1. Sleuthing the Isolated Compact Stars Jeremy Drake Smithsonian Astrophysical Observatory Compact Stars: Quest for New States of Dense Matter

2. Neutron/Quark stars

3. Outline What are the thermally-emitting isolated neutron stars (INS) and why are they important? What are the thermally-emitting isolated neutron stars (INS) and why are they important? Understanding the observed INS population Understanding the observed INS population X-ray spectroscopy of INS: harsh reality! X-ray spectroscopy of INS: harsh reality! The nature of RXJ1856-3754 The nature of RXJ1856-3754 Trying to understand the outer layers Trying to understand the outer layers Naked neutron stars or bare quark stars? - the missing physics Naked neutron stars or bare quark stars? - the missing physics Future prospects Future prospects

4. Physical Importance Ultradense matter EOS Ultradense matter EOS Cooling history Cooling history Mass, Radius estimates Mass, Radius estimates Insights into exotic states of matter - relatively(!) low cost - about $1M/observation Insights into exotic states of matter - relatively(!) low cost - about $1M/observation Test Galactic evolution, nucleosynthesis models Test Galactic evolution, nucleosynthesis models Probe SN Type II and Galactic nucleosynthesis history; interstellar medium heating Probe SN Type II and Galactic nucleosynthesis history; interstellar medium heating Lattimer & Prakash (2001) Thermally emitting NS uncomplicated by magnetospheric emission - spectral interpretation more straightforward

5. Thermally-emitting INS: Cooling Stellar nucleosynthesis yields, pulsar counts, extragalactic SNe rates => 10 8 -10 9 neutron stars in Galaxy - up to 1% of population Stellar nucleosynthesis yields, pulsar counts, extragalactic SNe rates => 10 8 -10 9 neutron stars in Galaxy - up to 1% of population From birth at ~10 11 K, NS cool and cease pulsar activity in 10 6 -10 7 yr From birth at ~10 11 K, NS cool and cease pulsar activity in 10 6 -10 7 yr Only ~1000 known objects; nearly all discovered as radio pulsars => young NS Only ~1000 known objects; nearly all discovered as radio pulsars => young NS Can only detect thermal emission from very closest sources Can only detect thermal emission from very closest sources Yakovlev & Haensel (2003) Become invisible at all wavelengths

6. Thermally-emitting INS: Re-heating Might be re-heated by ISM accretion Might be re-heated by ISM accretion 100-5000 sources expected in ROSAT all-sky X-ray survey (eg Treves & Colpi 1991; Blaes et al 1995) Treves et al (2000) In X-rays, important to understand if object is cooler or accretor - accretors will have pure H outer layer, coolers possibly dominated by metals - Fe, Si, Mg… In X-rays, important to understand if object is cooler or accretor - accretors will have pure H outer layer, coolers possibly dominated by metals - Fe, Si, Mg…

7. ROSAT counts up all those INS… Hmmm, now let me check that one more time… 1,2,3….7?

8. Complexities of Re-heating CURRENT ONLY 7 KNOWN THERMALLY-EMITTING INS - orders of magnitude less than predicted!! Why ? CURRENT ONLY 7 KNOWN THERMALLY-EMITTING INS - orders of magnitude less than predicted!! Why ? Accretion physics - how do objects really accrete? Accretion physics - how do objects really accrete? NS B field and evolution - do fields decay? NS B field and evolution - do fields decay? NS velocity distribution NS velocity distribution V 3 dependence in Bondi formula V 3 dependence in Bondi formula Birth “kick” velocities and evolution in Galactic potential Birth “kick” velocities and evolution in Galactic potential ISM morphology/density ISM morphology/density Bondi formula overestimates accretion rate based on more sophisticated simulations (eg Perna et al 2003)

9. Complexities of Re-heating Perna et al (2003) Popov et al (2000)

10. All ROSAT INS likely young Popov et al. (2003) argue that ROSAT INS can be explained by recent star formation in Gould belt Popov et al. (2003) argue that ROSAT INS can be explained by recent star formation in Gould belt All known thermally emitting INS young, cooling objects? All known thermally emitting INS young, cooling objects? Popov et al (2003)

11. Presently known thermally- emitting INS

12. X-ray Spectroscopy of INS Thermally-emitting INS at 10 6 K - best studied at soft X-ray wavelengths (10-100 A) Thermally-emitting INS at 10 6 K - best studied at soft X-ray wavelengths (10-100 A) Identify spectral lines due to metals in atmosphere Identify spectral lines due to metals in atmosphere Determine atmospheric composition Determine atmospheric composition Measure gravitational redshift --> M/R Measure gravitational redshift --> M/R Pressure broadening --> g --> M and R Pressure broadening --> g --> M and R Measure B field if proton/electron cyclotron lines detected Measure B field if proton/electron cyclotron lines detected Measure temperature, flux (+distance) --> radius Measure temperature, flux (+distance) --> radius IF we can understand the radiating outer layer!!

