NS masses from radio timing: Past, present and future. Paul Demorest (NRAO) Symposium on Neutron Stars, Ohio U., May 2016

Transcription

1 NS masses from radio timing: Past, present and future Paul Demorest (NRAO) Symposium on Neutron Stars, Ohio U., May 2016

2 Overview Review of how to measure neutron star masses via radio pulsar timing. Summary of recent (and not-so-recent) influential results. Progress in data analysis methods. Future instrumentation for radio searching and timing. and associated challenges.

3 Neutron star interior (Watts et al 2015)

4 Equation of state; Mass/radius plot (Watts et al 2015) EOS implies certain track in Mass-Radius plot. Maximum observed NS mass can rule out specific models.

5 Pulsar timing NS are visible as radio pulsars we can track phase of pulse train over many years or decades. MSPs have P ~ few ms. P Fluctuations in pulse arrival times are a direct measurement of variations in Earth PSR distance (light travel time); 1 us 0.3 km For pulsars in binary systems, measures projected orbital motion in most cases is many times pulse period! (plot: S. Ransom)

6 Characterizing binary pulsar orbits Besides the normal 5 Keplerian parameters (Porb, e, asin(i)/c, T0, ω), post-keplerian orbital effects are measurable: where: T GM /c3 = μs, M = m1 + m2 In general relativity, these are only functions of: - the precisely known Keplerian orbital parameters Pb, e, asin(i) - the mass of the pulsar m1 and the mass of the companion m2

7 Mass-mass diagram (PSR B ; plot: Will, 2014)

8 The original binary pulsar: PSR B The first binary pulsar, discovered in 1974 Arecibo pulsar survey. ~7-hour orbit, 60-ms pulse period, companion is another NS. Measured change in orbital period exactly matches GR prediction for emission of GW 1993 Nobel prize in physics for Hulse and Taylor. First evidence for GW! Three PK params measured Also provided accurate masses for both stars: M1 = (2) Msun M2 = (2) MSun (Figures and masses: Weisberg et al 2010)

9 J : A double pulsar system Discovered in 2003 in Parkes multibeam survey. Extreme NS-NS binary: 2.4-hr orbit, 22-ms period, viewed nearly edge-on (allows Shapiro delay). Excellent GR test! Only such system where both stars are (were) visible as pulsars. M1 = (7) Msun M2 = (7) MSun May eventually provide moment of inertia via spinorbit effects. (Kramer et al. 2006)

10 Masses from Shapiro delay Long-term, eccentric orbit Eccentric orbit Long-term, compact orbit Short-term, geometry Orbital evolution needs eccentric, compact orbit mainly possible in DNS systems. NS-WD binaries tend to have very circular orbits. Other non-gr effects (tidal effects, galactic acceleration) can contribute systematic errors to mass measurements. Shapiro delay avoids both issues, but is a smaller-amplitude effect, so harder to detect. Main requirement is ideal alignment of orbit wrt Earth.

11 Shapiro delay General relativistic time delay from propagation through curved spacetime. First detected in solar system radar experiments.

12 Shapiro delay in pulsar binaries First SD detection in a pulsar binary system: PSR B (Ryba & Taylor, 1991) NRAO / Bill Saxton MPSR = 1.27(20) Msun i = 88.3(7) deg

13 Shapiro delay in pulsar binaries Shape and amplitude of signal highly dependent on orbital inclination edge on (i = 90 deg) gives sharp peak, stronger signal.

14 PSR J ms pulsar in an 8.7-day orbit with a WD. Marginal Shapiro delay after ~7 years of GBT timing with older generation of backend instruments (Spigot, BCPM, GUPPI-1, etc):

15 ~1 week of dense timing observations with upgraded GUPPI: Orbital inclination = 89.17(2) deg! Companion mass = 0.500(6) solar! Pulsar mass = 1.97(4) solar! Improved instrument (better BW, time resolution) plus lots of telescope time dramatically improved data quality. (Demorest et al, 2010)

16 Radio plus optical: PSR J Extremely compact NS-WD binary (Antoniadis et al 2013) WD optical radial velocity + PSR radio timing Mass ratio Mass ratio + WD spectrum MPSR = 2.01(4) Msun Mass ratio + orbital period derivative + GR MPSR = 2.07(20) Msun

17 Mass-radius plot with observations (Figure: N. Wex)

18 Black widow pulsar systems PSR in binary with low-mass, puffy companion. Often show ecplises. Are being discovered in greater numbers via Fermi source followup. Messy binary does not permit GR-based mass measurement. Radio timing plus optical obs and companion modeling give intriguingly high masses! B : MPSR ~ 2.4 Msun (van Kerkwijk et al 2011) J : MPSR ~ 2.1 to 2.7 Msun (Romani et al 2012) (See R. Romani talk later this session for more!) (van Kerkwijk et al 2011)

19 NS mass compilation LMXB Optical DNS MSP (Watts et al 2015; AASKA)

20 Statistical analysis NS mass distribution LMXB Opt DNS MSP Growing total number of measurements. Eventually may be able to statistically constrain maximum mass. (Ozel et al 2012) (Watts et al 2015) Somehwat unclear whether statistical maximum mass determination will tell us more about physics or astrophysics. (See also Ozel et al 2012, Kiziltan et al 2010, )

