arXiv subjects

Abraham Loeb

Publications and source records attributed to Abraham Loeb.

At least 379 records · Page 21Linked to original sources

arXiv2016

The interaction of relativistic spacecrafts with the interstellar medium

The Breakthrough Starshot initiative aims to launch a gram-scale spacecraft to a speed of $v\sim 0.2$c, capable of reaching the nearest star system, $α$ Centauri, in about 20 years. However, a critical challenge for the initiative is the damage to the spacecraft by interstellar gas and dust during the journey. In this paper, we quantify the interaction of a relativistic spacecraft with gas and dust in the interstellar medium. For gas bombardment, we find that damage by track formation due to heavy elements is an important effect. We find that gas bombardment can potentially damage the surface of the spacecraft to a depth of $\sim 0.1$ mm for quartz material after traversing a gas column of $N_{\rm H}\sim 2\times 10^{18}\rm cm^{-2}$ along the path to $α$ Centauri, whereas the effect is much weaker for graphite material. The effect of dust bombardment erodes the spacecraft surface and produces numerous craters due to explosive evaporation of surface atoms. For a spacecraft speed $v=0.2c$, we find that dust bombardment can erode a surface layer of $\sim 0.5$ mm thickness after the spacecraft has swept a column density of $N_{\rm H}\sim 3\times 10^{17}\rm cm^{-2}$, assuming the standard gas-to-dust ratio of the interstellar medium. Dust bombardment also damages the spacecraft surface by modifying the material structure through melting. We calculate the equilibrium surface temperature due to collisional heating by gas atoms as well as the temperature profile as a function of depth into the spacecraft. Our quantitative results suggest methods for damage control, and we highlight possibilities for shielding strategies and protection of the spacecraft.

astro-ph.GA↗

arXiv2016

Interferometric Measurement of Acceleration at Relativistic Speeds

We show that an interferometer moving at a relativistic speed relative to a point source of light offers a sensitive probe of acceleration. Such an accelerometer contains no moving parts, and is thus more robust than conventional "mass-on-a-spring" accelerometers. In an interstellar mission to Alpha-Centauri, such an accelerometer could be used to measure the masses of planets around other stars as well as the mass distribution of the Milky Way Galaxy.

astro-ph.IM↗

arXiv2016

Optical-infrared flares and radio afterglows by Jovian planets inspiraling into their host stars

When a planet inspirals into its host star, it releases gravitational energy which is converted into an expanding bubble of hot plasma. We study the radiation from the bubble and show that it includes prompt optical-infrared emission and a subsequent radio afterglow. The prompt emission from M31 and Large Magellanic Cloud is detectable by optical-near infrared transient surveys with a large field of view. The subsequent radio afterglows are detectable for $10^{3-4}$~years. The event rate depends on uncertain parameters in the formation and dynamics of giant planets. Future observation of the rate will constrain related theoretical models. If the event rate is high (> a few events per year), the circumstellar disk must typically be massive as suggested by recent numerical simulations.

astro-ph.EP↗

arXiv2016

Ultra high energy cosmic rays from non-relativistic quasar outflows

It has been suggested that non-relativistic outflows from quasars can naturally account for the missing component of the extragalactic $γ$-ray background and explain the cumulative neutrino background through pion decay in collisions between protons accelerated by the outflow shock and interstellar protons. Here we show that the same quasar outflows are capable of accelerating protons to energies of $\sim 10^{20}$ eV during the early phase of their propagation. The overall quasar population is expected to produce a cumulative ultra high energy cosmic ray flux of $\sim10^{-7}\,\rm GeV\,cm^{-2}s^{-1}sr^{-1}$ at $E_{\rm CR}\gtrsim10^{18}$ eV. The spectral shape and amplitude is consistent with recent observations for outflow parameters constrained to fit secondary $γ$-rays and neutrinos without any additional parameter tuning. This indicates that quasar outflows simultaneously account for all three messengers at their observed levels.

astro-ph.HE↗

arXiv2016

On the Habitability of Our Universe

Is life most likely to emerge at the present cosmic time near a star like the Sun? We consider the habitability of the Universe throughout cosmic history, and conservatively restrict our attention to the context of "life as we know it" and the standard cosmological model, LCDM. The habitable cosmic epoch started shortly after the first stars formed, about 30 Myr after the Big Bang, and will end about 10 Tyr from now, when all stars will die. We review the formation history of habitable planets and find that unless habitability around low mass stars is suppressed, life is most likely to exist near 0.1 solar mass stars ten trillion years from now. Spectroscopic searches for biosignatures in the atmospheres of transiting Earth-mass planets around low mass stars will determine whether present-day life is indeed premature or typical from a cosmic perspective.

