arXiv subjects

Abraham Loeb

Publications and source records attributed to Abraham Loeb.

At least 145 records · Page 8Linked to original sources

arXiv2022

The Inferred Abundance of Interstellar Objects of Technological Origin

The local detection rate of interstellar objects can allow for estimations of the total number of similar objects bound by the Milky Way thin disk. If interstellar objects of artificial origin are discovered, the estimated total number of objects can be lower by a factor of about $10^{16}$ if they target the habitable zone around the Sun. We propose a model for calculating the quantity of natural or artificial interstellar objects of interest based on the object's velocity and observed density. We then apply the model to the case of chemically propelled rockets from extraterrestrial civilizations. Finally, we apply the model to three previously discovered interstellar objects -- the object 'Oumuamua of unknown origin and the first interstellar meteors CNEOS 2014-01-08 and CNEOS 2017-03-09.

physics.pop-ph↗

arXiv2022

The 2019 Discovery of a Meteor of Interstellar Origin

The earliest confirmed interstellar object, `Oumuamua, was discovered in the Solar System by Pan-STARRS in 2017, allowing for a calibration of the abundance of interstellar objects of its size $\sim 100\;$ m. This was followed by the discovery of Borisov, which allowed for a similar calibration of its size $\sim 0.4 - 1 \mathrm{\; km}$. One would expect a much higher abundance of significantly smaller interstellar objects, with some of them colliding with Earth frequently enough to be noticeable. Based on the CNEOS catalog of bolide events, we identify the $\sim 0.45$m meteor detected at 2014-01-08 17:05:34 UTC as originating from an unbound hyperbolic orbit with 99.999\% confidence. The U.S. Department of Defense has since verified that "the velocity estimate reported to NASA is sufficiently accurate to indicate an interstellar trajectory." We infer that the meteor had an asymptotic speed of $v_{\infty} \sim 42.1 \pm 5.5\; \mathrm{km \; s^{-1}}$ outside of the solar system. Its origin is approximately towards R.A. $49.4 \pm 4.1^{\circ}$ and declination $11.2 \pm 1.8^{\circ}$, implying that its initial velocity vector was $58\pm6\; \mathrm{km\;s^{-1}}$ away from the velocity of the Local Standard of Rest (LSR). Its high LSR speed implies a possible origin from the deep interior of a planetary system or a star in the thick disk of the Milky Way galaxy. The local number density of its population is $10^{6{^{+0.75}_{-1.5}}} \; \mathrm{AU^{-3}}$ or $9 \times 10^{21{^{+0.75}_{-1.5}}} \; \mathrm{pc^{-3}}$ (necessitating 0.2 -- 20 Earth masses of material to be ejected per local star). We show that the detections of CNEOS 2014-01-08, `Oumuamua, and Borisov collectively imply that the differential size distribution in good agreement with a collisional distribution, with a power-law slope is $q \sim 3.6 \pm 0.5$, where the quoted uncertainty corresponds to $2 σ$.

astro-ph.EP↗

arXiv2022

An Ocean Expedition by the Galileo Project to Retrieve Fragments of the First Large Interstellar Meteor CNEOS 2014-01-08

astro-ph.EP↗

arXiv2022

Implications for the Hubble tension from the ages of the oldest astrophysical objects

We use the ages of old astrophysical objects (OAO) in the redshift range $0 \lesssim z \lesssim 8$ as stringent tests of the late-time cosmic expansion history. Since the age of the Universe at any redshift is inversely proportional to $H_0$, requiring that the Universe be older than the oldest objects it contains at any redshift, provides an upper limit on $H_0$. Using a combination of galaxies imaged from the CANDELS program and various high-$z$ quasars, we construct an age-redshift diagram of $\gtrsim 100$ OAO up to $z \sim 8$. Assuming the $Λ$CDM model at late times, we find the 95\% confidence level upper limit $H_0<73.2\,{\rm km}/{\rm s}/{\rm Mpc}$, in slight disagreement with a host of local $H_0$ measurements. Taken at face value, and assuming that the OAO ages are reliable, this suggests that ultimately a combination of pre- and post-recombination ($z \lesssim 10$) new physics might be required to reconcile cosmic ages with early-time and local $H_0$ measurements. In the context of the Hubble tension, our results motivate the study of either combined global pre- and post-recombination modifications to $Λ$CDM, or local new physics which only affects the local $H_0$ measurements.

astro-ph.CO↗

arXiv2022

Overview of the Galileo Project

The Galileo Project is the first systematic scientific research program in search for potential astro8 archaeological artifacts or remnants of extraterrestrial technological civilizations (ETCs) or potentially active equipment near Earth. Taking a path not taken, it conceivably may pick some low-hanging fruit, and without asserting probabilities -- make discoveries of ETC-related objects, which would have far-reaching implications for science and our worldview.

physics.pop-ph↗

arXiv2022

Extinction of the TeV Gamma-Ray Background by Sunlight

I show that pair production on sunlight introduces a sizable anisotropy in the cosmic background of TeV gamma-rays. The anisotropy amplitude in the direction of the Sun exceeds the cosmic dipole anisotropy from the motion of the Sun relative to the cosmic rest-frame.

hep-ph↗

arXiv2022

Lensing in the Darkness: A Bayesian Analysis of 22 Chandra Sources at $z \gtrsim 6$ Shows No Evidence of Lensing

