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.

 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. THIS MEANS THAT, AS SINGLE OBJECTS THEY ARE INTERESTING BECAUSE:

X-ray Dim Isolated Neutron Star (INS) RINSs are the largest class of thermally emitting Neutron Stars (Treves et al, 2000) 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 (N H ~ cm -2 )  Constant X-ray flux on time scales of years  Some are X-ray pulsars ( s)  No radio emission ?  No obvious association with SNR  Optically faint

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 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 Accreting from ISM? Unlikely: high proper motion Cooling NS or descendant from AXP, SGRs (old magnetars?) 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) 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 Genuinely radio-quiet? (as Geminga, SGRs, AXPs)? Population synthesis models

The striking case of RX J ks DDT Chandra exposure 500 ks DDT Chandra exposure (i) RX J 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).

The striking case of RX J Optical excess of ~6 over the Rayleigh-Jeans tail of the X- ray best fitting bb (Walter & Lattimer, 2002) No X-ray pulsations: upper limit on the pulsed fraction  1% (Burwitz et al., 2003) previous d ~ pc (Kaplan et al, 2001; Walter & Lattimer, 2002)  revised  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 x10 13 G) (Turolla et al. 2003) (Pons et al, 2002; Walter & Lattimer, 2002 )

Pulsating neutron stars: 4 so far! RX J0420: previous pulsation 22.7s, 1  in ROSAT HRI (Haberl et al. 1999). 1 RXS 1308: previous pulsation s (Hambaryan et al. 2002)   Haberl et al. 2003: double peaked light curve, P=10.31 s   Haberl et al., 2004 in prep.: spurious. Instead, P= 3.45 s (4 XMM PN and 4 XMM MOS observations in 2003) RX J Epic-PN ( ) keV RX J Epic-PN ( ) keV RBS 1223 Epic-PN ( ) keV RX J Epic-PN ( ) keV

Spectral variations with pulse 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 x s/s  B  ( ) x G ; E cp  keV (Cropper et al in prep.) RXJ 0720 RBS 1223 Phase Normalised Flux Hardness ratio Phase Hardness keV Norm. Intens keV Norm. Intens.

Thermal Spectra: blackbody fits The situation changed only this year…. Energy (keV) Counts/s/keV SCRI VO Energy (keV) RX J1605: kT = 96 eV N H = 2.7x10 19 cm -2 RX J0420: kT = 44 eV N H = 1.3 x10 20 cm -2 Scrivo RX J0720: kT = 86 eV N H = 1.3 x10 20 cm -2 Scri RBS 1223: kT = 95 eV N H = 7.1 x10 20 cm -2

Absorption features: RBS 1223 (Haberl et al., 2003)  E line  0.3 keV ;   100 eV, EW  150 eV  B  5(1+z) x G P = 10.3 s; cooling age  5 x 10 5 yrs  dP/dt  P/2t  3 x s/s  B dip  6 x10 13 G  B consistent with what is required for a proton cyclotron line  Line parameters (EW, sigma) consistent with models (Zane et al. 2001) Energy (keV) Counts/s/keV Scri

Absorption features: RX J (van Kerkwijk et al., 2003) Two gaussians: E line  0.45 keV + a narrower marginally significant one at 0.55 keV   B  7(1+z) x G No detected pulsations to a limit of 3%   impossible to verify the B-field strength from timing measures RGS spectrum of RX J Overdrawn is the best fit model: a slightly extincted blackbody with two Gaussian absorption features. (Ǻ) n  (ks -1 cm -2 Ǻ -1 ) RX J

Absorption features and magnetic fields: Summary  RX J1605: The cyclotron line needs to be weaker at the pulse max to explain the observed correlation between hardness ratio/pulse max Vacuum polarization effects?  E line  0.3 keV  B  5(1+z) x G hardness ratio shifted in phase wrt pulse max  RX J0720:  no P, E line  0.45 keV  B  7(1+z) x G  RBS 1223:  no line yet, dP/dt = 1.4± 0.6 x s/s  B  ( )x10 13 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

An hotter isolated neutron star: 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: 1E no cyclotron, no H atmosphere   He atmosphere with B=1.5x10 14 G 2) Mereghetti et al. 2002:   Fe or other high Z atmosphere with B  G 3) Hailey and Mori 2002:   He-like oxygen or neon with B  G

(the longest EPIC observation of a galactic source) (the longest EPIC observation of a galactic source) 1E : 257,303 s with XMM-Newton Data and best fitting continuum spectral model 3 Multiple absorption features: i.0.72 ± 0.02 keV ii.1.37 ± 0.02 keV iii.2.11 ± 0.03 keV iv.less significant at 2.85 ± 0.06 keV (Bignami et al., 2003, Nature) (two bb at kT=0.211± keV and kT=0.40 ± 0.02 keV; N H = 1.0 ± 0.1 cm -2 ) Residuals in unit of standard deviations  from the best- fitting continuum MOS PN

P =0.424 s dP/dt = 1.4 ± s/s Evidence of cyclotron absorption  Proton cyclotron  B  1.6 x G:  Electron Cyclotron  B  8 x G   Better agreement if:   TOO HIGH! Additional breaking mechanisms (debris disk..); Cyclotron scattering at R ~3-4 stellar radii …. (but also t ~4.8 x 10 5 yrs, incompatible with that of the SNR < 10 4 yrs) B  (2-3) x G 1E :

1E : Lines vary in phase Comparison of 4 PN spectra at different phase intervals. Residuals of the phase-dependent spectra from the two- blackbody continuum fit.  The peak of the total light curve corresponds to the phase- interval where absorption lines are at their minimum;  Lines are more important at the light curve trough. Phase Norm. Intensity Peak Decline Trough Rise Energy (keV) Counts/s/keV

Pulsed phase spectroscopy of proton cyclotron lines: theory 1) Computing atmospheric models at different magnetic co-latitudes 2) Assuming surface temperature profile and B-field topology 3) Ray-tracking in the strong gravitational field. + + = 4) Predicting spin variation of the line parameters! GOAL: probe the surface properties of the neutron star via pulse-phase spectroscopy of cyclotron absorption lines  = 0 ˚  = 40 ˚  = 80 ˚ Zane, Turolla, Perna, Llyod, 2004 in prep.