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
   

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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
   

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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
   

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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
   

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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
   

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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
   

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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
   

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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
   

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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
   

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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
    

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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
    

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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
    

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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
    

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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
    

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15. A.L. Stevens • Michigan State U. & U. Michigan
    NICER in action
    15
    Video: NASA GSFC
    https://youtu.be/kk0ry3_R2pE
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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30. QPOs → Damped harmonic oscillators
    A.L. Stevens • Michigan State U. & U. Michigan
    y = cos(⍵t)
    30
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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