Transcript Astronomy news

Strange Star? Neutron Star?
XMM-Newton finds the leader of the Magnificent Seven in a spin
ESA-Space Science, 9 March 2007
XMM-NEWTON DISCOVERY OF 7 s PULSATIONS IN THE ISOLATED NEUTRON STAR RX
-3754
ApJ., 657: L101-L104, 2007 March 10
VLA image of RX J1856 sky region
Magnificent Seven?
豪勇七洨蛟龍?!
Properties: 1. Nearby radio-quiet isolated NSs discovered in ROSAT data and
characterized by thermal X-ray spectra.
2. Despite intensive searches their number remained constant since 2001.
3. Five of the stars exhibit pulsations in their X-ray flux with periods
in the range [3-12]s and pulsed fractions between 4% and 18% .
4. XMM-Newton observations indicate more complex X-ray spectra with
multiple absorption lines among these sources.
Neutron stars
Neutron Stars (~1400)
Companion?
Isolated Neutron Stars
In Binary systems
Radio emission ?
Radio-quiet INS
Isolated Radio Pulsars
AXPs (~6)
SGRs (~5)
With γ-ray ?
RQINs with γ-ray
CCOs (~5)
RQINs without γ-ray
XDINs (~?)
With SNR ?
Others (>7)
By J. L. Chiu; 2004 9/30
RX J1856.5-3754
Features: The soft X-ray source RX J1856.5-3754 belongs to a small group of
seven nearby, INSs discovered with ROSAT and often dubbed XDINS.
RX J1856.5-3754 received particular interest since it is the brightest
XDIN, its X-ray flux doesn’t show any apparent contamination from
non thermal magnetospheric emission, and a parallactic distance (117
12 pc) has been reported.
It is an ideal target to derive information on the neutron star radius and
thus constrain the equation of state of matter at super nuclear density
through detailed modeling of its surface thermal emission.
Even the X-ray spectra with the highest resolution and the best statistics
do not show any line feature and can be fit by a simple Planckian with
tempearture kT = 63 eV.
The tight upper limit on the pulsed fraction (<1.3% at 2) obtained for
periods between 0.02 and 1000s and another upper limit on the pulsed
fraction of 2.1% (at 1) was reported for periods in the range 1-20 ms.
Quark Star? Neutron Star?
The accurate parallactic distance (500 Lyrs) of RX J1856 which was supplied by HST
has allowed astronomers to use the brightness to estimate its radius.
The estimated radius came out to be smaller than 10 km and this was taken as possible
evidence that RX J1856 was an exotic object, known as a quark star.
Using data collected by XMM-Newton, A. Tiengo and S. Mereghetti found a repeating
7s pulsation over a 19 hr observations of the source performed in Oct. 2006.
˝The pulsations are a typical characteristic of a neutron star!˝ says Tiengo. The
pulsations in RX J1856 have very low amplitude; this explains why they were not seen
before.
XMM-Newton view of RX J1856.5-3754 and its pulation
Periodicity Search
A 70ks long observation of RX J1856 was carried out with XMM staring on 2006
Oct. 24 at 00:30 UT. After data reduction, the event files result in net exposure
times of 47 and 68 ks for the pn and MOS.
Due to the very soft spectrum of RX J1856, they started timing analysis using the
data of pn camera and selecting energy range 0.15-1.2 keV.
They used a circular extraction region centered at the source position and with
radius 40˝ and then resulted in ~356600 counts, of which ~2700 can be ascribed
to the background.
They rebinned the counts at ~0.278s and omputed the Fourier power spectrum.
A significant peak was found at the frequency 0.14174Hz. Taking into account
the number of searched periods, the peak value 38.46 corresponds to a prob.
6x10-4. To better estimate the period, they used the epoch folding technique and
fitted the peak in the 2 versus trial period distribution obtaining P = 7.05514±
0.00007s.
The pulsed fraction, derived by fitting sinusoid to the background subtracted pn
light curve is 1.6%±0.2%.
In order to confirm the periodicity seen in the pn data of 2006 Oct., they analyzed
the other XMM observations listed in the table with Z2-test by assuming P  5x10 10
s s-1.
Random
prob.
3x10-5
7x10-5
5x10-4
2x10-3
2x10-2
Periodicity Search
The values of chance probabilities indicate that the periodicity is detected in all the
observations, although sometimes with small Z2-values, as expected for the case of
a small pulsed fraction.
The verified all the folded light curves by means of Kolmogorov-Smirnov test, taking
into account the unknown relative phase alignment, all the figures are fully compatible
with the same profile.
Periodicity Search
All the period measurements, spanning more than 4 yr, are consistent with average
value of P = 7.05515±0.00004s. With a linear fit to the period values, they can limit
the period derivative to -1.2x10-12 s s-1 < dP/dt < 1.9 x 10-12 s s-1.(90% c.l.)
The upper limit derived for period derivative can be used to infer a characteristic
age  >6x104 yr, a magnetic field B < 1.2x1014G, and a spin-down luminosity <
2x1032 ergs s-1. So their results indicate that, expect for the striking absence of
deviations from a pure blackbody in X-ray spectrum, RX J1856.5-3754 shares most
of the properties of the other members of the XDINS group.
"We don’t rule out the quark star interpretation but the pulsations show that the object
is also consistent with neutron star models", says Tiengo.
More importantly, the discovery of pulsations opens the way for the determination of
timing-dependent parameters that are crucial for more detailed modeling of the star’s
surface emission properties.