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Thermal Emission from Isolated Neutron Stars:
Spectral Features and Featureless Spectra
Silvia Zane, MSSL, UCL, UK
Congresso Nazionale Oggetti Compatti |||
Osservatorio Astronomico di Roma, 9-11 Dicembre 2003
Over the last few years, intense
observational resources have been
devoted to study the faint thermal
emission from neutron stars and to
search for features in their spectrum.
Isolated neutron stars play a key role
in compact objects astrophysics:
these are the only sources in which
we can see directly the surface of the
compact star.
THIS MEANS THAT, AS SINGLE OBJECTS
THEY ARE INTERESTING BECAUSE:
we can measure physical parameters as
star mass, radius, probing our understanding
of the EOS.

 we can measure the surface temperature
and reconstruct the cooling history of the
source.
 we can detect/undetect spectral features,
constraining chemical composition and/or
magnetic field strength in the atmosphere.
X-ray Dim Isolated Neutron Star (INS)
D/pc
100
100
100
100
100
175
100
OPT.
B>25.5
B=26.6
B>24
m=28.6
m=26.8
V=25.7
R>23
COMMENT
44
85
96
86
96
60
(90)
LX/erg s-1
2.7 x 1030
2.6 x 1031
5.7 x 1030
5.1 x 1030
1.1 x 1031
3.6 x 1031
1.1 x 1031
RX J1836.2+5925 ?
(43)
5.4 x 1030
100
V>25.2
Variable?
Thermal emission
detected in more than
20 NSs (SGRs, AXPs,
PSRs, Radio-quiet NSs)
 Soft X-ray sources in ROSAT survey
 BB-like X-ray spectra, no non thermal hard emission
 Low absorption, nearby (NH ~1019-1020 cm-2)
 Constant X-ray flux on time scales of years
 Some are X-ray pulsars (3.45-11.37 s)
 No radio emission ?
 No obvious association with SNR
 Optically faint
OBJECT
RX J0420.0-5022
RX J0720.4-3125
RX J0806.4-4123
1RXS J13048.6+212708
RX J1605.3+3249
RX J1856.5-3754
1 RXS J214303.7+065419
KT/eV
RINSs are the largest
class of thermally
emitting Neutron Stars
(Treves et al, 2000)
PM
RBS 1223
RBS1556
PM, parallax
RBS1774
As a class, they are interesting because:





They imply the existence of a fair number of
neutron stars different from standard radio
pulsars and X-ray binaries
Accreting from ISM? Unlikely: high proper
motion
Cooling NS or descendant from AXP, SGRs
(old magnetars?)
Standard radio pulsars beamed away from
the Earth? (however: they are relatively
numerous and all close-by)
Genuinely radio-quiet? (as Geminga, SGRs,
AXPs)? Population synthesis models
The striking case of RX J1856.5-3754

