arXiv subjects

Abraham Loeb

Publications and source records attributed to Abraham Loeb.

At least 595 records · Page 33Linked to original sources

arXiv2009

Astro2010 Decadal Survey Whitepaper: Coordinated Science in the Gravitational and Electromagnetic Skies

It is widely expected that the coming decade will witness the first direct detection of gravitational waves (GWs). The ground-based LIGO and Virgo GW observatories are being upgraded to advanced sensitivity, and are expected to observe a significant binary merger rate. The launch of The Laser Interferometer Space Antenna (LISA) would extend the GW window to low frequencies, opening new vistas on dynamical processes involving massive (M >~ 10^5 M_Sun) black holes. GW events are likely to be accompanied by electromagnetic (EM) counterparts and, since information carried electromagnetically is complementary to that carried gravitationally, a great deal can be learned about an event and its environment if it becomes possible to measure both forms of radiation in concert. Measurements of this kind will mark the dawn of trans-spectral astrophysics, bridging two distinct spectral bands of information. The aim of this whitepaper is to articulate future directions in both theory and observation that are likely to impact broad astrophysical inquiries of general interest. What will EM observations reflect on the nature and diversity of GW sources? Can GW sources be exploited as complementary probes of cosmology? What cross-facility coordination will expand the science returns of gravitational and electromagnetic observations?

astro-ph.CO↗

arXiv2008

Using Millimeter VLBI to Constrain RIAF Models of Sagittarius A*

The recent detection of Sagittarius A* at lambda = 1.3 mm on a baseline from Hawaii to Arizona demonstrates that millimeter wavelength very long baseline interferometry (VLBI) can now spatially resolve emission from the innermost accretion flow of the Galactic center region. Here, we investigate the ability of future millimeter VLBI arrays to constrain the spin and inclination of the putative black hole and the orientation of the accretion disk major axis within the context of radiatively inefficient accretion flow (RIAF) models. We examine the range of baseline visibility and closure amplitudes predicted by RIAF models to identify critical telescopes for determining the spin, inclination, and disk orientation of the Sgr A* black hole and accretion disk system. We find that baseline lengths near 3 gigalambda have the greatest power to distinguish amongst RIAF model parameters, and that it will be important to include new telescopes that will form north-south baselines with a range of lengths. If a RIAF model describes the emission from Sgr A*, it is likely that the orientation of the accretion disk can be determined with the addition of a Chilean telescope to the array. Some likely disk orientations predict detectable fluxes on baselines between the continental United States and even a single 10-12 m dish in Chile. The extra information provided from closure amplitudes by a four-antenna array enhances the ability of VLBI to discriminate amongst model parameters.

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arXiv2008

Long-Term Evolution in Transit Duration of Extrasolar Planets from Magnetic Activity in their Parent Stars

Existing upper limits on variations in the photospheric radius of the Sun during the solar magnetic activity cycle are at a fractional amplitude of 2x10^{-4}. At that level, the transit duration of a close-in planet around a Sun-like star could change by a fraction of a second per year. This magnitude of variation is larger than that caused by other studied effects (owing to proper motion or general-relativistic effects), and should be included in the analysis of constraints on multi-planet systems from transit timing. A temporal correlation between the transit duration and spectroscopic measures of stellar activity can be used to separate the stellar radius change from other effects. The magnetic activity effect could be significantly larger for late-type stars, such as M-dwarfs, which are more variable than the Sun. In general, precision transit measurements provide a new tool for measuring long-term variations of stellar radii.

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arXiv2008

Imprint of Inhomogeneous Hydrogen Reionization on the Temperature Distribution of the Intergalactic Medium

We study the impact of inhomogeneous hydrogen reionization on the thermal evolution of the intergalactic medium (IGM) using hydrodynamic + radiative transfer simulations where reionization is completed either early (z ~ 9) or late (z ~ 6). In general, we find that low-density gas near large-scale overdensities is ionized and heated earlier than gas in the large-scale, underdense voids. Furthermore, at a later time the IGM temperature is inversely related to the reionization redshift because gas that is heated earlier has more time to cool through adiabatic expansion and Compton scattering. Thus, at the end of reionization the median temperature-density relation is an inverted power-law with slope gamma-1 ~ -0.2, in both models. However, at fixed density, there is up to order unity scatter in the temperature due to the distribution of reionization redshifts. Because of the complex equation-of-state, the evolved IGM temperature-density relations for the redshift range 4 < z < 6 can still have significant curvature and scatter. These features must be taken into account when interpreting the Lyman alpha absorption in high redshift quasar spectra.

