arXiv subjects

Abraham Loeb

Publications and source records attributed to Abraham Loeb.

At least 433 records · Page 24Linked to original sources

arXiv2015

Implications of the Eccentric Kozai-Lidov Mechanism for Stars Surrounding Supermassive Black Hole Binaries

An enhanced rate of stellar tidal disruption events (TDEs) may be an important characteristic of supermassive black hole (SMBH) binaries at close separations. Here we study the evolution of the distribution of stars around a SMBH binary due to the eccentric Kozai-Lidov (EKL) mechanism, including octupole effects and apsidal precession caused by the stellar mass distribution and general relativity. We identify a region around one of the SMBHs in the binary where the EKL mechanism drives stars to high eccentricities, which ultimately causes the stars to either scatter off the second SMBH or get disrupted. For SMBH masses 10^7 Msun and 10^8 Msun, the TDE rate can reach 10^{-2} yr and deplete a region of the stellar cusp around the secondary SMBH in ~0.5 Myr. As a result, the final geometry of the stellar distribution between 0.01 and 0.1 pc around the secondary SMBH is a torus. These effects may be even more prominent in nuclear stellar clusters hosting a supermassive and an intermediate mass black hole.

astro-ph.GA↗

arXiv2015

Water Formation During the Epoch of First Metal Enrichment

We demonstrate that high abundances of water vapor could have existed in extremely low metallicity ($10^{-3}$ solar) partially shielded gas, during the epoch of first metal enrichment of the interstellar medium of galaxies at high redshifts.

astro-ph.GA↗

arXiv2015

Probing the Spacetime Around Supermassive Black Holes with Ejected Plasma Blobs

Millimeter-wavelength VLBI observations of the supermassive black holes in Sgr A* and M87 by the Event Horizon Telescope could potentially trace the dynamics of ejected plasma blobs in real time. We demonstrate that the trajectory and tidal stretching of these blobs can be used to test general relativity and set new constraints on the mass and spin of these black holes.

astro-ph.IM↗

arXiv2015

How to collect matches that will catch fire

How can we select a cohort of promising astrophysicists before they have made their discoveries? This is the fundamental challenge of academic planning. I argue that science can only blossom if young researchers are rewarded for acquired skills and growth rather than inherited academic ancestry.

astro-ph.IM↗

arXiv2015

Using Atomic Clocks to Detect Gravitational Waves

Atomic clocks have recently reached a fractional timing precision of $<10^{-18}$. We point out that an array of atomic clocks, distributed along the Earth's orbit around the Sun, will have the sensitivity needed to detect the time dilation effect of mHz gravitational waves (GWs), such as those emitted by supermassive black hole binaries at cosmological distances. Simultaneous measurement of clock-rates at different phases of a passing GW provides an attractive alternative to the interferometric detection of temporal variations in distance between test masses separated by less than a GW wavelength, currently envisioned for the eLISA mission.

astro-ph.IM↗

arXiv2014

A Simple Physical Model for the Gas Distribution in Galaxy Clusters

The dominant baryonic component of galaxy clusters is hot gas whose distribution is commonly probed through X-ray emission arising from thermal bremsstrahlung. The density profile thus obtained has been traditionally modeled with a beta-profile, a simple function with only three parameters. However, this model is known to be insufficient for characterizing the range of cluster gas distributions, and attempts to rectify this shortcoming typically introduce additional parameters to increase the fitting flexibility. We use cosmological and physical considerations to obtain a family of profiles for the gas with fewer parameters than the beta-model but which better accounts for observed gas profiles over wide radial intervals.

astro-ph.CO↗

arXiv2014

New insights from deep VLA data on the potentially recoiling black hole CID-42 in the COSMOS field

