seems therefore more plausible that the B-field
of RXJ0720.4-3125 only had a relatively small
change over the evolution, in which case the
present source age is ? 106 yrs.
Recently, there are some indications that the
time-stamping of XMM-Newton data may need
fine-adjustments at the level of our derived
accuracy. Further investigations by the SOC are
in progress and, if necessary, a re-computation
of some of these results may be required.