Publications (58)
Testing Properties of the Galactic Center Black Hole Using Stellar Orbits
David Merritt, Tal Alexander, Seppo Mikkola +1
The spin and quadrupole moment of the supermassive black hole at the Galactic center can in principle be measured via astrometric monitoring of stars orbiting at milliparsec (mpc)…
Constraining the dark cusp in the Galactic Center by long-period binaries
Tal Alexander, Oliver Pfuhl
Massive black holes (MBHs) in galactic nuclei are believed to be surrounded by a high density stellar cluster, whose mass is mostly in hard-to-detect faint stars and compact remnan…
Resonant Relaxation near the Massive Black Hole in the Galactic Center
Clovis Hopman, Tal Alexander
The coherent torques between stars on orbits near massive black holes (MBHs) lead to resonant angular momentum relaxation. Due to the fact that orbits are Keplerian to good approxi…
Stellar dynamics and tidal disruption events in galactic nuclei
Tal Alexander
The disruption of a star by the tidal field of a massive black hole is the final outcome of a chain of complex dynamical processes in the host galaxy. I introduce the "loss cone pr…
Enhanced Microlensing by Stars Around the Black Hole in the Galactic Center
Tal Alexander, Abraham Loeb
The effect of stars on the lensing properties of the supermassive black hole in the Galactic Center is similar to the effect of planets on microlensing by a star. We show that the…
On Uncertainties in Cross-Correlation Lags and the Reality of Wavelength-Dependent Continuum Lags in Active Galactic Nuclei
Bradley M. Peterson, Ignaz Wanders, Keith Horne +4
We describe a model-independent method of assessing the uncertainties in cross-correlation lags determined from AGN light curves, and use this method to investigate the reality of…
Stellar Dynamics of Extreme-Mass-Ratio Inspirals
David Merritt, Tal Alexander, Seppo Mikkola +1
Inspiral of compact stellar remnants into massive black holes (MBHs) is accompanied by the emission of gravitational waves at frequencies that are potentially detectable by space-b…
Massive perturbers and the efficient merger of binary massive black holes
Hagai B. Perets, Tal Alexander
We show that dynamical relaxation in the aftermath of a galactic merger and the ensuing formation and decay of a binary massive black hole (MBH), are dominated by massive perturber…
Stellar Energy Relaxation around A Massive Black Hole
Ben Bar-Or, Gábor Kupi, Tal Alexander
[abridged] Energy relaxation around a massive black hole (MBH) is key to establishing the dynamical state of galactic nuclei, and the nature of close stellar interactions with the…
Infrared spectroscopy of NGC4151: Probing the obscured ionizing AGN continuum
Tal Alexander, Eckhard Sturm, Dieter Lutz +3
The ISO-SWS infrared spectroscopic observations of the nucleus of Seyfert galaxy NGC4151, which are described in a companion paper, are used together with a compilation of UV to IR…
Bloated Stars as AGN Broad Line Clouds: The Emission Line Profiles
Tal Alexander, Hagai Netzer
The Bloated Stars Scenario proposes that AGN broad line emission originates in the winds or envelopes of bloated stars (BS). Alexander and Netzer (1994) established that ~ 5e4 BSs…
The statistical mechanics of relativistic orbits around a massive black hole
Ben Bar-Or, Tal Alexander
Stars around a massive black hole (MBH) move on nearly fixed Keplerian orbits, in a centrally-dominated potential. The random fluctuations of the discrete stellar background cause…
The effect of mass-segregation on gravitational wave sources near massive black holes
Clovis Hopman, Tal Alexander
Gravitational waves (GWs) from the inspiral of compact remnants (CRs) into massive black holes (MBHs) will be observable to cosmological distances. While a CR spirals in, 2-body sc…
Squeezars: Tidally powered stars orbiting a massive black hole
Tal Alexander, Mark Morris
We propose that there exists a class of transient sources, "squeezars", which are stars caught in highly eccentric orbits around a massive (m<10^8 Mo) black hole (MBH), whose atypi…
Orbital capture of stars by a massive black hole via exchanges with compact remnants
Tal Alexander, Mario Livio
