Bard Colloquium: Mapping Matter in Strong Gravity

Colloquium for the Physics Program at Bard College, my alma mater.

Title:
Mapping Matter in Strong Gravity: Spectral-Timing of Black Holes and Neutron Stars

Abstract:
One of the best laboratories to study strong-field gravity is the inner 100s of kilometers around black holes and neutron stars in binary systems with low-mass stars like our Sun. The X-ray light curves of these binary systems show variability on timescales from milliseconds to months — the shorter (sub-second) variability can appear as quasi-periodic oscillations (QPOs), which may be produced by general relativistic effects. My research looks at QPOs from black holes and neutron stars (as well as coherent X-ray pulsations from neutron stars) by fitting the phase-resolved energy spectra of these signals to constrain their physical origin and track their evolution in time. In this talk, I will introduce why black holes and neutron stars are interesting and discuss state-of-the-art “spectral-timing” analysis techniques for understanding more about them. I will also highlight open-source astronomy research software and the importance of mental wellbeing among students and early-career researchers.

Transcript

1. Mapping Matter in Strong Gravity: Spectral-Timing of Black Holes and Neutron Stars Dr. Abbie Stevens NSF Astronomy & Astrophysics Postdoctoral Fellow Michigan State University & University of Michigan alstev@msu.edu @abigailstev github.com/abigailstev Image: NASA/JPL-Caltech
2. Abbie since Bard § Graduated from Bard in 2011 in physics (sproj advisor: Peter Skiff), “minored” in math A.L. Stevens • Michigan State U. & U. Michigan
3. Abbie since Bard § Graduated from Bard in 2011 in physics (sproj advisor: Peter Skiff), “minored” in math § 2011-2013: MSc in (astro)physics from the Univ. of Alberta in Edmonton, AB, Canada A.L. Stevens • Michigan State U. & U. Michigan
4. Abbie since Bard § Graduated from Bard in 2011 in physics (sproj advisor: Peter Skiff), “minored” in math § 2011-2013: MSc in (astro)physics from the Univ. of Alberta in Edmonton, AB, Canada § 2013-2017: PhD in astronomy from the Univ. of Amsterdam in the Netherlands A.L. Stevens • Michigan State U. & U. Michigan
5. Abbie since Bard § Graduated from Bard in 2011 in physics (sproj advisor: Peter Skiff), “minored” in math § 2011-2013: MSc in (astro)physics from the Univ. of Alberta in Edmonton, AB, Canada § 2013-2017: PhD in astronomy from the Univ. of Amsterdam in the Netherlands § 2018-now: NSF Astronomy & Astrophysics Postdoctoral Fellow at Michigan State Univ. and Univ. of Michigan A.L. Stevens • Michigan State U. & U. Michigan
6. Outline A.L. Stevens • Michigan State U. & U. Michigan § Compact objects, low-mass X-ray binaries § X-ray astronomy: timing, spectroscopy § Quasi-periodic oscillations (QPOs) § STROBE-X: proposed X-ray observatory § Stingray: open-source spectral-timing software
7. Outline A.L. Stevens • Michigan State U. & U. Michigan § Compact objects, low-mass X-ray binaries § X-ray astronomy: timing, spectroscopy § Quasi-periodic oscillations (QPOs) § STROBE-X: proposed X-ray observatory § Stingray: open-source spectral-timing software
8. Stellar remnants A.L. Stevens • Michigan State U. & U. Michigan Image: R.N. Bailey, CC BY 4.0, WikiMedia 2 2 Compact objects
9. § Radius ~ 12 km (7.5 mi), mass ~ 1.5 MSun § Densityavg. ~ 1014 g/cm3 (the density of an atomic nucleus!) § Surface accelerationgravity ~ 1012 m/s2 § Magnetic field ~ 108 - 1015 Gauss (104-1011 Tesla) § Spin frequencies up to 100’s of Hz Neutron stars (NSs) A.L. Stevens • Michigan State U. & U. Michigan Watts+16
10. Stellar black holes (BHs) § Ultracompact remnant of a massive star >20 MSun § Event horizon radius ~15 km for a 10 MSun BH A.L. Stevens • Michigan State U. & U. Michigan Image adapted from LIGO/Caltech X-ray binary black holes LIGO-Virgo black holes 80 40 20 10 5 Mass (MSun ) as of Oct. 2017
11. A.L. Stevens • Michigan State U. & U. Michigan Image: NASA/CXC/M. Weiss Companion star ≲ 1 MSun Hot inner flow/ corona Compact object (BH or NS) Accretion disk Low-mass X-ray binaries How does matter behave in strong gravitational fields?