500 ks DDT Chandra exposure
(i) RX J1856.5-3754 has a featureless X-ray
continuum
(ii) better fit with a simple bb than with more
sophisticated atmospheric models (Burwitz et
al 2001, Drake et al 2002, Burwitz et al, 2002)
XMM-Newton and Chandra
spectra of RXJ1856 together with
the best single blackbody fit to
each instrument (see table).
Mission
Instrument
kT 
R BB
10 cm
eV
km (d/120 pc)
1.46 ±0.20
0.95±0.02
0.12±0.02
0.60 ±0.02
0.87 ±0.08
56.7±1.0
62.5±0.2
62.3±0.2
62.6±0.4
62.4±0.3
7.5 ± 0.5
4.4± 0.1
4.2± 0.1
4.4± 0.1
4.0± 0.2
NH
22
Rosat
Chandra
XMM
XMM
XMM
PSPC
LETGS
EPIC-PN
EPIC-MOS2
RGS1+RGS2
-2
2 /D.O.F.
0.9/16
1.2/114
2.2/122
6.1/41
1.1/717
The striking case of RX J1856.5-3754
Optical excess of ~6 over the Rayleigh-Jeans tail of the Xray best fitting bb (Walter & Lattimer, 2002)
No X-ray pulsations: upper limit on the pulsed fraction
1% (Burwitz et al., 2003)
previous d ~120-140 pc (Kaplan et al, 2001; Walter & Lattimer, 2002)
 revised d ~175 pc (Kaplan et al., 2003, Korea meeting)
 radiation radius of only 7-8 km!
• Two-T model: x-ray = caps; optical = star surface (Pons et al. 2002;
Walter & Lattimer, 2002)
• Is it the first quark/strange star discovered? (Drake et al, 2002; Xu, 2002)
• Phase transition to a solid surface (B>few x1013 G) (Turolla et al. 2003)
Pulsating neutron stars: 4 so far!
RX J072.4-3125
Epic-PN (0.12-1.2) keV
RBS 1223
Epic-PN (0.12-1.2) keV
RX J0806.4-4123
Epic-PN (0.12-1.2) keV
RX J0420.0-5022
Epic-PN (0.12-0.7) keV
RX J0420: previous pulsation 22.7s , 1 in ROSAT HRI (Haberl et al. 1999).
 Haberl et al., 2004 in prep.: spurious. Instead, P= 3.45 s
(4 XMM PN and 4 XMM MOS observations in 2003)
1 RXS 1308: previous pulsation 5.157 s (Hambaryan et al. 2002)
 Haberl et al. 2003: double peaked light curve, P=10.31 s
Spectral variations with pulse phase
0.12-0.5 keV
Norm. Intens.
RBS 1223
0.5-1.0 keV
Norm. Intens.
Normalised Flux
Hardness
Hardness ratio
RXJ 0720
Phase
Phase
 Hardness ratio is max at the pulse maximum: counter-intuitive!
 Same observed in RX J0420 and RXJ0806 (Haberl et al., 2004, in prep.)
 Beaming effects ? (Cropper et al. 2001)
 Phase-dependent cyclotron absorption? (Haberl et al., 2003)
 dP/dt measured in 1 case: the brightest pulsating source RXJ0720.
 dP/dt = 1.4 ± 0.6 x10-13 s/s  B  (2.8-4.2) x 1013 G ; Ecp  0.2-0.3 keV
(Cropper et al. 2004 in prep.)
Thermal Spectra: blackbody fits
RX J1605:
kT = 96 eV
NH = 2.7x1019 cm -2
Counts/s/keV
Counts/s/keV
RX J0420:
kT = 44 eV
NH = 1.3 x1020 cm -2
SCRI
VO
Energy (keV)
Energy (keV)
RX J0720:
kT = 86 eV
NH = 1.3 x1020 cm -2
RBS 1223:
kT = 95 eV
NH = 7.1 x1020 cm -2
Scrivo
Scri
The situation changed only this year….
Counts/s/keV
Absorption features: RBS 1223
(Haberl et al., 2003)
Scri
Energy (keV)
Eline  0.3 keV ;   100 eV , EW  150 eV  B  5(1+z) x 1013 G
P = 10.3 s; cooling age  5 x 105 yrs
dP/dt  P/2t  3 x 10-13 s/s  Bdip  6 x1013 G
 B consistent with what is required for a proton cyclotron line
 Line parameters (EW, sigma) consistent with models (Zane et al. 2001)
n (ks-1 cm-2 Ǻ-1)
Absorption features: RX J1605.3+3249
(van Kerkwijk et al., 2003)
RX J1605.3+3249
 (Ǻ)
RGS spectrum of RX J1605.3+3249. Overdrawn is the best fit model: a
slightly extincted blackbody with two Gaussian absorption features.
Two gaussians:
Eline 0.45 keV + a narrower marginally significant one at 0.55 keV
 B  7(1+z) x 1013 G
No detected pulsations to a limit of 3%
 impossible to verify the B-field strength from timing measures
Absorption features and magnetic fields:
Summary
 RX J1605:
• no P, Eline 0.45 keV  B  7(1+z) x 1013 G
 RBS 1223:
• Eline 0.3 keV  B  5(1+z) x 1013 G
• hardness ratio shifted in phase wrt pulse max
 RX J0720:
• no line yet, dP/dt = 1.4± 0.6 x 10-13 s/s  B  (2.8-4.2)x1013 G
• hardness ratio shifted in phase wrt pulse max
 RX J0420:
• no dP/dt, no line yet
• hardness ratio shifted in phase wrt pulse max
The cyclotron line needs to be weaker at the pulse max to explain the
observed correlation between hardness ratio/pulse max
Vacuum polarization effects?
An hotter isolated neutron star:
1E1207.4-5209
1E1207: still radio-silent, but hottest and associated with a SNR
2 Multiple absorption features at ~0.7 and ~1.4 keV in Chandra and XMM data
+ 1 marginal feature at ~2 keV
1) Sanwal et al. 2002:
no cyclotron, no H atmosphere
 He atmosphere with B=1.5x1014 G
2) Mereghetti et al. 2002:
 Fe or other high Z atmosphere with B1012 G
3) Hailey and Mori 2002:
 He-like oxygen or neon with B1012 G
1E1207.4-5209: 257,303 s with XMM-Newton
(the longest EPIC observation of a galactic source)
3 Multiple absorption features: i. 0.72 ± 0.02 keV
ii. 1.37 ± 0.02 keV
iii. 2.11 ± 0.03 keV
PN
iv.
less significant at 2.85 ± 0.06 keV
• Data and best fitting
continuum spectral model
MOS
(two bb at kT=0.211± 0.0001
keV and kT=0.40 ± 0.02 keV;
NH = 1.0 ± 0.1 cm -2)
• Residuals in unit of standard
deviations  from the bestfitting continuum
(Bignami et al., 2003, Nature)
1E1207.4-5209:
Evidence of cyclotron absorption
• P =0.424 s
• dP/dt = 1.4 ± 0.3 10-14 s/s
B  (2-3) x 1012 G
(but also t ~4.8 x 105 yrs, incompatible with that of the SNR < 104 yrs)
• Proton cyclotron  B  1.6 x 1014 G:
 TOO HIGH!
• Electron Cyclotron  B  8 x 1010 G
 Better agreement if:
Additional breaking mechanisms (debris disk..);
Cyclotron scattering at R ~3-4 stellar radii
….
1E1207.4-5209: Lines vary in phase
Counts/s/keV
Rise
Decline
Peak
Norm. Intensity
Trough
Comparison of 4 PN spectra at different phase intervals.
Energy (keV)
Phase
Residuals of the phase-dependent spectra from the twoblackbody continuum fit.
 The peak of the total light curve corresponds to the phaseinterval where absorption lines are at their minimum;
 Lines are more important at the light curve trough.
Pulsed phase spectroscopy of
proton cyclotron lines: theory
1) Computing atmospheric
models at different
magnetic co-latitudes
 = 0˚
 = 40˚
 = 80˚
=
4) Predicting spin
variation of the
line parameters!
2) Assuming surface
temperature profile
and B-field topology
+
3) Ray-tracking in the
strong gravitational
field.
+
GOAL: probe the
surface
properties of the
neutron star via
pulse-phase
spectroscopy of
cyclotron
absorption lines
Zane, Turolla, Perna, Llyod, 2004 in prep.