astro-ph↗

arXiv2008

Evidence for Merger-Driven Activity in the Clustering of High Redshift Quasars

Recently, a very large clustering length has been measured for quasars at a redshift of z~4. In combination with the observed quasar luminosity function we assess the implications of this clustering for the relationship between quasar luminosity and dark matter halo mass. Our analysis allows for non-linearity and finite scatter in the relation between quasar luminosity and halo mass, as well as a luminosity dependent quasar lifetime. The additional novel ingredient in our modelling is the allowance for an excess in the observed bias over the underlying halo bias owing to the merger driven nature of quasar activity. We find that the observations of clustering and luminosity function can be explained only if both of the following conditions hold: (i) The luminosity to halo mass ratio increases with halo mass; (ii) The observed clustering amplitude is in excess of that expected solely from halo bias. The latter result is statistically significant at the 99% level. Taken together, the observations provide compelling evidence for merger driven quasar activity, with a black-hole growth that is limited by feedback. In difference from previous analyses, we show that there could be scatter in the luminosity halo mass relation of up to 1 dex, and that quasar clustering can not be used to estimate the quasar lifetime.

astro-ph↗

arXiv2008

Implications of Recent Measurements of the Milky Way Rotation for the Orbit of the Large Magellanic Cloud

We examine the implications of recent measurements of the Milky Way rotation for the trajectory of the Large Magellanic Cloud. The ~ 14+/-6% increase in the MW circular velocity relative to the IAU standard of 220 km/s changes the qualitative nature of the inferred LMC orbit. Instead of the LMC being gravitationally unbound, as has been implied based on a recent measurement of its proper motion, we find that the past orbit of the LMC is naturally confined within the virial boundary of the MW, although still not as tightly bound as in previous estimates.

astro-ph↗

arXiv2008

Constraints on the Local Sources of Ultra High-Energy Cosmic Rays

Ultra high-energy cosmic rays (UHECRs) are believed to be protons accelerated in magnetized plasma outflows of extra-Galactic sources. The acceleration of protons to ~10^{20} eV requires a source power L>10^{47} erg/s. The absence of steady sources of sufficient power within the GZK horizon of 100 Mpc, implies that UHECR sources are transient. We show that UHECR "flares" should be accompanied by strong X-ray and gamma-ray emission, and that X-ray and gamma-ray surveys constrain flares which last less than a decade to satisfy at least one of the following conditions: (i) L>10^{50} erg/s; (ii) the power carried by accelerated electrons is lower by a factor >10^2 than the power carried by magnetic fields or by >10^3 than the power in accelerated protons; or (iii) the sources exist only at low redshifts, z<<1. The implausibility of requirements (ii) and (iii) argue in favor of transient sources with L>10^{50} erg/s.

astro-ph↗

arXiv2008

Methods for detecting flaring structures in Sagittarius A* with high frequency VLBI

The super massive black hole candidate, Sagittarius A*, exhibits variability from radio to X-ray wavelengths on time scales that correspond to < 10 Schwarzschild radii. We survey the potential of millimeter-wavelength VLBI to detect and constrain time variable structures that could give rise to such variations, focusing on a model in which an orbiting hot spot is embedded in an accretion disk. Non-imaging algorithms are developed that use interferometric closure quantities to test for periodicity, and applied to an ensemble of hot-spot models that sample a range of parameter space. We find that structural periodicity in a wide range of cases can be detected on most potential VLBI arrays using modern VLBI instrumentation. Future enhancements of mm/sub-mm VLBI arrays including phased array processors to aggregate VLBI station collecting area, increased bandwidth recording, and addition of new VLBI sites all significantly aid periodicity detection. The methods described herein can be applied to other models of Sagittarius A*, including jet outflows and Magneto-Hydrodynamic accretion simulations.

astro-ph↗

arXiv2008

Lyman Alpha Driven Outflows Around Star Forming Galaxies

We present accurate Monte-Carlo calculations of Lyman Alpha (hereafter Lya) radiation pressure in a range of models which represent galaxies during various epochs of our Universe. We show that the radiation force that Lya photons exert on hydrogen gas in the neutral intergalactic medium (IGM), that surrounds minihalos that host the first stars, may exceed gravity by orders of magnitude and drive supersonic winds. Lya radiation pressure may also dominate over gravity in the neutral IGM that surrounds the HII regions produced by the first galaxies. However, the radiation force is likely too weak to result in supersonic outflows in this case. Furthermore, we show that Lya radiation pressure may drive outflows in the interstellar medium of star forming galaxies that reach hundreds of km/s. This mechanism could also operate at lower redshifts z<6, and may have already been indirectly detected in the spectral line shape of observed Lya emission lines.

astro-ph↗

arXiv2008

Effects of gravitational-wave recoil on the dynamics and growth of supermassive black holes