astro-ph.CO↗

arXiv2016

Constraining Milky Way mass with Hypervelocity Stars

Context. Although a variety of techniques have been employed for determining the Milky Way dark matter halo mass distribution, the range of allowed masses spans both light and heavy values. Knowing the precise mass of our Galaxy is important for placing the Milky Way in a cosmological $Λ$CDM context. Aims. We show that hypervelocity stars (HVSs) ejected from the center of the Milky Way galaxy can be used to constrain the mass of its dark matter halo. Methods. We use the asymmetry in the radial velocity distribution of halo stars due to escaping HVSs, which depends on the halo potential (escape speed) as long as the round trip orbital time is shorter than the stellar lifetime, to discriminate between different models for the Milky Way gravitational potential. Results. Adopting a characteristic HVS travel time of $330$ Myr, which corresponds to the average mass of main sequence HVSs, we find that current data favors a mass for the Milky Way in the range $(1.2$-$1.9)\times 10^{12} \mathrm{M}_\odot$.

astro-ph.GA↗

arXiv2016

Constraining the CMB Optical Depth Through the Dispersion Measure of Cosmological Radio Transients

The dispersion measure of extragalactic radio transients, such as of recently discovered Fast Radio Burst FRB150418, can be used to measure the column density of free electrons in the intergalactic medium. The same electrons also scatter the Cosmic Microwave Background (CMB) photons, affecting precision measurements of cosmological parameters. We explore the connection between the dispersion measure of radio transients existing during the Epoch of Reionization (EoR) and the total optical depth for the CMB, $τ_{CMB}$, showing that the existence of such transients would provide a new sensitive probe of $τ_{CMB}$. As an example, we consider the population of FRBs. Assuming they exist during the EoR, we show that: (i) such sources can probe the reionization history by measuring $τ_{CMB}$ to sub-percent accuracy, and (ii) they can be detected with high significance by an instrument such as the Square Kilometer Array.

astro-ph.CO↗

arXiv2016

Jetted Tidal Disruptions of Stars as a Flag of Intermediate Mass Black Holes at High Redshifts

Tidal disruption events (TDEs) of stars by single or binary supermassive black holes (SMBHs) brighten galactic nuclei and reveal a population of otherwise dormant black holes. Adopting event rates from the literature, we aim to establish general trends in the redshift evolution of the TDE number counts and their observable signals. We pay particular attention to two types of TDEs which are expected to be observable out to high redshifts, namely (i) jetted TDEs whose luminosity is boosted by relativistic beaming, and (ii) TDEs around binary black holes. We show that the brightest (jetted) TDEs are expected to be produced by massive black hole binaries if the occupancy of intermediate mass black holes (IMBHs) in low mass galaxies is high. The same binary population will also provide gravitational wave sources for eLISA. In addition, we find that the shape of the X-ray luminosity function of TDEs strongly depends on the occupancy of IMBHs and could be used to constrain scenarios of SMBH formation. Finally, we make predictions for the expected number of TDEs observed by future X-ray telescopes as a function of their sensitivity limits. We find that an instrument which is 50 times more sensitive than the Burst Alert Telescope (BAT) on board the Swift satellite is expected to trigger ~10 times more events than BAT with 50% of the events coming from z>2. Because of their long decay times, high-redshift TDEs can be mistaken for fixed point sources in deep field surveys such as the 4Ms survey with the Chandra X-ray Observatory and targeted observations of the same deep field with year-long intervals could reveal TDEs. If the occupation fraction of IMBHs is high, 6-20 TDEs are expected in each deep field observed by a telescope 50 times more sensitive than Chandra.

astro-ph.HE↗

arXiv2016

Prospects for Characterizing the Atmosphere of Proxima Centauri b

The newly detected Earth-mass planet in the habitable zone of Proxima Centauri could potentially host life - if it has an atmosphere that supports surface liquid water. We show that thermal phase curve observations with the James Webb Space Telescope (JWST) from 5-12 microns can be used to test the existence of such an atmosphere. We predict the thermal variation for a bare rock versus a planet with 35% heat redistribution to the nightside and show that a JWST phase curve measurement can distinguish between these cases at $4σ$ confidence, assuming photon-limited precision. We also consider the case of an Earth-like atmosphere, and find that the ozone 9.8 micron band could be detected with longer integration times (a few months). We conclude that JWST observations have the potential to put the first constraints on the possibility of life around the nearest star to the Solar System.

