UW: Mapping Matter in Strong Gravity

Colloquium given for the University of Washington Department of Astronomy on May 14, 2020.

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 in academia.

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. 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 Q&A Time #1 Q&A Time #2 Q&A Time #3 2
3. 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 3
4. Stellar remnants A.L. Stevens • Michigan State U. & U. Michigan Image: R.N. Bailey, CC BY 4.0, WikiMedia 2 2 Compact objects 4
5. § 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 5
6. 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 6
7. 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? 7
8. 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? 8
9. 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? 9
10. 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? 10
11. 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 11
12. X-ray telescopes X-rays UV IR Microwaves Radio Image: Cool Cosmos/Caltech X-ray astronomy A.L. Stevens • Michigan State U. & U. Michigan 12
13. 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 13
14. X-ray telescopes: NICER A.L. Stevens • Michigan State U. & U. Michigan § 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 Image: NASA 14
15. A.L. Stevens • Michigan State U. & U. Michigan NICER in action 15 Video: NASA GSFC https://youtu.be/kk0ry3_R2pE
16. 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 16
17. 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 17
18. “Soft” spectrum: strong accretion disk emission “Hard” spectrum: strong Comptonized emission A.L. Stevens • Michigan State U. & U. Michigan Same object, different spectra Images: Done+07; NASA/JPL-Caltech; NASA/GSFC 1000 18
19. “Soft” spectrum: strong accretion disk emission “Hard” spectrum: strong Comptonized emission A.L. Stevens • Michigan State U. & U. Michigan Same object, different spectra Images: Done+07; NASA/JPL-Caltech; NASA/GSFC 1000 19
20. “Soft” spectrum: strong accretion disk emission “Hard” spectrum: strong Comptonized emission A.L. Stevens • Michigan State U. & U. Michigan Same object, different spectra Images: Done+07; NASA/JPL-Caltech; NASA/GSFC 1000 20
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 21
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 ^ Image: L. Barbosa via wikiMedia 22
23. 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
24. 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
25. 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 25
26. 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 26
27. Q&A Time #1: § Compact objects § X-ray binaries § X-ray observations § General spectral analysis § General timing analysis A.L. Stevens • Michigan State U. & U. Michigan 27
28. 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 28
29. 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) 29
30. QPOs → Damped harmonic oscillators A.L. Stevens • Michigan State U. & U. Michigan y = cos(⍵t) 30
31. QPOs → Damped harmonic oscillators A.L. Stevens • Michigan State U. & U. Michigan y = cos(⍵t) x e-bt b=0 b=0.08 31
32. 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 32
33. 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 33
34. 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 34
35. 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 35
36. 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 36
37. 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 37
38. LF QPOs: Lense-Thirring precession? A.L. Stevens • Michigan State U. & U. Michigan See recent work by: Chattarjee, Liska, Markoff, Tchekhovskoy Movie: A. Tchekhovskoy × 38
39. 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? 39
40. 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 40
41. 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 41
42. 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 Dynamical power spectrum: evolution of power spectrum in time 42 Stevens+18
43. Power (rms2/Hz) Elapsed time (in 64 s segments) A.L. Stevens • Michigan State U. & U. Michigan NICER observations of MAXI J1535 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 43 Stevens+18
44. 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. 44
45. 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. Due to lag sign and binary inclination, we predict a compact, smaller scale-height emitting region Image: Ingram+09 45
46. LF QPOs linked to discrete jet ejecta A.L. Stevens • Michigan State U. & U. Michigan § Transition between 2 types of LF QPOs (along with broadband spectral changes) correlates with radio jet “turning off” and discrete jet ejecta § Long suspected! Radio flare within ~day of QPOs for 6 BHs studied in Fender+09, Miller-Jones+12, Russell+19 § MAXI J1820+070 (right): Type-B QPO turning off at start of flare (using NICER, AMI-LA) Homan+20 (incl. ALS) 46
47. LF QPOs linked to discrete jet ejecta A.L. Stevens • Michigan State U. & U. Michigan § Transition between 2 types of LF QPOs (along with broadband spectral changes) correlates with radio jet “turning off” and discrete jet ejecta § Long suspected! Radio flare within ~day of QPOs for 6 BHs studied in Fender+09, Miller-Jones+12, Russell+19 § MAXI J1820+070 (right): Type-B QPO turning off at start of flare (using NICER, AMI-LA) Homan+20 (incl. ALS) § QPO emission mechanism/region closely connected with jet emission region (see Homan+20, incl. ALS) § Meg Davis tried phase-resolving the “Type B” QPO (purple arrow) to dig deeper, too low amplitude for full phase-resolved spectroscopy 47
48. Q&A Time #2: § QPOs (quasi-periodic oscillations) § Emission mechanisms like precession § Observational and analytical techinques A.L. Stevens • Michigan State U. & U. Michigan 48
49. 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 49
50. 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) 50
51. 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 § 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: 51
52. 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 research: <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: 52
53. 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 53
54. § 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 & JOSS § Google Summer of Code students in 2016-2020 (including S. Sharma* in 2018) Stingray A.L. Stevens • Michigan State U. & U. Michigan StingraySoftware.github.io * Student mentored by ALS 54
55. 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 55
56. 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? § Variability transitions linked to discrete jet ejecta § NICER: soft X-ray telescope on International Space Station § STROBE-X: proposed large-area X-ray observatory § Stingray: github.com/ StingraySoftware