We propose a dynamical mechanism for capturing stars around a massive black hole (MBH), which is based on the accumulation there of a very dense cluster of compact stellar remnants…
The Galactic Center as a laboratory for extreme mass ratio gravitational wave source dynamics
Tal Alexander
The massive Galactic black hole and the stars around it are a unique laboratory for studying how relaxation processes lead to close interactions of stars and compact remnants with…
Near-Infrared Microlensing of Stars by the Super-Massive Black Hole in the Galactic Center
Tal Alexander, Amiel Sternberg
We investigate microlensing amplification of faint stars in the dense stellar cluster in the Galactic Center (GC) by the super-massive black hole (BH). Such events would appear ver…
The torquing of circumnuclear accretion disks by stars and the evolution of massive black holes
Michal Bregman, Tal Alexander
An accreting massive black hole (MBH) in a galactic nucleus is surrounded by a dense stellar cluster. We analyze and simulate numerically the evolution of a thin accretion disk due…
EMRIs and the relativistic loss-cone: The curious case of the fortunate coincidence
Tal Alexander
Extreme mass ratio inspiral (EMRI) events are vulnerable to perturbations by the stellar background, which can abort them prematurely by deflecting EMRI orbits to plunging ones tha…
Resonant relaxation near a massive black hole: the stellar distribution and gravitational wave sources
Clovis Hopman, Tal Alexander
Resonant relaxation (RR) of orbital angular momenta occurs near massive black holes (MBHs) where the stellar orbits are nearly Keplerian and so do not precess significantly. The re…
Massive perturbers in the galactic center
Hagai B. Perets, Clovis Hopman, Tal Alexander
We investigate the role of massive perturbers, such as giant molecular clouds or stellar clusters, in supplying low-angular momentum stars that pass very close to the central massi…
Ultraluminous X-ray Sources as Intermediate Mass Black Holes Fed by Tidally Captured Stars
Clovis Hopman, Simon F. Portegies Zwart, Tal Alexander
The nature of ultraluminous X-ray sources (ULXs) is presently unknown. A possible explanation is that they are accreting intermediate mass black holes (IBHs) that are fed by Roche…
A universal minimal mass scale for present-day central black holes
Tal Alexander, Ben Bar-Or
Intermediate-mass black holes (IMBHs) of mass solar masses, , are the long-sought missing link between stellar black holes, born of…
Stars and singularities: Stellar phenomena near a massive black hole
Tal Alexander
This is a pedagogical review of recent results on the interactions of central massive black holes with stars very near them, focused on the black hole in the center of the Milky Wa…
Bloated Stars as AGN Broad Line Clouds: The Emission Lines Response to Continuum Variations
Tal Alexander
The `Bloated Stars Scenario' proposes that AGN broad line emission originates in the winds or envelopes of bloated stars (BS). Alexander and Netzer (1994, 1996) established that ~…
Probing Post-Newtonian Gravity near the Galactic Black Hole with Stellar Doppler Measurements
Shay Zucker, Tal Alexander, Stefan Gillessen +2
Stars closely approaching the massive black hole in the center of the Galaxy provide a unique opportunity to probe post-Newtonian physics in a yet unexplored regime of celestial me…
Stellar Relaxation Processes Near the Galactic Massive Black Hole
Tal Alexander
The massive black hole (MBH) in the Galactic Center and the stars around it form a unique stellar dynamics laboratory for studying how relaxation processes affect the distribution…
Stellar Dynamics and Stellar Phenomena Near A Massive Black Hole
Tal Alexander
Most galactic nuclei harbor a massive black hole (MBH), whose birth and evolution are closely linked to those of its host galaxy. The unique conditions near the MBH: high velocity…
Pinpointing the massive black hole in the Galactic Center with gravitationally lensed stars
Tal Alexander
A new statistical method for pinpointing the massive black hole (BH) in the Galactic Center on the IR grid is presented and applied to astrometric IR observations of stars close to…