astro-ph.EP↗

arXiv2016

Lyman-alpha radiation hydrodynamics of galactic winds before cosmic reionization

The dynamical impact of Lyman-alpha (Lyα) radiation pressure on galaxy formation depends on the rate and duration of momentum transfer between Lyα photons and neutral hydrogen gas. Although photon trapping has the potential to multiply the effective force, ionizing radiation from stellar sources may relieve the Lyα pressure before appreciably affecting the kinematics of the host galaxy or efficiently coupling Lyα photons to the outflow. We present self-consistent Lyα radiation-hydrodynamics simulations of high-$z$ galaxy environments by coupling the Cosmic Lyα Transfer code (COLT) with spherically symmetric Lagrangian frame hydrodynamics. The accurate but computationally expensive Monte-Carlo radiative transfer calculations are feasible under the one-dimensional approximation. The initial starburst drives an expanding shell of gas from the centre and in certain cases Lyα feedback significantly enhances the shell velocity. Radiative feedback alone is capable of ejecting baryons into the intergalactic medium (IGM) for protogalaxies with a virial mass of $M_{\rm vir} \lesssim 10^8~{\rm M}_\odot$. We compare the Lyα signatures of Population III stars with $10^5$ K blackbody emission to that of direct collapse black holes with a nonthermal Compton-thick spectrum and find substantial differences if the Lyα spectra are shaped by gas pushed by Lyα radiation-driven winds. For both sources, the flux emerging from the galaxy is reprocessed by the IGM such that the observed Lyα luminosity is reduced significantly and the time-averaged velocity offset of the Lyα peak is shifted redward.

astro-ph.GA↗

arXiv2016

Detection of Gravitational Wave Emission by Supermassive Black Hole Binaries Through Tidal Disruption Flares

Galaxy mergers produce supermassive black hole binaries, which emit gravitational waves prior to their coalescence. We perform three-dimensional hydrodynamic simulations to study the tidal disruption of stars by such a binary in the final centuries of its life. We find that the gas stream of the stellar debris moves chaotically in the binary potential and forms accretion disks around both black holes. The accretion light curve is modulated over the binary orbital period owing to relativistic beaming. This periodic signal allows to detect the decay of the binary orbit due to gravitational wave emission by observing two tidal disruption events that are separated by more than a decade.

astro-ph.HE↗

arXiv2016

Cumulative neutrino background from quasar-driven outflows

Quasar-driven outflows naturally account for the missing component of the extragalactic $γ$-ray background through neutral pion production in interactions between protons accelerated by the forward outflow shock and interstellar protons. We study the simultaneous neutrino emission by the same protons. We adopt outflow parameters that best fit the extragalactic $γ$-ray background data and derive a cumulative neutrino background of $\sim10^{-7}\,\rm GeV\,cm^{-2}\,s^{-1}\,sr^{-1}$ at neutrino energies $E_ν\gtrsim 10$ TeV, which naturally explains the most recent IceCube data without tuning any free parameters. The link between the $γ$-ray and neutrino emission from quasar outflows can be used to constrain the high-energy physics of strong shocks at cosmological distances.

astro-ph.HE↗

arXiv2016

Circularization of Tidally Disrupted Stars around Spinning Supermassive Black Holes

We study the circularization of tidally disrupted stars on bound orbits around spinning supermassive black holes by performing three-dimensional smoothed particle hydrodynamic simulations with Post-Newtonian corrections. Our simulations reveal that debris circularization depends sensitively on the efficiency of radiative cooling. There are two stages in debris circularization if radiative cooling is inefficient: first, the stellar debris streams self-intersect due to relativistic apsidal precession; shocks at the intersection points thermalize orbital energy and the debris forms a geometrically thick, ring-like structure around the black hole. The ring rapidly spreads via viscous diffusion, leading to the formation of a geometrically thick accretion disk. In contrast, if radiative cooling is efficient, the stellar debris circularizes due to self-intersection shocks and forms a geometrically thin ring-like structure. In this case, the dissipated energy can be emitted during debris circularization as a precursor to the subsequent tidal disruption flare. The possible radiated energy is up to ~2*10^{52} erg for a 1 Msun star orbiting a 10^6 Msun black hole. We also find that a retrograde (prograde) black hole spin causes the shock-induced circularization timescale to be shorter (longer) than that of a non-spinning black hole in both cooling cases. The circularization timescale is remarkably long in the radiatively efficient cooling case, and is also sensitive to black hole spin. Specifically, Lense-Thirring torques cause dynamically important nodal precession, which significantly delays debris circularization. On the other hand, nodal precession is too slow to produce observable signatures in the radiatively inefficient case. We also discuss the relationship between our simulations and the parabolic TDEs that are characteristic of most stellar tidal disruptions.