13. RXJ 1856.5-3754 Brightest INS candidate in X-rays Brightest INS candidate in X-rays HST parallax => 110-170 pc (Walter & Lattimer 2002; Kaplan et al 2002; 175pc - Kaplan 2003!) HST parallax => 110-170 pc (Walter & Lattimer 2002; Kaplan et al 2002; 175pc - Kaplan 2003!) Proper motion points to Upper Scorpius OB association => age ~10 6 yr Proper motion points to Upper Scorpius OB association => age ~10 6 yr N.B. This is only a guess! Discovered serendipitously in study of pre-main-sequence stars in R CrA star forming region Discovered serendipitously in study of pre-main-sequence stars in R CrA star forming region Bright in X-rays (3.6 count/s ROSAT PSPC), but optically very faint (V>23; Walter et al 1996) Bright in X-rays (3.6 count/s ROSAT PSPC), but optically very faint (V>23; Walter et al 1996) Walter et al. (1996) ROSAT HRI error circle 3”

14. Chandra X-ray Observatory

15. RX J1856.5-3754 in Visible Light and X-rays ESO VLT Chandra LETGS 0th Order

16. 500ks of RX J1856.5-3754 in Chandra LETGS Drake et al (2002) T eff =60 eV R inf =3.8-8.2 km N H =1x10 20 cm 2 No pulsations: Limit < 4% (now < 1%; Burwitz et al 2003) - emission from whole of stellar surface ? IF object radiates as blackbody, R too small for neutron star… R inf =4-8.2 km Drake et al (2002) suggested possible strange quark star interpretation (see also Haensel (2001)

17. The failure of conventional models Pons et al. (2002)

18. Stop the Press! “Simply a discovery that defies all known laws of physics” “Simply a discovery that defies all known laws of physics” - France 2, national TV news - France 2, national TV news “Astronomers have discovered two quarks with the Hubble Space Telescope” “Astronomers have discovered two quarks with the Hubble Space Telescope” - The Times (London) “Quark stars signify unstable Universe” “Quark stars signify unstable Universe” - Harvard Gazette

19. They just fit into the Grand Canyon…

20. Optical Excess Walter & Lattimer (2002) Two backbody fit: T 1 =64 eV T 2 =30 eV R inf =12-26 km Pure H model

21. Argument for a conventional NS Walter & Lattimer (2002) with HST parallax of 117 pc get R inf =12-26 km with: Walter & Lattimer (2002) with HST parallax of 117 pc get R inf =12-26 km with: Two-component blackbody Two-component blackbody BUT no pulsations observed Si or Fe model Si or Fe model BUT no spectral features observed Walter & Lattimer (2002)

22. RX J0720.4-3125 First INS observed with high resolution X-ray spectrometer (XMM RGS) First INS observed with high resolution X-ray spectrometer (XMM RGS) Shows thermal, featureless spectrum! Shows thermal, featureless spectrum! Best matched by blackbody rather than eg H atmospheric models Best matched by blackbody rather than eg H atmospheric models BUT blackbody optical flux 5x lower than observed (Kulkarni & van Kerwijk 1998) BUT blackbody optical flux 5x lower than observed (Kulkarni & van Kerwijk 1998) Similar results for other INS Similar results for other INS Paerels et al (2001)

23. Are some compact stars “naked”? Solid surface does not necessarily emit as a blackbody (cf Brinkman 1980) Solid surface does not necessarily emit as a blackbody (cf Brinkman 1980) Fe condensate condensate H condensate Turolla, Zane & Drake (2003) At low T and/or high B, outer layer can be a solid crust - there is no atmosphere (eg Lai 2001; cf Ruderman 1974) Applies also to eg strange star crusts if B field can be sustained RX J1856, RX J0720 best candidates RX J1856 RX J0720

24. Emergent spectrum from solid crust Turolla, Zane & Drake (2003) T eff =10 6 K B=2x10 13 B=5x10 13 10 6 K blackbody Best fit blackbody Model Electron-phonon damping Reduced emissivity cf blackbody --> larger radius

25. Explaining the optical excess Reprocessing within thin H layer produces optical excess up to x4- 5 Reprocessing within thin H layer produces optical excess up to x4- 5 If layer can be heated above surface Teff --> larger excess If layer can be heated above surface Teff --> larger excess Radiative cooling time 10 -6 s - too fast for acoustic heating (cf. Solar chromosphere) Radiative cooling time 10 -6 s - too fast for acoustic heating (cf. Solar chromosphere) Drake, Turolla & Zane Emission from thin H “corona”

26. Is RX J1856.5-3754 a Quark Star? Gondek-Rosinska et al. (2002) New distance of 175pc (Kaplan this meetng) => R inf =8.3+/-1 km Naked star => R inf ~10.7 km 175pc 175pc naked Until we can better understand the surface character interpretation is open

27. Cyclotron resonance scattering in e+,e- pair plasma?! Ruderman (2003)

28. Future Chandra serendipitous survey

29. Summary Predictions of large numbers of INS re-heated to X-ray temperatures were grossly wrong because of incorrect assumptions: accretion physics; NS velocity distributions Predictions of large numbers of INS re-heated to X-ray temperatures were grossly wrong because of incorrect assumptions: accretion physics; NS velocity distributions The 7 known radio-quiet, thermally emitting INS are likely young, cooling objects The 7 known radio-quiet, thermally emitting INS are likely young, cooling objects Uncertainties as to surface characteristics means current spectral interpretation is very uncertain Uncertainties as to surface characteristics means current spectral interpretation is very uncertain Naked stars ? Naked stars ? Strongly magnetised H atmospheres ? Strongly magnetised H atmospheres ? RX J1856 remains one of the strongest quark star candidates RX J1856 remains one of the strongest quark star candidates Future progress lies in: Future progress lies in: improvements in these models improvements in these models Finding more INS Finding more INS