Beaming Binaries - a New Observational Category of Photometric Binary Stars
Shay Zucker, Tsevi Mazeh, Tal Alexander
The new photometric space-borne survey missions CoRoT and Kepler will be able to detect minute flux variations in binary stars due to relativistic beaming caused by the line-of-sig…
Improved AGN light curve analysis with the z-transformed discrete correlation function
Tal Alexander
The cross-correlation function (CCF) is commonly employed in the study of AGN, where it is used to probe the structure of the broad line region by line reverberation, to study the…
The orbital statistics of stellar inspiral and relaxation near a massive black hole: characterizing gravitational wave sources
Clovis Hopman, Tal Alexander
We study the orbital parameters distribution of stars that are scattered into nearly radial orbits and then spiral into a massive black hole (MBH) due to dissipation, in particular…
Tidal spin-up of stars in dense stellar cusps around massive black holes
Tal Alexander, Pawan Kumar
We show that main-sequence stars in dense stellar cusps around massive black holes are likely to rotate at a significant fraction of the centrifugal breakup velocity due to spin-up…
Analytic study of mass segregation around a massive black hole
Uri Keshet, Clovis Hopman, Tal Alexander
We analyze the distribution of stars of arbitrary mass function xi(m) around a massive black hole (MBH). Unless xi is strongly dominated by light stars, the steady-state distributi…
Steady state relativistic stellar dynamics around a massive black hole
Ben Bar-Or, Tal Alexander
A massive black hole (MBH) consumes stars whose orbits evolve into the small phase-space volume of unstable orbits, the "loss-cone", which take them directly into the MBH, or close…
Rapid growth of seed black holes in the early universe by supra-exponential accretion
Tal Alexander, Priyamvada Natarajan
Mass accretion by black holes (BHs) is typically capped at the Eddington rate, when radiation's push balances gravity's pull. However, even exponential growth at the Eddington-limi…
The Signature of Microlensing in QSO Variability-Redshift Correlations
Tal Alexander
A recently discovered inverse correlation between QSO redshift and long-term continuum variability timescales was suggested to be the signature of microlensing on cosmological scal…
Probing Stellar Dynamics in Galactic Nuclei
M. Coleman Miller, Tal Alexander, Pau Amaro-Seoane +7
Electromagnetic observations over the last 15 years have yielded a growing appreciation for the importance of supermassive black holes (SMBH) to the evolution of galaxies, and for…
The Cosmic Baryon Fraction and the Extragalactic Ionizing Background
Lam Hui, Zoltan Haiman, Matias Zaldarriaga +1
We reassess constraints on the cosmological baryon density from observations of the mean decrement and power spectrum of the Lyman-alpha forest, taking into account uncertainties i…
Relativistic Stellar Dynamics Around a Massive Black Hole in Steady State
Tal Alexander
I briefly review advances in the understanding and modeling of relativistic stellar dynamics around massive black holes (MBHs) in galactic nuclei, following the inclusion of cohere…
Strong mass segregation around a massive black hole
Tal Alexander, Clovis Hopman
We show that the mass-segregation solution for the steady state distribution of stars around a massive black hole (MBH) has two branches: the known weak segregation solution (Bahca…
The distribution of stars near the super-massive black hole in the Galactic Center
Tal Alexander
We analyze three sets of infrared star counts in the inner ~0.5 pc of the Galactic Center. We perform statistical tests on the star counts and model in detail the extinction field…
Infrared spectroscopy of NGC 1068: Probing the obscured ionizing AGN continuum
Tal Alexander, Dieter Lutz, Eckhard Sturm +3
The ISO-SWS 2.5-45 um infrared spectroscopic observations of the nucleus of the Seyfert 2 galaxy NGC 1068 (see companion paper) are combined with a compilation of UV to IR narrow e…
Orbital inspiral into a massive black hole in a galactic center
Tal Alexander, Clovis Hopman