Simulations of binary black hole mergers indicate that asymmetrical gravitational wave (GW) emission can cause black holes to recoil at speeds up to thousands of km/s. These GW recoil events can dramatically affect the coevolution of recoiling supermassive black holes (SMBHs) and their host galaxies. However, theoretical studies of SMBH-galaxy evolution almost always assume a stationary central black hole. In light of the numerical results on GW recoil velocities, we relax that assumption here and consider the consequences of recoil for SMBH evolution. We follow the trajectories of SMBHs ejected in a smooth background potential that includes both a stellar bulge and a multi-component gaseous disk. In addition, we calculate the accretion rate onto the SMBH as a function of time using a hybrid prescription of viscous (alpha-disk) and Bondi accretion. We find that recoil kicks between 100 km/s and the escape speed cause SMBHs to wander through the galaxy and halo for about 1 Myr - 1 Gyr before settling back to the galactic center. However, the mass accreted during this time is roughly constant at about 10% of the initial mass, independent of the recoil velocity. This indicates that while large recoils may disrupt active galactic nuclei feedback processes, recoil itself is an effective means of regulating SMBH growth. Recoiling SMBHs may be observable as spatially or kinematically offset quasars, but finding such systems could be challenging, because the largest offsets correspond to the shortest quasar lifetimes.

astro-ph↗

arXiv2008

Imprint of Distortions in the Oort Cloud on the CMB Anisotropies

We study the effect of a close encounter of a passing star on the shape of the inner Oort Cloud, using the impulse approximation. The deviation of the perturbed Oort Cloud from sphericity adds angular fluctuations to the brightness of the Cosmic Microwave Background (CMB) due to thermal emission by the comets. An encounter with a solar-mass star at an impact parameter of $1750 \au$, as expected based on the abundance and velocity dispersion of stars in the solar neighborhood, leads to a quadrupole moment in the square of the fractional CMB intensity fluctuation of $C_2 = 4.5 \times 10^{-15}, 6.7 \times 10^{-12}, 1.1 \times 10^{-9}$ at $ν= 30, 353, 545 \GHz$ (these being the frequency bands of the upcoming Planck satellite). We also quantify the quadrupole spectral distortions produced by the Scattered Disc, which will exist regardless of any perturbation and the subsequent shape of the Oort Cloud. For comparison, the square fractional temperature fluctuation quadrupole moment predicted by the current cosmological model is $C_2 = 1.76 \times 10^{-10}$, which corresponds to fluctuations in the CMB intensity of $C_2 = 2.9 \times 10^{-10}, 6.8 \times 10^{-9}, 1.6 \times 10^{-8}$ at $ν= 30, 353, 545 \GHz$. Finally, we discuss how a measurement of the anisotropic spectral distortions could be used to constrain the trajectory of the closest stellar fly-by.

astro-ph↗

arXiv2008

The Polarization of Scattered Lyman Alpha Radiation Around High-Redshift Galaxies

The high-redshift Universe contains luminous Lyman Alpha (hereafter Lya) emitting sources such as galaxies and quasars. The emitted Lya radiation is often scattered by surrounding neutral hydrogen atoms. We show that the scattered Lya radiation obtains a high level of polarization for a wide range of likely environments of high-redshift galaxies. For example, the back-scattered Lya flux observed from galaxies surrounded by a superwind-driven outflow may reach a fractional polarization as high as ~40%. Equal levels of polarization may be observed from neutral collapsing protogalaxies. Resonant scattering in the diffuse intergalactic medium typically results in a lower polarization amplitude (<7%), which depends on the flux of the ionizing background. Spectral polarimetry can differentiate between Lya scattering off infalling gas and outflowing gas; for an outflow the polarization should increase towards longer wavelengths while for infall the opposite is true. Our numerical results suggest that Lya polarimetry is feasible with existing instruments, and may provide a new diagnostic of the distribution and kinematics of neutral hydrogen around high-redshift galaxies. Moreover, polarimetry may help suppress infrared lines originating in the Earth's atmosphere, and thus improve the sensitivity of ground-based observations to high-redshift Lya emitting galaxies outside the currently available redshift windows.

astro-ph↗

arXiv2008

Detecting Flaring Structures in Sagittarius A* with (Sub)Millimeter VLBI

Multiwavelength monitoring observations of Sagittarius A* exhibit variability on timescales of minutes to hours, indicating emission regions localized near the event horizon. (Sub)Millimeter-wavelength VLBI is uniquely suited to probe the environment of the assumed black hole on these scales. We consider a range of orbiting hot-spot and accretion-disk models and find that periodicity in Sgr A* flares is detectable using closure quantities. Our methods are applicable to any model producing source structure changes near the black hole, including jets and magnetohydrodynamic disk instabilities, and suggest that (sub)millimeter VLBI will play a prominent role in investigating Sgr A* near the event horizon.

