Session Y10: Active Galaxies, Star Clusters and Exoplanets

Live

Markarian 421, like most of the TeV high-frequency-peaked BL Lacs (HBLs), shows slow or no motion of the VLBI radio-knot structures in its jet, in stark contrast to its known fast variability. This problem, known as ``the bulk Lorentz factor crisis,'' can be resolved if we consider that these strings of knots are successive recollimation shocks in the jet. Successive shocks predict that a unique pattern of the non-thermal emission variability should appear after each strong flare. Using the 13-year-long X-ray dataset from the Swift X-ray Telescope, we find evidence of such a distinct pattern with a statistical significance of more than 3 standard deviations compared to variability from stochastic fluctuations. We show how this pattern can be used in an innovative way to unveil the physical properties of the Mrk 421 jet, such as the apparent flow speed, the size of jet perturbations, and the jet beaming parameters. We discuss how this study can be extended to other wavelengths.   

Live

We present comprehensive modeling of the SEDs of the ensemble of the Fermi-LAT detected blazar spectra, commonly known as 'the blazar sequence'. As shown by Fermi, the entire sequence is encapsulated in a single diagram that relates the LAT spectral index to the blazar synchrotron peak frequency. The former ranges between values ~1.5 and ~3 while the latter spans five decades from $10^{12}$ to $10^{17}$ Hz, to form a one parameter family. We show that this parameter is basically the (normalized to the Eddington value) dimensionless blazar accretion rate. An aspect of this model, crucial in reducing the many parameters of the spectra of individual objects, is the presence of an accretion disk MHD wind which spans the entire accretion disk domain, of order $10^6$ gravitational radii; this reprocesses the central engine continuum to similar distances along the jet axis and thus provide significant $\gamma$-ray emission at distances from the black hole as large as ~ 1pc, as implied by the observations. We will also discuss the issue of blazar neutrinos within the context of this same model.   

Live

Tidal disruption events occur rarely in any individual galaxy. Over the last decade, however, time-domain surveys have begun to accumulate statistical samples of these flares. What dynamical processes are responsible for feeding stars to supermassive black holes? At what rate are stars tidally disrupted in realistic galactic nuclei? What may we learn about supermassive black holes and broader astrophysical questions by estimating tidal disruption event rates from observational samples of flares? We address these questions in an upcoming review which summarizes current theoretical knowledge about rates of stellar tidal disruption, and compares theoretical predictions to the current state of observations.   

Live

To account for the observed flat rotation curves of galaxies, Alexander and Smolin[1] have proposed that dark matter emerges from a dark superfluid, background state. They invoke a ``dark magnetic potential''. On the other hand, Vikram[2] proposes that the dark sector is superconducting. A dark vector potential is a central feature. The present work builds on this prior work as follows: 1) An intuitive mechanism for the generation of a dark magnetic field is proposed. Specifically, a ``BCS-like'' vector potential is generated via ``Faraday'' induction by the observable rotating galactic mass current. 2) This potential couples to a dark superconducting background causing massive dark photons to emerge, defining a ``dark'' supercurrent and resulting in higher total galactic mass current. In one scenario, the coupling admits an interesting picture in which the emergent dark photons would have negative mass, as postulated by Farnes[3]. Finally, a laboratory scale experiment is proposed which might bolster the notion of galactic ``supercurrent''. [1] Stephon Alexander and Lee Smolin, arXiv:1804.09573 [2] Alexander Vikram, arXiv:1712.10311v1 [3] J.S. Farnes, arXiv:1712.07962v2   

Live

The Dark Energy Spectroscopic Instrument (DESI) is a redshift survey that will observe more than 30 million galaxies and quasars over a five year period. Using the resulting redshift catalog and the effects of baryon acoustic oscillations on galaxy and quasar clustering, DESI will chart the expansion history of the universe out to a redshift of z$=$4. This analysis looks at the effects of radial observational systematics on redshift measurements of luminous red galaxies, emission line galaxies, and quasars, and how this ultimately affects DESI cosmology by examining the impact on the two point correlation function.   

Uncovering the extremes of black hole accretion in a local Seyfert galaxy

In this golden age of Time Domain Astronomy, we are discovering supermassive black holes that defy long-held ideas on how material accretes on to black holes. We continue to discover extreme accretion episodes due to tidal disruption events or disc instabilities, where the physics is not yet understood. In this talk, I will present ongoing work from a multi-wavelength follow-up campaign of a local Seyfert galaxy that has recently become the brightest AGN in the soft X-ray sky. These new observations are helping us understand the nature of the heating mechanism of the X-ray corona and its connection to the accretion disc and the launching of relativistic jets.   

Deep Transfer Learning for Star Cluster Classification

We present the results of a proof-of-concept experiment which demonstrates that deep learning can successfully be used for production-scale classification of compact star clusters detected in HST UV-optical imaging of nearby spiral galaxies (D < 20 Mpc) in the PHANGS-HST survey. Given the relatively small and unbalanced nature of existing, human-labelled star cluster samples, we transfer the knowledge of state-of-the-art neural network models for real-object recognition to classify star clusters candidates into four morphological classes. We perform a series of experiments to determine the dependence of classification performance on: neural network architecture; training data sets curated by either a single expert or three astronomers; and the size of the images used for training. We find that the overall classification accuracies are not significantly affected by these choices. The networks are used to classify star cluster candidates in the PHANGS-HST galaxy NGC 1559, which was not included in the training samples. The performance is competitive with that achieved in previously published human and automated quantitative classification of star cluster candidate samples. The methods introduced herein lay the foundations to automate classification for star clusters at scale.   

Detection of Exoplanet as a Binary Source of Microlensing Events in WFIRST Survey

We investigate the possibility of exoplanet detection orbiting source stars in microlensing events through WFIRST observations. We perform a Monto Carlo simulation on the detection rate of exoplanets via microlensing, assuming that each source star has at least one exoplanet. The exoplanet can reflect part of the light from the parent star or emit internal thermal radiation. In this new detection channel, we use microlensing as an amplifier to magnify the reflection light from the planet. In the literature, this mode of detecting exoplanets has been investigated much less than the usual mode in which the exoplanets are considered as one companion in binary lens events. Assuming 72 days of observation per season with the cadence of 15 minutes, we find the probability of rocky planet detection with this method to be virtually zero. However, there is non-zero probability, for the detection of Jovian planets. We estimate the detection rates of the exoplanets by this method, using WFIRST observation to be 0.012% in single lens events and 0.9% in the binary lens events.   

Not Participating

The blazar 1ES~0229+200 is the archetypical member of the extreme high-frequency peaked BL Lacertae (EHBL) class of objects. Observations by imaging atmospheric Cherenkov telescopes (IACTs) have revealed that the very high-energy (VHE) spectrum of 1ES~0229+200 is surprisingly hard and only weakly variable. Several studies have used the observed IACT spectrum to place constraints on both the extragalactic background light and the intergalactic magnetic field. However, these studies rely on the assumption that the spectrum as measured by the IACTs is a good representation of the long-term VHE emission from 1ES~0229+200. With its wide field of view and near-100\% duty cycle, the High Altitude Water Cherenkov (HAWC) Observatory provides complementary observations of 1ES~0229+200 that can be used to check this assumption. In this presentation, we report on observations of 1ES~0229+200 using more than three years of data from HAWC. We discuss the implications of the HAWC non-detection of 1ES~0229+200 in the context of the spectra reported by the IACTs.