Constraints on the equation-of-state of neutron stars from nearby neutron star observations Constraints on the equation-of-state of neutron stars from nearby neutron star observations Ralph Neuhäuser with post-docs Valeri Hambaryan and Markus Hohle, and PhD students Thomas Eisenbeiss, Ludwig Trepl, and Nina Tetzlaff Astrophysikalisches Institut und Universitäts-Sternwarte Friedrich-Schiller-Universität Jena collaboration with Valeri Suleimanov (U Tübingen) (i) X-ray and optical observations  radii of neutron stars RXJ1856 and RXJ0720 (ii) Rotational phase-resolved X-ray spectroscopy  compactness (M/R) of neutron star RBS1223 (iii)Kinematic identification of neutron star birth sites  age and distance of SN and mass of progenitor star (also for the SN that placed 60Fe on Earth)

RXJ0420 RXJ0720 RXJ0806 RXJ1308 RXJ1605 RXJ1856 RXJ B = 26.6 V = 26.8 B > 24 ~ 28.6 B = 27.2 V = 25.7 B = 27.0 < < ? Object energy period Pulsed Optical Proper Motion kT [eV] [sec] fraction % [mag] [mas/yr] The Magnificent Seven Neutron stars (M7 NS):  Isolated neutron stars without supernova remnants and without companions  Soft blackbody X-ray spectra with kT = eV (we observe surface)  nearby: pc (distance and temperature  radius)  no radio emission, but X-ray pulsation (  periods 3-12 seconds)  long-term variability in RXJ 0720: precession or glitch (  Grav. Waves ?)  spectral lines: broad proton cyclotron, highly ionized atomic lines (  M / R) <3 1 4

M7 Neutron Stars in the P – P dot diagram binary X-ray Dim Isolated Neutron Stars (XDINS) = M7 neutron stars may have evolved from AXPs or SGRs Related to RRATs ? Relevant for studying Pulsar birth rates M7 (from ATNF 15 May 2010)

allowed area allowed area Equation of State (EoS): Mass and Radius - Here masses of neutron stars in binaries: Most around 1.35 solar masses Motivation to Study M7 Neutron stars:  M7 Neutron stars may be a different population (same or different mass ?)  Radius determination also possible (in addition to mass) recently: 1.97 ± 0.04 M sol Demorest et al., 2010

Goal: Constraining equation-of-state Radius of RXJ1856 from surface observations M7 are radio-quiet thermally emitting Neutron Stars, we observe their surface. XMM & Chandra X-ray spectra give temperature T from spectral fitting (+/- ~ 1 %) Optical imaging photometry (e.g. Hubble Space Telescope) gives brightness (+/- ~ 1 %) Multiple optical imaging gives parallaxe or distance (+/- 10 %) Distance and brightness give luminosity L (+/- 20 %) Luminosity L and temperature T give radius R (+/- 20 %) L = 4  R^2 T^4 blue: b-b fit ROSAT PSPC: k T = /- 1.0 eV red: b-b fit to Chandra LETGS: /- 0.2 eV (Burwitz, …, RN, et al A&A) Top: b-b fit to XMM spectra with EPIC-pn: k T = /- 0.3 eV MOS-2: /- 0.4 eV RGS 1+2: /- 0.3 eV (Burwitz, …, RN, et al A&A)

Goal: Constraining equation-of-state Radius of RXJ1856 from surface observations M7 are radio-quiet thermally emitting Neutron Stars, we observe their surface. XMM & Chandra X-ray spectra give temperature T from spectral fitting: k T = 63 +/- 1 eV Optical imaging photometry (e.g. Hubble Space Telescope) gives brightness (+/- ~ 1 %) Multiple optical imaging gives parallaxe or distance (+/- 10 %) Distance and brightness give luminosity L (+/- 20 %) Luminosity L and temperature T give radius R (+/- 20 %) L = 4  R^2 T^4 Parallaxe 8 mas  distance 122 +/- 13 pc (10%) (Walter, Eisenbeiss, …, RN, 2010, ApJ)

