Talks & Ideas



Antonia Bevan (UCL) In Search of Stardust

  • dust grains are critical for forming molecules
  • dust forms in the winds of AGB stars (e.g., CW Leo)
    • however, it is difficult to explain large quantities of dust in early Universe (z>6)
    • Massive stars can produce some dust in novae and outbursts. -> not enough
  • The dust budget problem
    • Dust injection rates cannot account for the large quantity of dust in early (local) Universe
    • Possible: Core-collapse SNe (CCSNe) solve the dust budget problem?
    • Dust nuclearation models predicted that large masses of dust form in the ejecta of CCNSe ($0.1-1 M_{\odot}$ of dust)
    • SN 1987A, Crab Nebula, Cassiopeia A—> nice source for dust formation from CCSNe
  • Three ways to trace dust formation in CCSNe
    1. Decrease in UV/visible luminosity (Leibundgut & Suntzeff 03)
    2. Increased IR emission at same time as drop in UV-visible
      • Difficult to distinguish between pre-existing dust and newly formed dust
    3. Apparently blue-shifted line profile in optical/IR (Lucy+89)
  • DAMOCLES code: monte-carlo radiative transfer code (Bevan+16)
    • What is the dust mass?
    • How fast does it forms?
    • What is the composition of the dust?
    • How big are the dust grains?
  • Dust growth curve (Gall+14)


EHT telecon:

Andrew Chael MAD simulation of M87 jets

  • Uncertainty about what to do from radiation from high magnetization parameter $\sigma$ region because of the artificial density floor inserted through jet.

Simulation Library

  • INSANE(experimental): high B but no MAD
  • Monica: synchrotron emissivity function


  • simple "two-ring" model to simulate the jet bases -> shadow and hot spots
    • no consideration of radial motion of jet
    • assumption of optically thin
    • Huang-Yi: ring location is likely related to the black hole spin.

Seminar: Jakob van den Eijnden (API) Impossible Jets, and where to find them

  • Jets
    • lunch region
    • collimation
    • acceleration of particles
    • connection to the infalling gas
  • Jet formation
    • BH: Blandford-Znajek or Blandford-Payne
    • neutron star: no Blandford-Znajek
  • alternative jet formation mechanism for the neutron star in high magnetic field (Parfrey+16)
    • Strongly magnetized neutron stars cannot eject jets by Blandford-Payne mechanism. ($B > 10^{9}$ G)
  • radio: tracing outflow, x-ray: tracing the gas inflow. (Gusinskaia+17)
  • prove of jet
    • spectral index & spectral evolution
    • X-ray vs. radio coupling
    • polarization
    • spatial coincidence & initial non-detection
  • Implications
    • BP jet models are ruled out for slow X-ray pulsars
    • slow x-ray pulsars can from a large hidden class of radio sources
    • HMXBs can probe relation between spin and jet power
    • UX pulsars might lunch jets as well.


Seminar: Gregg Hallinan (Caltech) Superluminal Motion of the Relativistic Jet of GW170817

  • The full DSA radio dish array (2019-2020): 100 FRBs/yr to > 3 arcseconds
    • entire sky survey
  • (Abbott+17)
  • GW170817: Gamma-ray burst afterglow
  • Ejecta
    • Dynamical ejecta (Months-Years) (Hotokeza & Piran 15)
    • relativistic jet (days-weeks) (Granot 02)
    • cocoon (weeks-months) (Lazzati+17)
  • Widely off-axis jets
  • Light curve: Mooley+18, Nature
  • Early Afterglow consistent with wide-angle outflow
  • Light Curve: has energy ejection stopped (Nakar & Piran 18)
  • Polarization (Corsi+18)
  • Constraints on the Hubble constant (Hotokezaka+18)


Fe Krauss (GRAPPA) TXS 0506+056, IC 170922A, and multimessenger observations of blazers

  • neutrino alert from Ice-cube and coincide with the $\gamma$-ray.
  • Why do we think AGN produce neutrinos?
    • Ultra High energy cosmic-ray
  • Broadband spectral energy distributions (SEDs) (Kraub F.+14, Bottcher 13)
  • $\gamma$-ray flux does not correlate w/ neutrino flux.
    • X-ray may be alternative tracer of neutrino flux.


