Thermal Emission from Isolated Neutron Stars:

1. 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.

2. 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.

3. X-ray Dim Isolated Neutron Star (INS) 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 RINSs are the largest class of thermally emitting Neutron Stars (Treves et al, 2000)

4. 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

5. The striking case of RX J1856.5-3754 (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) • 500 ks DDT Chandra exposure XMM-Newton and Chandra spectra of RXJ1856 together with the best single blackbody fit to each instrument (see table).

6. The striking case of RX J1856.5-3754 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 ~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) (Pons et al, 2002; Walter & Lattimer, 2002 )

7. RX J0806.4-4123 Epic-PN (0.12-1.2) keV RX J072.4-3125 Epic-PN (0.12-1.2) keV RX J0420.0-5022 Epic-PN (0.12-0.7) keV RBS 1223 Epic-PN (0.12-1.2) keV Pulsating neutron stars: 4 so far! 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

8. 0.12-0.5 keV Norm. Intens. Normalised Flux 0.5-1.0 keV Norm. Intens. Hardness Hardness ratio Phase Phase Spectral variations with pulse phase RBS 1223 RXJ 0720 • 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.)

9. Counts/s/keV Counts/s/keV SCRIVO Energy (keV) Energy (keV) Scrivo Scri Thermal Spectra: blackbody fits RX J0420: kT = 44 eV NH = 1.3 x1020 cm -2 RX J1605: kT = 96 eV NH = 2.7x1019 cm -2 RX J0720: kT = 86 eV NH = 1.3 x1020 cm -2 RBS 1223: kT = 95 eV NH = 7.1 x1020 cm -2 The situation changed only this year….

10. Counts/s/keV Scri Energy (keV) Absorption features: RBS 1223 (Haberl et al., 2003) 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)

11. RX J1605.3+3249 n (ks-1 cm-2Ǻ-1)  (Ǻ) Absorption features: RX J1605.3+3249 (van Kerkwijk et al., 2003) 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

12. 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)x1013G • 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?

13. 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

14. 3 Multiple absorption features: • 0.72 ± 0.02 keV • 1.37 ± 0.02 keV • 2.11 ± 0.03 keV • less significant at 2.85 ±0.06 keV PN MOS 1E1207.4-5209: 257,303 s with XMM-Newton (the longest EPIC observation of a galactic source) • Data and best fitting continuum spectral model (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 best-fitting continuum (Bignami et al., 2003, Nature)

15. 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 ….

16. Rise Trough Counts/s/keV Norm. Intensity Decline Peak Energy (keV) Phase 1E1207.4-5209: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.

17.  = 0˚  = 40˚  = 80˚ 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. + + GOAL: probe the surface properties of the neutron star via pulse-phase spectroscopy of cyclotron absorption lines = 4) Predicting spin variation of the line parameters! Zane, Turolla, Perna, Llyod, 2004 in prep.