We present deep 3 GHz VLA observations of the potentially recoiling black hole CID-42 in the COSMOS field. This galaxy shows two optical nuclei in the HST/ACS image and a large velocity offset of ~ 1300 km/s between the broad and narrow H beta emission line although the spectrum is not spacially resolved (Civano et al. 2010). The new 3 GHz VLA data has a bandwidth of 2 GHz and to correctly interpret the flux densities imaging was done with two different methods: multi-scale multi-frequency synthesis and spectral windows stacking. The final resolutions and sensitivities of these maps are 0.7" with rms = 4.6 muJy/beam and 0.9" with rms = 4.8 muJy/beam respectively. With a 7 sigma detection we find that the entire observed 3 GHz radio emission can be associated with the South-Eastern component of CID-42, coincident with the detected X-ray emission. We use our 3 GHz data combined with other radio data from the literature ranging from 320 MHz to 9 GHz, which include the VLA, VLBA and GMRT data, to construct a radio synchrotron spectrum of CID-42. The radio spectrum suggests a type I unobscured radio-quiet flat-spectrum AGN in the South-Eastern component which may be surrounded by a more extended region of old synchrotron electron population or shocks generated by the outflow from the supermassive black hole. Our data are consistent with the recoiling black hole picture but cannot rule out the presence of an obscured and radio-quiet SMBH in the North-Western component.

astro-ph.GA↗

arXiv2014

Effects of Intermediate Mass Black Holes on Nuclear Star Clusters

Nuclear star clusters (NSCs) are dense stellar clusters observed in galactic nuclei, typically hosting a central massive black hole. Here we study the possible formation and evolution of NSCs through the inspiral of multiple star clusters hosting intermediate mass black holes (IMBHs). Using an N-body code we examine the dynamics of the IMBHs and their effects on the NSC. We find that IMBHs inspiral to the core of the newly formed NSC and segregate there. Although the IMBHs scatter each other and the stars, none of them is ejected from the NSC. The IMBHs are excited to high eccentricities and their radial density profile develops a steep power-law cusp. The stars also develop a power-law cusp (instead of the central core that forms in their absence), but with a shallower slope. The relaxation rate of the NSC is accelerated due to the presence of IMBHs, which act as massive-perturbers. This in turn fills the loss-cone and boosts the tidal disruption rate of stars both by the MBH and the IMBHs to a value excluded by rate estimates based on current observations. Rate estimates of tidal disruptions can therefore provide a cumulative constraint on the existence of IMBHs in NSCs.

astro-ph.GA↗

arXiv2014

A two-component jet model for the tidal disruption event Swift J164449.3+573451

We analyze both the early and late time radio and X-ray data of the tidal disruption event Swift J1644+57. The data at early times ($\lesssim 5$ days) necessitates separation of the radio and X-ray emission regions, either spatially or in velocity space. This leads us to suggest a two component jet model, in which the inner jet is initially relativistic with Lorentz factor $Γ\approx 15$, while the outer jet is trans-relativistic, with $Γ\lesssim 1.2$. This model enables a self-consistent interpretation of the late time radio data, both in terms of peak frequency and flux. We solve the dynamics, radiative cooling and expected radiation from both jet components. We show that while during the first month synchrotron emission from the outer jet dominates the radio emission, at later times radiation from ambient gas collected by the inner jet dominates. This provides a natural explanation to the observed re-brightening, without the need for late time inner engine activity. After 100 days, the radio emission peak is in the optically thick regime, leading to a decay of both the flux and peak frequency at later times. Our model's predictions for the evolution of radio emission in jetted tidal disruption events can be tested by future observations.

astro-ph.HE↗

arXiv2014

Event-Horizon-Telescope Evidence for Alignment of the Black Hole in the Center of the Milky Way with the Inner Stellar Disk

Observations of the black hole in the center of the Milky Way with the Event Horizon Telescope at 1.3 mm have revealed a size of the emitting region that is smaller than the size of the black-hole shadow. This can be reconciled with the spectral properties of the source, if the accretion flow is seen at a relatively high inclination (50-60 degrees). Such an inclination makes the angular momentum of the flow, and perhaps of the black hole, nearly aligned with the angular momenta of the orbits of stars that lie within 3 arcsec from the black hole. We discuss the implications of such an alignment for the properties of the black hole and of its accretion flow. We argue that future Event-Horizon-Telescope observations will not only refine the inclination of Sgr A* but also measure precisely its orientation on the plane of the sky.