A massive black hole (MBH) in a galactic center drives a flow of stars into nearly radial orbits to replace those it destroyed. Stars whose orbits cross the event horizon r_s or th…
Intrinsic Absorption in the Spectrum of Mrk 279: Simultaneous Chandra, FUSE, and STIS Observations
Jennifer E. Scott, Gerard A. Kriss, Julia C. Lee +12
We present a study of the intrinsic X-ray and far-ultraviolet absorption in the Seyfert 1.5 galaxy Markarian 279 using simultaneous observations from the Chandra X-ray Observatory,…
Spectroscopic Binary Mass Determination using Relativity
Shay Zucker, Tal Alexander
High-precision radial-velocity techniques, which enabled the detection of extrasolar planets are now sensitive to relativistic effects in the data of spectroscopic binary stars (SB…
Massive perturber-driven interactions of stars with a massive black hole
Hagai B. Perets, Clovis Hopman, Tal Alexander
We study the role of massive perturbers (MPs) in deflecting stars and binaries to almost radial ("loss-cone") orbits, where they pass near the central massive black hole (MBH), int…
Dynamical evolution of the young stars in the Galactic center: N-body simulations of the S-stars
Hagai B. Perets, Alessia Gualandris, Gabor Kupi +2
We use N-body simulations to study the evolution of the orbital eccentricities of stars deposited near (<0.05 pc) the Milky Way massive black hole (MBH), starting from initial cond…
The Galactic potential and the asymmetric distribution of hypervelocity stars
Hagai B. Perets, Xufen Wu, HongSheng Zhao +3
In recent years several hypervelocity stars (HVSs) have been observed in the halo of our Galaxy. Such HVSs have possibly been ejected from the Galactic center and then propagated i…
The efficiency of resonant relaxation around a massive black hole
Ehud Eilon, Gábor Kupi, Tal Alexander
Resonant relaxation (RR) is a rapid relaxation process that operates in the nearly-Keplerian potential near a massive black hole (MBH). RR dominates the dynamics of compact remnant…
Key questions about Galactic Center dynamics
Tal Alexander
I discuss four key questions about Galactic Center dynamics, their implications for understanding both the environment of the Galactic MBH and galactic nuclei in general, and the p…
Dynamical evolution of the young stars in the Galactic center
Hagai. B. Perets, Alessia Gualandris, David Merritt +1
Recent observations of the Galactic center revealed a nuclear disk of young OB stars near the massive black hole (MBH), in addition to many similar outlying stars with higher eccen…
Stellar Processes Near the Massive Black Hole in the Galactic Center
Tal Alexander
A massive black hole resides in the center of most, perhaps all galaxies. The one in the center of our home galaxy, the Milky Way, provides a uniquely accessible laboratory for stu…
Tidal scattering of stars on supermassive black holes in galactic centers
Tal Alexander, Mario Livio
Some of the mass that feeds the growth of a massive black hole (BH) in a galactic center is supplied by tidal disruption of stars that approach it on unbound, low angular momentum…
Stellar Tidal Processes Near Massive Black Holes
Tal Alexander
Close tidal interactions of stars with a central massive black hole (MBH) or with other stars in the high density cusp around it can affect a significant fraction of the stellar po…
Getting a kick out of the stellar disk(s) in the galactic center
Hagai B. Perets, Gabor Kupi, Tal Alexander
Recent observations of the Galactic center revealed a nuclear disk of young OB stars, in addition to many similar outlying stars with higher eccentricities and/or high inclinations…
Bloated Stars as AGN Broad Line Clouds: The Emission Line Spectrum
Tal Alexander, Hagai Netzer
The Bloated Stars Scenario proposes that AGN broad line emission originates in the winds or envelopes of bloated stars (BS). Its main advantage over BLR cloud models is the gravita…
Accretion disk warping by resonant relaxation: The case of maser disk NGC4258
Michal Bregman, Tal Alexander
The maser disk around the massive black hole (MBH) in active galaxy NGC 4258 exhibits an O(10 deg) warp on the O(0.1 pc) scale. The physics driving the warp are still debated. Sugg…