Goal: Constraining equation-of-state Radius of RXJ1856 from surface observations M7 are radio-quiet thermally emitting Neutron Stars, we observe their surface. XMM & Chandra X-ray spectra give temperature T from spectral fitting: k T = 63 +/- 1 eV Optical imaging photometry (e.g. Hubble Space Telescope) gives brightness (+/- ~ 1 %) Multiple optical imaging gives parallaxe or distance: 122 +/- 13 pc (+/- 10 %) Distance and brightness give luminosity L (+/- 20 %) Luminosity L and temperature T give radius R (+/- 20 %) L = 4  R^2 T^4  Radius R = 4.4 km Solution: small hot spot for X-rays: kT = 63 +/- 1 eV larger warm surface for optical emission: kT = 40 +/- few eV Radius R at least 17 +/- 3 km (at infinity) (+/- 20 %) (Walter, Eisenbeiss, …, RN, 2010, ApJ) Problem: Optical excess.

Equation of State (EoS): Mass and Radius hence radius > 17 km (at infinity), +/- 3 km for both RXJ1856 and RXJ0720

RXJ0420 RXJ0720 RXJ0806 RXJ1308 RXJ1605 RXJ1856 RXJ B = 26.6 V = 26.8 B > 24 ~ 28.6 B = 27.2 V = 25.7 B = 27.0 < < ? Object energy period Pulsed Optical Proper Motion kT [eV] [sec] fraction % [mag] [mas/yr] <3 1 4 (a) Optical faint, but detected  we observe the surface, i.e. flux, distance, and temperature can yield radius (b) X-ray bright  X-ray spectral lines can yield Mass / Radius

Phase-resolved spectroscopy T 1 = keV T 2 = keV a 1 = 0.6 a 2 = 0.25 i = 90° θ B = 45° κ = 4° B p = 8.9 x G τ 0 = 2.77 σ Line = 0.20 keV N H = cm -2 g r = 0.16 Spectra at different Rotational phases Light curves in different energy bands RBS 1223 = RXJ 1308 double-hamped light curve (but largest pulse fraction, 18 %)

Model computes rotational phase-resolved spectra for Condensed Fe surface + thin partially ionized H atmo Suleimanov, Hambaryan, … RN et al. (2010) A&A Our new model: temperature distribution (emitting area)  and mag. field distribution (bottom)

Spectrum and light curve or phase-resolved spectroscopy Hambaryan, … RN et al. (in prep) Rotation phase vs. X-ray energy (flux color-coded)

Model with condensed Fe surface Plus thin partially ionized H atmosphere Compactness from phase-resolved spectroscopy: RBS1223 T 1 = keV T 2 = keV a 1 = 0.6 a 2 = 0.25 i = 45° θ B = 90° κ = 4° B p = 8.9 x G τ 0 = 2.77 σ Line = 0.20 keV N H = cm -2 g r = 0.16 (+/- 10 %)  Grav. redshift g r  Mass / Radius = (+/- 10 %) i.e. another constrain on the EoS (Hambaryan, … RN, in prep.) Spectra at different Rotational phases Markov-Chain Monte-Carlo fitting of XMM data on RBS1223 with our model

Constraining the equation of state Next: further improvements on the model and phase-resolved spectroscopy to get M/R for 6 more M7 Neutron Stars; In particular phase-resolved spectra for RXJ1856 and RXJ0720  M / R (in addition to R) Radius of RX J1856: R > 17 +/- 3 km Needs distance to +/- 10 % M / R = for X7 47 Tuc (Heinke et al. 2006) M / R = for LMXRBs (Suleimanov & Poutanen 2006) M / R = for Cas A (Wyn & Heinke 2009) M / R = (+/- 10 %) for RBS 1223 (Suleimanov, …RN et al. 2010, Hambaryan, …, RN et al. in prep.) Independent of distance !!! M / R to +/- 10 % and R to +/- 20 %  M to +/- 30 % M / R = for 1.4 M_sun  R = 16 km ! (consistent with 17 +/- 3 km, but not 10 km) NS

SN in ScoCenLup triggered more star formation, cleared Local Bubble (?), and … Breitschwerdt et al etc. Breitschwerdt & Berghöfer 2002 Maiz-Appelaniz et al Benitez et al Fuchs et al LCC: Lower Centaurus Crux UCL: Upper Centaurus Lupus US: Upper Scorpius Local Bubble Knie et al. 2004: black points Fitoussi et al. 2008: green pts. Peak at Myrs … and deposited 60Fe on Earth: Half life of 60 Fe newly determined: Was / Myrs before, Now / Myrs (Rugel et al PRL)  Fluence of 60Fe on Earth, But one unknown: distance towards SN ! We search for the neutron star that was formed in that SN, so that we can determine distance towards SN and mass of the progenitor star See poster Neuhäuser et al.