21 Statistical analysis orbital inclination distribution Assuming Pb Mc relation (Tauris & Savonije 1999) and NS mass distribution (Ozel et al 2012), can test distribution of cos(i). Does not directly use detected Shapiro delay or other PK params. Shows either inconsistency with flat cos(i) or wrong assumption. Non-flat cos(i) could be due to PSR beaming direction effects. (Sanpa-arsa, Ransom et al)

22 Fourier decomposition of Shapiro delay Full H1 Introduced by Freire & Wex (2010). H2 First two harmonics (H1, H2) totally covariant with other basic orbital params. H3 H4 H3+ Unabsorbed (ie, detectable) Shapiro signal encoded in H3 and higher harmonics.

23 Fourier decomposition of Shapiro delay (Freire & Wex 2010) Provides better parameterization than traditional (r,s) especially for low-i or low-significance detections. If used consistently may help inform statistical population analyses; this is now beginning to be done (e.g. Fonseca et al. NANOGrav binary analysis).

24 Noise modeling and Bayesian analysis Recent work in PTA/GW context has provided tools for advanced noise analysis and Bayesian parameter estimation (Ellis, van Haasteren, et al) will lead to more robust mass measurements from timing. (plots: J. Ellis)

25 Noise modeling and Bayesian analysis (NANOGrav, Arzoumanian et al 2015)

26 Pulsar timing arrays Large ongoing effort worldwide to detect nhzfrequency GW using MSP timing. (IPTA 2014 meeting, Banff, Canada)

27 Pulsar timing arrays PTA projects are conducting regular observations of many MSPs (~50 in upcoming NANOGrav 11-year data set). ~70% are in binary systems. Data are taken long-term using very consistent observational setup/procedures; analyzed using consistent methodologies. Data volume results in more NS mass measurements; consistency may also help provide a relatively unbiased data set for statistical population analysis. (NANOGrav 9-year data; plot D. Nice) (See next talk by E. Fonseca for binaries in the NANOGrav 9-year data set!)

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29 The future improving NS mass measurements Conceptually easy: Surveys to increase number known MSPs; small fraction will be suitable for mass measurement or otherwise exotic. Improve instrumentation for better timing precision on known pulsars; detect smaller timing effects. Upcoming instrumental improvements on current telescopes: Backend (digital) development mostly done. Ultra-wide band receiver systems for improved timing. Multi-pixel feeds for better survey speed. New (larger) telescopes.

30 Ultra-wideband receiver systems Standard receivers have octave (2:1) BW. New quad-ridge designs go up to ~6:1. Could improve S/N and DM estimation; however likely noise penalty. Ongoing developments at Effelsberg, Parkes, FAST, etc.

31 Telescopes for searching and timing Current US pulsar telescopes: Green Bank Telescope: 100-m single steerable dish Arecibo: 305-m fixed reflector Other upcoming GHz-freq telescopes: FAST (China): 500-m, Arecibo-like; ~ MeerKAT (SA): 64 x 13.5-m; 2017 SKA1-Mid (SA): 133 x 15-m + MeerKAT; ~2020 Lower-freq instruments: LOFAR, MWA, CHIME, SKA1Low, Complementary searches and ISM monitoring. Further off (~2030s): SKA2, next-gen VLA,?

32 Future telescopes FAST (China) 500-m Arecibo-style fixed reflector; 40 deg zenith angle range; currently under construction, first light expected this year, drift-scan mode for at least first ~year. Sensitivity should be ~2x Arecibo.

33 Future telescopes MeerKAT (South Africa) 64-element array of 13.5-m dishes; under construction. 16-element array this year, full array mid Sensitivity ~ GBT; southern hemisphere location ideal for pulsar observing.

34 SKA pulsar searches Searches with SKA telescopes should increase total known pulsar population by a factor of ~2 5. SKA2 should find majority of detectable pulsars in the galaxy. (Keane et al, AASKA14)

35 Shapiro delay detectability (Watts et al 2015) Improved timing precision higher fraction of systems with detectable Shapiro delay. ~80% of orientations have > 50 ns signal (assuming flat cos(i) distribution).

36 Challenges Pulsar searches with arrays (Plot: S. Ransom)

37 Challenges ISM timing effects (NANOGrav, Arzoumanian, et al 2015) Wavelength-dependent timing effects due to multi-path propagation through ISM. Influences optimal frequency choice for PTA and other timing projects (higher freq less ISM influence but PSRs are fainter).

38 Challenges Intrinsic pulse jitter Extreme example, individual pulses from magnetar J : (Bower et al. 2014) Jitter averages down over time but not over BW or collecting area. MSPs likely show similar behavior, only detectable statistically (e.g., Shannon et al 2014; Dolch et al 2014)

39 Summary Radio timing of MSPs offers a number of direct measurements of NS mass In recent years, measurements of several ~2-solarmass NS. PTA projects currently motivating advances in timing data analysis and observations. Moving into regime of large numbers of masses, allows statistical analysis. Many future telescopes planned and under construction will perform radio pulsar searches and timing over the next years. Will increase number of measured masses by at least factor of several.