Ying Zu (Shanghai Jiao Tong Univ.) Galaxy-Halo Connection for Next Generation Galaxy Surveys

  • connecting galaxy (light) & halo (density)
  • Mapping galaxy & Halo: SDSS, BOSS EBOSS
  • Main Question:
    1. What drives "galaxy quenching", the active-to-passive, blue-to-red transition of galaxies?
    2. How did galaxies acquire and lose cold gas? did they happen over the same timescale as star formation and quenching?
    3. What drives the $M_{\rm BH}-\sigma$ relation? Merger stochastically?
  • Halo quenching (Halo mass is the main driver of quenching) other than stellar mass quenching & age-matching (Zu&Mandelbaum 17, Alam,Zu+18)
  • What drives the cold gas in star formation?


Jian-Min Wang (Institue of High Energy Physics) Reverberation Mapping (RM) of AGNs: Supermassive Black Holes

  • RM:
    • Mass / Spin and evoution
    • link to Binary black holes
    • Standard candles
  • Maximum redsift quasars: z=7.5, $M_{\rm BH} \sim 10^{9} M_{\odot}$ already very huge in earlier time.
  • SDSS (2.5m) -> DESI (>2018; 4m)
  • Metals in AGN: always quite rich
  • $H_{\beta}-\lambda L_{\lambda}$ (R-L) scaling relation (Kaspi+00; Bentz+13)
  • BLR-size scaling relation (Bentz+13, Du+15,18, Garier+17)
    • Assumptions to be justified
      • Isotropic
      • Point of ionization
      • Spins
      • Single or binaries
  • super-Eddington BHs (SEAMBH) !!
    • Monitoring AGNs with Hbeta Asymmetry (MAHA collaboration)
  • SMBH spins
    • Line profile affected by gravity and doppler shift
    • Reynolds+14
    • Duty cycle (Wang+06,07)
    • Radiative efficiency for spins (Wang+09)
    • Cosmic evolution: spin-down (maximum at z~2 and slow down as z decreases)
      • ##red| why $\eta$ is flat before z~2$ and downsizing after then?]]
      • w/ accretion: mass increases, but spin goes down!
      • Secular evolution? Or minor mergers?
      • Numerical simulations: Volonteri+13
      • Spin & Accretion: Wang+13
      • R-L relation in retro-grade accretion (Du+18)
    • eRosita (2019) for studying black hole spin by detecting (Iorn $K_{\alpha}$)
    • Athena (2025)
  • BH mass
    • Classical model (Abramowicz+88)
    • Accretion disk (Shakura-Sunyaev disk model) + Broad-line region (R-L relation)
  • Rubin(2014): No SNIa in early universe
  • Detection Sensitivity of Gravitational Waves (Moore+2015)


Jerry Sellwood (Steward Observatory/Arizona) Two-body scattering and relaxation in disk systems


Xinwen SHU (Anhui Normal University) Supersoft AGNs: signature of super Eddington accretion, IMBH, TDE or new AGN spectral state

  • supersoft AGN w/o hard X-ray emission above ~2 kev
    • Likely signature of direction accretion disk thermal emission, an AGN high/soft state
    • Radio emission: unexpected for "soft-state" AGN
  • what is the mechanism for driving jets in soft state?
    • —> like sporadic jets in transient XRBs
  • super soft spectrum from disk generated by TDE is quite normal since it has initially no intrinsic corona.
  • slim disk


