A direct confrontation between neutron star X-ray spectra and nuclear physics models

Transcription

1 A direct confrontation between neutron star X-ray spectra and nuclear physics models Sebastien Guillot Post-doc CNES collaboration with N. Baillot d Etivaux J. Margueron N. Webb

2 Neutron stars are amazing laboratories for extreme physics. Extreme B-fields Extreme gravity Extreme Accretion High temperatures Extreme densities

3 The behaviour of ultra-dense matter is a key problem of astrophysics and fundamental physics or Supernovae NS-NS mergers nucleons or quarks? Stable ultra-dense matter cannot be produced in labs!

4 Neutron stars come in a variety of flavours, with different properties and observational signatures.

5 The internal structure of neutron stars is still unknown and many theories are proposed. Weber et al. 2007

6 Density (fm -3 ) Lattimer and Prakash 2001 Dense nuclear matter is described by an equation of state P(ρ). But what is it? Pressure (MeV fm -3 ) Nuclear density MNS (M ) RNS (km)

7 NICER will provide radius measurements for four millisecond pulsars (that have known masses) MNS (M ) Examples of expected NICER measurements RNS (km)

8 Measuring the radius precisely is rather difficult for neutron stars. To measure the radius, we need to: observe the surface thermal emission, correctly model this emission, know the distance independently. Neutron stars in quiescent low-mass X-ray binaries

9 Quiescent low-mass X-ray binaries are ideal systems for radius measurements. Surface thermal emission NS at T eff ~ 10 6 K, powered by residual heat with L X = erg/sec ~70% hydrogen ~28% helium ~2% metals X-ray Spectral fitting of this surface emission gives us T eff and F X (R /D) 2

10 Globular clusters host an overabundance of LMXB systems... EINSTEIN Observatory 1980s ROSAT 1990s Optical Image Chandra X-ray Obs. 2000s...and they have independently measured distances.

11 Flux The first globular cluster qlmxb was discovered in Omega Centauri. Typical X-ray spectrum of qlmxb Rutledge et al. 2002

12 Because of gravitational redshift, the radius is degenerate with the unknown mass. Lines of constant R

13 We want to find which equation of state is common to all these M-R measurements. MNS (M ) qlmxbs in the clusters R NS (km)

14 A solution consists in combining these observations in a statistical analysis. Constant R NS i.e., the radius is the same for all neutron stars Guillot et al. 2013, 2014, 2016 Analytical parameterizations Ozel et al. 2016

15 Using a realistic, physically-driven, parameterisation of the equation of state is preferable. P = f (,E sat,e sym,l sym,k sym,k sat,q sat,...) MNS (M ) MNS (M ) R NS (km) RNS (km) Margueron et al. 2018

16 Sampling L sym, K sym and Q sat by MCMC permits determining the parameters values that fits the M NS -R NS contours MNS (M ) MNS (M ) R NS (km) R NS (km) Measurements of L sym, K sym and Q sat

17 Our measurements of these four parameters improve on previous estimates. Preliminary Results Our measurements (2σ): L sym ~ MeV K sym ~ MeV Q sat ~ -200 MeV Qsat (MeV) Ksym (MeV) For comparison: Ranges of value from experimental and theoretical estimates L sym ~ MeV K sym ~ MeV L sym (MeV) K sym (MeV) Q sat (MeV) Q sat ~ MeV

18 Some future perspectives to improve these measurements on nuclear parameters: Using improved neutron stars distances, i.e., globular clusters distances measured by GAIA ± d (%) used ± d (%) GAIA DR2 47Tuc ±3.2 ±1.3 M13 ±5.6 ±1.4 M28 ±5.5 ±2.4 M30 ±5.6 ±1.5 NGC 6304 ±4.2 ±2.4 NGC 6397 ±2.7 ±2.2 Using deeper X-ray observations ωcen ±3.2 ±1.7 Helmi et al. 2018

19 The ATHENA X-ray Observatory will drastically improve constraints! Currently, with Chandra and XMM: ~1500 ksec accumulated: R NS /R NS ~ ±15 % R-SCIOBJ-331 Athena shall constrain the equation of state of neutron stars by obtaining X-ray spectra of seven quiescent low mass X-ray binaries with a good distance estimate. Observations of 7 qlmxbs with ATHENA total: 450 ksec R NS R NS 1.4 M = ±1.7% Guillot (2015)

20 Brief summary The X-ray spectra of neutron stars in low-mass X-ray binaries provides M NS -R NS measurements. Constraints on the equation of state of dense matter requires parameterising the equation of state when combining the X-ray spectra. A physically justified parameterisation gives us a direct measurements on the nuclear physics parameters.