astro-ph↗

arXiv2008

Verifying the Identity of High-Redshift Massive Galaxies Through the Clustering of Lower Mass Galaxies Around Them

Massive high-redshift galaxies form in over-dense regions where the probability of forming other galaxies is also strongly enhanced. Given an observed flux of a galaxy, the inferred mass of its host halo tends to be larger as its inferred redshift increases. As the mass and redshift of a galaxy halo increase, the expected clustering of other galaxies around it gets stronger. It is therefore possible to verify the high-redshift identity of a galaxy (prior to an unambiguous spectral identification) from the clustering of other galaxies around it. We illustrate this method for the massive galaxy suggested by Mobasher et al. (2005) to be at redshift z~6.5. If this galaxy were to exist at z~6.5, there should have been a mean of ~10 galaxies larger than a hundredth of its mass and having z-band magnitudes less than ~25 detected as i-dropouts in the HUDF. We calculate an approximate probability distribution for neighbor galaxies and determine that there is less than a ~0.3% chance of detecting no massive neighbor galaxies. The lack of other massive z~6.5 galaxies in the HUDF image argues that the Mobasher et al. (2005) galaxy is instead a low redshift interloper. We generalize our results to other galaxy masses and redshifts.

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arXiv2008

Brightening of an Accretion Disk Due to Viscous Dissipation of Gravitational Waves During the Coalescence of Supermassive Black Holes

Mergers of supermassive black hole binaries release peak power of up to ~10^57 erg/s in gravitational waves (GWs). As the GWs propagate through ambient gas, they induce shear and a small fraction of their power is dissipated through viscosity. The dissipated heat appears as electromagnetic (EM) radiation, providing a prompt EM counterpart to the GW signal. For thin accretion disks, the GW heating rate exceeds the accretion power at distances farther than 10^3 Schwarzschild radii, independently of the accretion rate and viscosity coefficient.

astro-ph↗

arXiv2008

The Density Contrast of the Shapley Supercluster

We calculate the density contrast of the Shapley Supercluster (SSC) based on the enhanced abundance of X-ray clusters in it using the extended Press-Schechter formalism. We derive a total SSC mass of M_tot=(4.4+-0.44)x10^{16} M_sun within a sphere of 50 Mpc centered at a distance of about 160 Mpc. The nonlinear fractional density contrast of the sphere is (1+delta)=1.76+-0.17 relative to the mean matter density in the Universe, but the contrast increases in the interior of the SSC. Including the cosmological constant, the SSC region is found to be gravitationally unbound. The SSC contributes only a minor portion (9.0%+-2.1%) of the peculiar velocity of the local group.

astro-ph↗

arXiv2008

Let there be Light: the Emergence of Structure out of the Dark Ages in the Early Universe

The initial conditions of our Universe can be summarized on a single sheet of paper. Yet the Universe is full of complex structures today, such as stars, galaxies and groups of galaxies. In this review I describe the standard theoretical model for how complexity emerged from the simple initial state of the Universe at early cosmic times through the action of gravity. In order to test and inform the related theoretical calculations, large-aperture telescopes and arrays of radio antennae are currently being designed and constructed. The actual transition from simplicity to complexity has not been observed as of yet. The simple initial conditions were already traced in maps of the microwave background radiation, but the challenge of detecting the first generation of galaxies defines one of the exciting frontiers in the future of cosmology. Once at hand, the missing images of the infant Universe might potentially surprise us and revise our current ideas.

astro-ph↗

arXiv2008

Limits on the Position Wander of Sgr A*

We present measurements with the VLBA of the variability in the centroid position of Sgr A* relative to a background quasar at 7-mm wavelength. We find an average centroid wander of 71 +/- 45 micro-arcsec for time scales between 50 and 100 min and 113 +/- 50 micro-arcsec for timescales between 100 and 200 min, with no secular trend. These are sufficient to begin constraining the viability of the hot-spot model for the radio variability of Sgr A*. It is possible to rule out hot spots with orbital radii above 15GM_SgrA*/c^2 that contribute more than 30% of the total 7-mm flux. However, closer or less luminous hot spots remain unconstrained. Since the fractional variability of Sgr A* during our observations was ~20% on time scales of hours, the hot-spot model for Sgr A*'s radio variability remains consistent with these limits. Improved monitoring of Sgr A*'s centroid position has the potential to place significant constraints upon the existence and morphology of inhomogeneities in a supermassive black hole accretion flow.

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