Jerry Sellwood (Univ. of Arizona) The origin of spirals in galaxies

  • Spirals can be self-excited by clearly collective instabilities.
  • decades of disputes in theory
    • C C Lin: quasi-steady spiral density waves
    • Toomre: swing-amplified transients
  • Spirals are one of the major drivers of secular evolution in disk galaxies
    • transport angular momentum
    • scatter stars away from circular orbits
    • cause radial mixing
    • smooth rotation curves and disk density profiles
    • help with magnetic dynamos
  • Why is gas important if the spiral is driven by gravity from old stars?
    • Spirals disappears after 10 galaxy rotations due to increasing random motion in absence of gas (Sellwood & Carlberg 84)
    • Unlike stars, gas can dissipate energy
    • new stars form from the gas on nearly circular orbits
    • add new stars at a steady rate
    • gas -> re-juvenate the star
  • How spirals form??
    • collective motions of the stars that creat a wave-like pattern in the disk density "density wave"
    • gravitation deflections of the stars must cause and sustain it
  • Swing Amplification (Toomre)
    • Linear perturbation theory calculation
    • spiral added "by hand"
    • Leading wave shears into trailing with a brief flourish
      • phase velocity in orbital direction
      • group velocity is radial
  • Need a reflection to make the pattern last
    • what if the disk is not smoot -> the equivalent of a "frayed" region to create a partial reflection
    • -> resonance with a previous wave!!!
  • corotation where $\Omega = \Omega_{p}$
  • Lindblad resonance where forcing freq. = epicycle freq.
  • Scattering at LRs -> heating - well known (LBK 72)
  • gravity torques extract energy from potential well
  • But stars scattered at ILR stay close to resonance.
  • Pour-Imani+ find that spiral pitch angle depends on the observed wavelength


Andrzej Zdziarski (Centrum Astronomiczne Kopernika) Jets in black-hole binaries

  • Jet appearance on the hardness-luminosity diagram (Fender & Belloni)
  • Jet-formation mechanism
    • Tchekhovskoy+11, McKinney+12: extraction of the spin energy of a black hole (Blandfold-Znajek) in a magnetically arrested disc (gas ram pressure is balanced by the magnetic pressure; MAD, Narayan+03)
    • collimation and acceleration by disc poloidal magnetic field (Blandfold-Payne)
  • Models of jet radiation
    • A magnetized blob w/ relativistic electrons moving through the jet. This is a popular for blazar emission. Can apply to ballistic blob ejection in transitional states of microquasars. Not for steady jets.
    • A conical jet with maintained power-law electron distribution and constant magnetic energy flux (Blandford & Konigl 79). Can apply to hard-state jets. In its original version, it neglects energy looses.
    • The main radiative processes are synchrotron emission and self-absorption, and Compton scattering of either synchrotron (SSC) or external photons.
  • A strong recollimation shock appears to be formed in Cyg X-3 due to the jet power being below critical.
  • GeV emission modulation in Cyg X-3 -> Compton anisotropy (Zdziarski+18)
    • gamma-ray: scattering of stellar photons
    • radio, X-ray: wind absorption
  • Puzzling of Cyg X-3
    • In its hard state, a radio/X-ray correlation similar to that of BH binaries, but major radio flares (<20 Jy) and strong $\gamma$-ray emission in the soft state unlike the jet quenching in the standard soft state.
    • the presence of strong magnetic fields in the soft-state accretion flow !!


Colloq: Zheng Zheng (U of Utah) The curious case of Lyman-alpha emitting galaxies

  • Ly$\alpha$ Emitters (LAEs) (Partridge & Peebles 67)
    • studying young star-forming galaxies (Fardal+06)
    • probing CGM
    • probing cosmic reionization
    • constraining cosmology
  • Lyman Break Galaxies (Dropout galaxies)
  • Ly$\alpha$ line
    • 1s-2p transition
    • wavelength = 1216 A
    • lifetime $10^{-8}$ sec
  • Solve the radiative transfer of Ly$\alpha$
    • Monte Carlo Code (Stephen Todd & Douglas Pierce-Price)
  • Extended Lyman-alpha Emission around Star-forming galaxies (Steidel+11) <- stacking method
  • Anisotropic Lyman-alpha Emission (Zheng & Wallace 14)


Colloq: Daniel Wang (UMASS) The Galactic Center Ecosystem

  • Mass of SMBH + Nuclear stellar cluster is anti-correlated to the mass of spheroid (bulge) (Graham & Spitler 09)
    • little is known about the interplay among the ism, stars and SMBHs.
    • which kinds of stellar cluster exist near the galactic center?
    • How energetic is the AGN outflow?
    • How energetic is the outflow of the Radiative Inefficient accretion flow (RIAF)?
    • How do the strong radiation and outflow affect the environment?
  • X-ray reverberation of a Sgr A* burst ~ 100 years ago (Ponti+10)
    • Best candidate is Sgr A* rather than magnetar, XRBs
    • Possibly illuminated by Tidal Disruption Event (TDE) $L_{\rm X-ray} \sim 10^{39}$ (i.e. current luminosity $L_{\rm X-ray} \sim 10^{33}$)