astro-ph.HE↗

arXiv2014

A size-duration trend for gamma-ray burst progenitors

Gamma-ray bursts (GRBs) show a bimodal distribution of durations, separated at a duration of ~2 s. Observations have confirmed the association of long GRBs with the collapse of massive stars. The origin of short GRBs is still being explored. We examine constraints on the emission region size in short and long GRBs detected by Fermi/GBM. We find that the emission region size during the prompt emission, R, and the burst duration, T$_{90}$, are consistent with the relation R ~ c x T$_{90}$, for both long and short GRBs. We find the characteristic size for the prompt emission region to be ~2 x 10$^{10}$ cm, and ~4 x 10$^{11}$ cm for short and long GRBs, respectively.

astro-ph.HE↗

arXiv2014

Finding rocky asteroids around white dwarfs by their periodic thermal emission

Since white dwarfs are small, the contrast between the thermal emission of an orbiting object and a white dwarf is dramatically enhanced compared to a main sequence host. Furthermore, rocky objects much smaller than the moon have no atmospheres and are tidally locked to the white dwarf. We show that this leads to temperature contrasts between their day and night side of order unity that should lead to temporal variations in infrared flux over an orbital period of $\sim 0.2$ to $\sim 2$ days. Ground based telescopes could detect objects with a mass as small as $1\%$ of the lunar mass $M_L$ around Sirius B with a few hours of exposure. The James Webb Space Telescope (JWST) may be able to detect objects as small as $10^{-3} M_L$ around most nearby white dwarfs. The tightest constraints will typically be placed on 12,000 K white dwarfs, whose Roche zone coincides with the dust sublimation zone. Constraining the abundance of minor planets around white dwarfs as a function of their surface temperatures (and therefore age) provides a novel probe for the physics of planetary formation.

astro-ph.EP↗

arXiv2014

Detecting floating black holes as they traverse the gas disk of the Milky Way

A population of intermediate-mass black holes (BHs) is predicted to be freely floating in the Milky Way (MW) halo, due to gravitational wave recoil, ejection from triple BH systems, or tidal stripping in the dwarf galaxies that merged to make the MW. As these BHs traverse the gaseous MW disk, a bow shock forms, producing detectable radio and mm/sub-mm synchrotron emission from accelerated electrons. We calculate the synchrotron flux to be $\sim \rm 0.01-10\, mJy$ at GHz frequency, detectable by Jansky Very Large Array, and $\sim 10-100\,μ\rm Jy$ at $\sim10^{10}-10^{12} \,\rm Hz$ frequencies, detectable by Atacama Large Millimeter/sub-millimter Array. The discovery of the floating BH population will provide insights on the formation and merger history of the MW as well as on the evolution of massive BHs in the early Universe.

astro-ph.GA↗

arXiv2014

Signatures of the M31-M32 Galactic Collision

The unusual morphologies of the Andromeda spiral galaxy (M31) and its dwarf companion M32 have been characterized observationally in great detail. The two galaxies' apparent proximity suggests that Andromeda's prominent star-forming ring as well as M32's compact elliptical structure may result from a recent collision. Here we present the first self-consistent model of the M31-M32 interaction that simultaneously reproduces observed positions, velocities, and morphologies for both galaxies. Andromeda's spiral structure is resolved in unprecedented detail, showing that a rare head-on orbit is not necessary to match Andromeda's ring-like morphology. The passage of M32 through Andromeda's disk perturbs the disk velocity structure. We find tidal stripping of M32's stars to be inefficient during the interaction, suggesting that some cEs are intrinsically compact. Additionally, the orbital solution implies that M32 is currently closer to the Milky Way than models have typically assumed, a prediction that may be testable with upcoming observations.