12. A.L. Stevens • Michigan State U. & U. Michigan Low-mass X-ray binaries Image: NASA/CXC/M. Weiss 10 5 20 0.1 1 keV2 (Photons cm−2 s−1 keV−1) Energy (keV) Comptonized blackbody reflection Companion star ≲ 1 MSun Accretion disk Compact object (BH or NS) Hot inner flow/ corona How does matter behave in strong gravitational fields?
13. A.L. Stevens • Michigan State U. & U. Michigan Low-mass X-ray binaries Image: NASA/CXC/M. Weiss 1700 1702 1704 1706 1708 1710 2000 4000 6000 8000 104 1.2×104 Count/sec T im e (s) S tart T im e 12339 7:28:14:566 S top T im e 12339 7:29:32:683 B in tim e: 0.7812E −02 s Accretion disk X-ray variability Companion star ≲ 1 MSun Hot inner flow/ corona Compact object (BH or NS) How does matter behave in strong gravitational fields?
14. A.L. Stevens • Michigan State U. & U. Michigan Low-mass X-ray binaries Image: NASA/CXC/M. Weiss 10 5 20 0.1 1 keV2 (Photons cm−2 s−1 keV−1) Energy (keV) Comptonized blackbody reflection Companion star ≲ 1 MSun Accretion disk Compact object (BH or NS) Hot inner flow/ corona 1700 1702 1704 1706 1708 1710 2000 4000 6000 8000 104 1.2×104 Count/sec T im e (s) S tart T im e 12339 7:28:14:566 S top T im e 12339 7:29:32:683 B in tim e: 0.7812E −02 s X-ray variability How does matter behave in strong gravitational fields?
15. Outline A.L. Stevens • Michigan State U. & U. Michigan § Compact objects, low-mass X-ray binaries § X-ray astronomy: timing, spectroscopy § Quasi-periodic oscillations (QPOs) § STROBE-X: proposed X-ray observatory § Stingray: open-source spectral-timing software
16. X-ray telescopes X-rays UV IR Microwaves Radio Image: Cool Cosmos/Caltech X-ray astronomy A.L. Stevens • Michigan State U. & U. Michigan
17. X-ray telescopes: NICER A.L. Stevens • Michigan State U. & U. Michigan Image: NASA § Neutron star Interior Composition ExploreR § Launched 3 June 2017, attached to Int’l Space Station § All-in-one: better timing than RXTE and AstroSat, energy resolution of XMM-Newton, high throughput
18. A.L. Stevens • Michigan State U. & U. Michigan NICER in action
19. X-ray spectroscopy § X-ray binaries are too small and far away for imaging § Like trying to image a human hair on the surface of Mars § Physical processes have characteristic energy distributions § Blackbody, multi-color blackbody § Inverse Compton up-scattering § “Reflection” A.L. Stevens • Michigan State U. & U. Michigan TOTAL COMPTONIZED REFLECTED DISK Image: Gilfanov10
20. X-ray spectroscopy A.L. Stevens • Michigan State U. & U. Michigan 0.5 1 1.5 Line profile Gravitational redshift General relativity Transverse Doppler shift Beaming Special relativity Newtonian Eobs/Eem Newtonian Special relativity Transverse Doppler shift Beaming Gravitational redshift Line profile General relativity TOTAL COMPTONIZED REFLECTED DISK
21. “Soft” spectrum: strong accretion disk emission “Hard” spectrum: strong Comptonized emission Spectrum changes in “outburst” over weeks to months A.L. Stevens • Michigan State U. & U. Michigan Same object, different spectra Images: Done+07; NASA/JPL-Caltech; NASA/GSFC 1000
22. Fourier transforms A.L. Stevens • Michigan State U. & U. Michigan § X-rays from BHs and NSs vary on timescales from microseconds to years § Shorter (< 1 minute) variability: Fourier analysis! § Study time domain f in the frequency domain f § Break down light curve into sine waves, take amplitude of sines at each frequency ^ Problem solution solve (hard) Transformed problem Transformed solution solve (easy) Fourier transform inverse Fourier transform Image: L. Barbosa via wikiMedia