astro-ph.HE↗

arXiv2016

Testing General Relativity with Accretion-Flow Imaging of Sgr A*

The Event Horizon Telescope is a global very-long baseline interferometer capable of probing potential deviations from the Kerr metric, which is believed to provide the unique description of astrophysical black holes. Here we report an updated constraint on the quadrupolar deviation of Sagittarius A* within the context of a radiatively inefficient accretion flow model in a quasi-Kerr background. We also simulate near-future constraints obtainable by the forthcoming eight-station array and show that in this model already a one-day observation can measure the spin magnitude to within $0.005$, the inclination to within $0.09^\circ$, the position angle to within $0.04^\circ$, and the quadrupolar deviation to within $0.005$ at $3σ$ confidence. Thus, we are entering an era of high-precision strong gravity measurements.

astro-ph.HE↗

arXiv2016

A novel approach for identifying host galaxies of nearby FRBs

We report on a search for host galaxies of a subset of Rotating Radio Transients (RRATs) that possess a dispersion measure (DM) near or above the maximum Galactic value in their direction. These RRATs could have an extragalactic origin and therefore be Fast Radio Bursts (FRBs). The sizes of related galaxies on the sky at such short distances are comparable to the beam size of a single-dish telescope (for example, the $7.0'$ radius of the Parkes beam). Hence the association, if found, could be more definitive as compared to finding host galaxies for more distant FRBs. We did not find any host galaxy associated with six RRATs near the maximum Galactic DM. This result is consistent with the fact that the probability of finding an FRB host galaxy within this volume is also very small. We propose that future follow-up observations of such RRATs be carried out in searching for local host galaxies as well as the sources of FRBs.

astro-ph.HE↗

arXiv2016

Lessons from Mayan Astronomy

The Mayan culture collected exquisite astronomical data for over a millennium. However, it failed to come up with the breakthrough ideas of modern astronomy because the data was analyzed within a mythological culture of astrology that rested upon false but mathematically sophisticated theories about the Universe. Have we learned the necessary lessons to prevent our current scientific culture from resembling Mayan Astronomy? Clearly, data collection by itself is not a guarantee for good science as commonly assumed by funding agencies. A vibrant scientific culture should cultivate multiple approaches to analyzing existing data and to collecting new data.

physics.pop-ph↗

arXiv2016

Relative Likelihood for Life as a Function of Cosmic Time

Is life most likely to emerge at the present cosmic time near a star like the Sun? We address this question by calculating the relative formation probability per unit time of habitable Earth-like planets within a fixed comoving volume of the Universe, dP(t)/dt, starting from the first stars and continuing to the distant cosmic future. We conservatively restrict our attention to the context of "life as we know it" and the standard cosmological model, LCDM. We find that unless habitability around low mass stars is suppressed, life is most likely to exist near 0.1 solar-mass stars ten trillion years from now. Spectroscopic searches for biosignatures in the atmospheres of transiting Earth-mass planets around low mass stars will determine whether present-day life is indeed premature or typical from a cosmic perspective.

astro-ph.CO↗

arXiv2016

Quasar-driven outflows account for the missing extragalactic gamma-ray background

The origin of the extragalactic $γ$-ray background permeating throughout the Universe remains a mystery forty years after its discovery. The extrapolated population of blazars can account for only half of the background radiation at the energy range of ~ 0.1-10 GeV. Here we show that quasar-driven outflows generate relativistic protons that produce the missing component of the extragalactic $γ$-ray background and naturally match its spectral fingerprint, with a generic break above ~ 1 GeV. The associated $γ$-ray sources are too faint to be detected individually, explaining why they had not been identified so far. However, future radio observations may image their shock fronts directly. Our best fit to the Fermi-LAT observations of extragalactic $γ$-ray background spectrum provides constraints on the outflow parameters that agree with observations of these outflows and theoretical predictions.

astro-ph.HE↗