astro-ph.GA↗

arXiv2014

Pulsar Timing Constraints on Cumulative and Individual Mass of Stars in the Galactic Center

We consider the time derivatives of the period $P$ of pulsars at the Galactic Center due to variations in their orbital Doppler shifts. We show that in conjunction with a measurement of a pulsar's proper motion and its projected separation from the supermassive black hole, Sgr A*,measuring two of the three derivatives $\dot{P}$, $\ddot{P}$, or $\dddot{P}$ sets a constraint that allows for the recovery of the complete six phase space coordinates of the pulsar's orbit, as well as the enclosed mass within the orbit. Thus, one can use multiple pulsars at different distances from Sgr A* to determine the radial mass distribution of stars and stellar remnants at the Galactic center. Furthermore, we consider the effect of passing stars on the pulsar's period derivatives and show how it can be exploited to measure the characteristic stellar mass in the Galactic Center.

astro-ph.IM↗

arXiv2014

Reionization on Large Scales IV: Predictions for the 21 cm signal incorporating the light cone effect

We present predictions for the 21 cm brightness temperature power spectrum during the Epoch of Reionization (EoR). We discuss the implications of the "light cone" effect, which incorporates evolution of the neutral hydrogen fraction and 21 cm brightness temperature along the line of sight. Using a novel method calibrated against radiation-hydrodynamic simulations, we model the neutral hydrogen density field and 21 cm signal in large volumes ($L = 2$ Gpc/$h$). The inclusion of the light cone effect leads to a relative decrease of about 50% in the 21 cm power spectrum on all scales. We also find that the effect is more prominent at the midpoint of reionization and later. The light cone effect also can introduce an anisotropy along the line of sight. By decomposing the 3D power spectrum into components perpendicular to and along the line of sight, we find that in our fiducial reionization model, there is no significant anisotropy. However, parallel modes can contribute up to 40% more power for shorter reionization scenarios. The scales on which the light cone effect is relevant are comparable to scales where one measures the baryon acoustic oscillation. We argue that due to its large comoving scale and introduction of anisotropy, the light cone effect is important when considering redshift space distortions and future application to the Alcock-Paczynski test for the determination of cosmological parameters.

astro-ph.CO↗

arXiv2014

Detecting industrial pollution in the atmospheres of earth-like exoplanets

Detecting biosignatures, such as molecular oxygen in combination with a reducing gas, in the atmospheres of transiting exoplanets has been a major focus in the search for alien life. We point out that in addition to these generic indicators, anthropogenic pollution could be used as a novel biosignature for intelligent life. To this end, we identify pollutants in the Earth's atmosphere that have significant absorption features in the spectral range covered by the James Webb Space Telescope (JWST). We focus on tetrafluoromethane CF4 and trichlorofluoromethane (CCl3F), which are the easiest to detect chlorofluorocarbons (CFCs) produced by anthropogenic activity. We estimate that ~1.2 days (~1.7 days) of total integration time will be sufficient to detect or constrain the concentration of CCl3F (CF4) to ~10 times current terrestrial level.

astro-ph.EP↗

arXiv2014

Chaos in the Test Particle Eccentric Kozai-Lidov Mechanism

The Kozai-Lidov mechanism can be applied to a vast variety of astrophysical systems involving hierarchical three-body systems. Here, we study the Kozai-Lidov mechanism systematically in the test particle limit at the octupole level of approximation. We investigate the chaotic and quasiperiodic orbital evolution by studying surfaces of section and the Lyapunov exponents. We find that the resonances introduced by the octupole level of approximation cause orbits to flip from prograde to retrograde and back as well as cause significant eccentricity excitation, and the chaotic behaviors occur when the mutual inclination between the inner and the outer binary is high. We characterize the parameter space that allows large amplitude oscillations in eccentricity and inclination.

astro-ph.EP↗