23. Applying Fourier transforms to data 1016 1018 1020 1022 1024 5000 104 1.5×104 Count/sec Time (s) Start Time 10168 18:16:52:570 Stop Time 10168 18:17:08:180 Bin time: 0.1562E−01 s Time domain Light curve Frequency/Fourier domain Power density spectrum FOURIER TRANSFORM2 A.L. Stevens • Michigan State U. & U. Michigan Light curve broken into equal-length chunks (64 seconds), take power spectrum of each chunk, average those together
24. X-ray variability: Hard to see by eye 1014 1016 1018 1020 1022 5000 104 1.5×104 Count/sec Time (s) CYGNUS_X−1 Start Time 10168 18:16:52:578 Stop Time 10168 18:17:02:547 Bin time: 0.3125E−01 s A.L. Stevens • Michigan State U. & U. Michigan 1696 1698 1700 1702 1704 4000 5000 6000 7000 Count/sec Time (s) GRS1915+105 Start Time 12339 7:28:14:582 Stop Time 12339 7:28:24:542 Bin time: 0.4000E−01 s Light curves Power density spectra Noise: Cygnus X-1 Signal: GRS 1915+105
25. Outline A.L. Stevens • Michigan State U. & U. Michigan § Compact objects, low-mass X-ray binaries § X-ray astronomy: timing, spectroscopy § Quasi-periodic oscillations (QPOs) § STROBE-X: proposed X-ray observatory § Stingray: open-source spectral-timing software
26. Strohmayer ‘01 § High-frequency: 100’s Hz § Hot blobs in Keplerian orbit at inner disk edge? § Low-frequency: ~0.1-10’s Hz § Precession of corona/hot flow? Magnetic warps in disk? Comptonized disk fluctuations? A.L. Stevens • Michigan State U. & U. Michigan Quasi-periodic oscillations (QPOs)
27. QPOs → Damped harmonic oscillators A.L. Stevens • Michigan State U. & U. Michigan y = cos(⍵t)
28. QPOs → Damped harmonic oscillators A.L. Stevens • Michigan State U. & U. Michigan y = cos(⍵t) x e-bt b=0 b=0.08
29. QPOs → Damped harmonic oscillators A.L. Stevens • Michigan State U. & U. Michigan y = cos(⍵t) x e-bt b=0 b=0.08 b=0.22
30. QPOs → Damped harmonic oscillators A.L. Stevens • Michigan State U. & U. Michigan y = cos(⍵t) x e-bt b=0 b=0.08 b=0.22 b=0.5
31. QPOs → Damped harmonic oscillators A.L. Stevens • Michigan State U. & U. Michigan y = cos(⍵t) x e-bt b=0 b=0.08 b=0.22 b=0.5 b=1.0 The stronger the damping, the wider the peak
32. QPOs → Damped harmonic oscillators A.L. Stevens • Michigan State U. & U. Michigan y = cos(⍵t) x e-bt The stronger the damping, the wider the peak Astrophysics: What is the cause of the oscillation? What is the cause of the damping/dissipation? What else are we not accounting for? b=0 b=0.08 b=0.22 b=0.5 b=1.0
33. LF QPOs: Lense-Thirring precession? A.L. Stevens • Michigan State U. & U. Michigan Stella+Vietri ‘98; Fragile+Anninos ‘05; Schnittman, Homan+Miller ‘06; Ingram+09; Ingram+van der Klis ‘15; Fragile+16; Ingram+16a,b; Liska+18 Lense-Thirring precession/frame dragging with nodal precession Movie: L. Stein
34. LF QPOs: Lense-Thirring precession? × A.L. Stevens • Michigan State U. & U. Michigan Hot inner flow (Comptonizing region) Accretion disk Disk color pattern: Doppler shifting and boosting of emission Stella+Vietri ‘98; Fragile+Anninos ‘05; Schnittman, Homan+Miller ‘06; Ingram+09; Ingram+van der Klis ‘15; Fragile+16; Ingram+16a,b; Liska+18
35. LF QPOs: Lense-Thirring precession? A.L. Stevens • Michigan State U. & U. Michigan See recent work by: Chattarjee, Liska, Markoff, Tchekhovskoy Movie: A. Tchekhovskoy ×
36. LF QPOs: Lense-Thirring precession? × A.L. Stevens • Michigan State U. & U. Michigan Expect changing energy spectrum on sub-QPO timescale: • Normalization • Blackbody • Iron line profile Want to: • Determine low-freq. QPO emission mechanism • Different mechanism for QPO types?