Isolated young neutron stars traced back to their place of origin OB associations Hubble Space Telescope V = 25.6 mag (and blue) M7 Neutron Star with known proper motion M7 Neutron Star w/o known proper motion Now 1 Myr ago Massive stars

Example: RX J no constraint on radial velocity, relative large distance error → several potential birth associations three former companion candidates (for three different birth associations) PLOTS, OTHER CONSTRAINTS  Al26, Fe60, distance needed PROGENITOR MASS 60 Fe on Earth? 2+ birth associations possible, i.e. not unique. Hence, search for other indicators, e.g. run-away stars as former companion Tetzlaff, Eisenbeiss, Hohle, RN, 2011, subm.

HIP 43158: B0II/III single star, v sin i = 96 ± 15 km/s → need further observations to confirm its status State of the art (re origin): no constraint on radial velocity, relative large distance error → several potential birth associations three former companion candidates (for three different birth associations) → need further indicators, e.g. 26 no constraint on radial velocity, relative large distance error → several potential birth associations three former companion candidates (for three different birth associations) → need further indicators, e.g. 26 Al sources Example: RX J measured value: π = 3.6 ± 1.6 mas [Eisenbeiss 2011, PhD, Jena] Tetzlaff, Eisenbeiss, Hohle, RN, 2011, subm. Neutron Star RXJ0720, run-away star HIP 43158, and Trumpler-10 association were at the same place together at the same time: SN in binary ?

 Constraints on the EoS possible from X-ray and optical observations, more coming soon …  Identification of birth places of young nearby NSs seems possible. More evidence being searched for, e.g. run-away stars and gamma sources  If the Neutron Star can be found that was born in the SN that placed 60Fe on Earth crust, then we get time and distance of SN.  See poster Neuhäuser et al. M / R = & g_r = 0.16 from phase-resolved spectra (RBS 1223) R > 17 km (infinity) From distance, flux, and temperature RXJ1856 & RXJ0720 Summary

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RXJ1856 – optical faint (HST) O VII K-alpha line red-shifted for g r = 0.16 O VII K-alpha line at rest (lab) Possibly a few O VII lines at rest (interstellar) instrumental First identification of atomic line in M7 neutron star (Hambaryan, Neuhäuser et al A&A Letters)  compactness, i.e. mass / radius, i.e. a constraint for EoS X-ray spectroscopy: RXJ 1856: no lines ! Now: RXJ 0720 co-added XMM-RGS1

co-added XMM-RGS1 (Hambaryan, RN 2010)co-added HRC-S/LETG (Chandra) OVII K α or OVI (ISM) or OVIII at g r =1.16 OVII K α or OVI at g r =1.16 K edge of OI (ISM) ? lines from the neutron star, circumstellar, or ISM? X-ray spectroscopy: RXJ 0720 (M. Hohle, PhD Jena 2010) RXJ 1856: no lines - best blackbody

Goal: Constraining equation-of-state Radius of RXJ0720 from surface observations M7 are radio-quiet thermally emitting Neutron Stars, we observe their surface. XMM & Chandra X-ray spectra give temperature from spectral fitting Optical imaging photometry (e.g. Hubble Space Telescope) gives brightness Multiple optical imaging gives parallaxe or distance Distance and brightness give luminosity L Luminosity L and temperature T give radius R L = 4  R^2 T^4 Distance 280 pc (200 – 500 pc)  radius > 17 +/- 3 km (at 280 pc) Eisenbeiss, PhD Jena 2011 (1856: > 17 +/- 3 km)