37. Quasi-periodic signals: § not coherent enough to fold light curve § in time domain, signal would smear out! è average together signals in frequency domain § ephemeris not needed Phase-resolved spectroscopy Periodic signals: § fold light curve at pulse period, stack signal in time domain § need to know ephemeris of source See Miller+Homan ‘05; Ingram+van der Klis ‘15; Stevens+Uttley ‘16 A.L. Stevens • Michigan State U. & U. Michigan
38. A.L. Stevens • Michigan State U. & U. Michigan Power (rms2/Hz) Elapsed time (in 64 s segments) NICER observations of MAXI J1535 Stevens+18 Dynamical power spectrum: evolution of power spectrum in time
39. Power (rms2/Hz) Elapsed time (in 64 s segments) Broadband/band-limited noise Low-freq. QPO QPO harmonic Low-amplitude low-freq. QPO hiding here? A.L. Stevens • Michigan State U. & U. Michigan NICER observations of MAXI J1535 Stevens+18 Dynamical power spectrum: evolution of power spectrum in time
40. Power (rms2/Hz) Elapsed time (in 64 s segments) A.L. Stevens • Michigan State U. & U. Michigan NICER observations of MAXI J1535 Stevens+18 Yes! Needed to average together data segments for it to be visible. QPO Harmonic Broadband noise Dynamical power spectrum: evolution of power spectrum in time
41. MAXI J1535 phase spectra A.L. Stevens • Michigan State U. & U. Michigan § Ratio of QPO- phase-resolved spectra with the mean spectrum → how spectral components vary with QPO phase § ~25% phase lag gives it the “doing the wave” appearance Stevens+ in prep. § Compared with previous “Type B” QPO: predict tall jet-like emitting region § But, lag sign is opposite and binary has different inclination! § Compact, smaller scale- height emitting region Image: Ingram+09
42. Outline A.L. Stevens • Michigan State U. & U. Michigan § Compact objects, low-mass X-ray binaries § X-ray astronomy: timing, spectroscopy § Quasi-periodic oscillations (QPOs) § STROBE-X: proposed X-ray observatory § Stingray: open-source spectral-timing software
43. Future X-ray telescope: A.L. Stevens • Michigan State U. & U. Michigan X-ray Concentrator Array (0.2-12 keV) Wide Field Monitor (2-50 keV) Large Area Detector (2-30 keV) Solar panels RXTE Electronics, antenna, etc. See Ray+18 (incl. ALS)
44. A.L. Stevens • Michigan State U. & U. Michigan RXTE Electronics, antenna, etc. Science drivers: spin distribution of BHs, accretion disk winds, disk-jet connection, NS equation of state, burst oscillations, GRBs, LIGO EM counterparts, TDEs, discovering new sources, etc! Video from NASA Mission Design Lab, April 2018 For my science: <1ms time resolution + CCD energy resolution + soft X-ray coverage è Resolve how physical components vary, where they’re located § Combines strengths of NICER and LOFT: high throughput X-ray timing with good spectroscopy § All components already at high tech. readiness level § Highly modular design Future X-ray telescope:
45. Outline A.L. Stevens • Michigan State U. & U. Michigan § Compact objects, low-mass X-ray binaries § X-ray astronomy: timing, spectroscopy § Quasi-periodic oscillations (QPOs) § STROBE-X: proposed X-ray observatory § Stingray: open-source spectral-timing software
46. § Open-source timing and spectral-timing software (Astropy affiliated package!) § Stingray: Python library of analysis tools § HENDRICS: shell scripting interface § DAVE: graphical user interface § Tutorials in Jupyter(/iPython) notebooks § Well-documented, automated unit tests, 95% test coverage § Huppenkothen, Bachetti, ALS+2019, ApJ (arXiv: 1901.07681) § Google Summer of Code students: S. Sharma* (‘18); O. Hammad and H. Rashid (‘17); U. Khan, H. Mishra, and D. Sodhi (‘16) Stingray A.L. Stevens • Michigan State U. & U. Michigan StingraySoftware.github.io * Student mentored by ALS
47. Mental wellbeing is important A.L. Stevens • Michigan State U. & U. Michigan § There is a mental health crisis among academics § ~1/3 of grad students meet criteria for depression § Gender minorities had much higher rates of problems than cis-men § Work and organizational context (incl. satisfaction with advising and work environment) are significant predictors of mental health § Stats, tools, resources: abigailstevens.com/mental- wellbeing-academia § Be aware, take care of yourself, look out for friends and colleagues
48. Come to MSU or U of M for grad! § Observational & computational astro § Nuclear physics, CM theory, CM experiment, biophysics, astroparticle, high-energy § How to apply: astro.natsci.msu.edu/graduate /how-to-apply/ A.L. Stevens • Michigan State U. & U. Michigan § Separate departments of astronomy and physics § Nearly all areas of astronomy § How to apply: lsa.umich.edu/astro/graduate s/application-info.html Deadline: December 15th
49. Summary GitHub: abigailStev Email: alstev@msu.edu Twitter: @abigailStev ✉ A.L. Stevens • Michigan State U. & U. Michigan § X-ray binaries are awesome! One of the best tools to study matter in strong gravitational fields § Use X-ray spectral-timing analysis to decipher emission mechanisms for rapid variability § Low-freq. quasi-periodic oscillations: precessing hot inner flow/base of jet? § NICER: soft X-ray telescope on International Space Station § STROBE-X: proposed large-area X-ray observatory § Stingray: github.com/StingraySoftware