More than $200$ quasars have been detected so far at $z > 6$, with only one showing clear signs of strong gravitational lensing. Some studies call for a missing population of lensed high-$z$ quasars, but their existence is still in doubt. A large fraction of high-$z$ quasars being lensed would have a significant effect on the shape of the intrinsic quasar luminosity function (QLF). Here, we perform the first systematic search for lensed X-ray-detected quasars at $z \gtrsim 6$ employing a Bayesian analysis, with the code BAYMAX, to look for morphological evidence of multiple images that may escape a visual inspection. We analyzed a sample of 22 quasars at $z > 5.8$ imaged by the Chandra X-ray observatory and found none with statistically significant multiple images. In the sub-sample of the 8 sources with photon counts $>20$ we exclude multiple images with separations $r>1''$ and count ratios $f>0.4$, or with separations as small as $0.''7$ and $f>0.7$ at $95\%$ confidence level. Comparing this non-detection with predictions from theoretical models suggesting a high and a low lensed fraction, we placed upper limits on the bright-end slope, $β$, of the QLF. Using only the sub-sample with 8 sources, we obtain, in the high-lensing model, a limit $β< 3.38$. Assuming no multiple source is present in the full sample of 22 sources, we obtain $β< 2.89$ and $β< 3.53$ in the high and low lensing models, respectively. These constraints strongly disfavor steep QLF shapes previously proposed in the literature.

astro-ph.GA↗

arXiv2022

Implication of spin constraints by the Event Horizon Telescope on stellar orbits in the Galactic Center

The center of the Milky Way hosts the closest supermassive black hole, SgrA$^*$. Decades of near-infrared observations of our Galactic Center have shown the presence of a small population of stars (the so called S-star cluster) orbiting SgrA$^*$, which were recently reported to be arranged in two orthogonal disks. In this case, the timescale for Lense-Thirring precession of S-stars should be longer than their age, implying a low spin for SgrA$^*$. In contrast, the recent results by the Event Horizon Telescope favor a highly-spinning SgrA$^*$, which seem to suggest that the S-stars could not be arranged in disks. Alternatively, the spin of SgrA$^*$ must be small, suggesting that the models for its observed image are incomplete.

astro-ph.GA↗

arXiv2022

Merger rates of intermediate-mass black hole binaries in nuclear star clusters

Repeated mergers of stellar-mass black holes (BHs) in dense star clusters can produce intermediate-mass black holes (IMBHs). In particular, nuclear star clusters at the centers of galaxies have deep enough potential wells to retain most of the BH merger products, in spite of the significant recoil kicks due to anisotropic emission of gravitational radiation. These events can be detected in gravitational waves (GWs), which represent an unprecedented opportunity to reveal IMBHs. In this paper, we analyze the statistical results of a wide range of numerical simulations, which encompass different cluster metallicities, initial BH seed masses, and initial BH spins, and we compute the merger rate of IMBH binaries. We find that merger rates are in the range $0.01$-$10$\,Gpc$^{-3}$\,yr$^{-1}$ depending on IMBH masses. We also compute the number of multi-band detections in ground-based and space-based observatories. Our model predicts that a few merger events per year should be detectable with LISA, DECIGO, ET, and LIGO for IMBHs with masses $\lesssim 1000\msun$, and a few tens of merger events per year with DECIGO, ET, and LIGO only.

astro-ph.HE↗

arXiv2022

Galactic Kites

I show that interstellar films of material thinner than a micron, drift away from the Galactic plane as a result of stellar radiation pressure. Such films, whether produced naturally by dust coagulation in proto-planetary disks or artificially by technological civilizations, would accumulate over the age of the Milky-Way and hover above the Galactic disk at a scale-height set gravitationally by the dark matter halo. Limits on scattered starlight imply that this population carries a fraction below 2x10^{-3} of the interstellar medium mass.

astro-ph.GA↗

arXiv2022

Eccentricity Evolution in Gaseous Dynamical Friction

We analyse how drag forces modify the orbits of objects moving through extended gaseous distributions. We consider how hydrodynamic (surface area) drag forces and dynamical friction (gravitational) drag forces drive the evolution of orbital eccentricity. While hydrodynamic drag forces cause eccentric orbits to become more circular, dynamical friction drag can cause orbits to become more eccentric. We develop a semi-analytic model that accurately predicts these changes by comparing the total work and torque applied to the orbit at periapse and apoapse. We use a toy model of a radial power-law density profile, $ρ\propto r^{-γ}$, to determine that there is a critical $γ= 3$ power index which separates the eccentricity evolution in dynamical friction: orbits become more eccentric for $γ< 3$ and circularize for $γ> 3$. We apply these findings to the infall of a Jupiter-like planet into the envelope of its host star. The hydrostatic envelopes of stars are defined by steep density gradients near the limb and shallower gradients in the interior. Under the influence of gaseous dynamical friction, an infalling object's orbit will first decrease in eccentricity, then increase. The critical separation that delineates these regimes is predicted by the local density slope and is linearly dependent on polytropic index. More broadly, our findings indicate that binary systems may routinely emerge from common envelope phases with non-zero eccentricities that were excited by the dynamical friction forces that drove their orbital tightening.

astro-ph.EP↗

arXiv2022

New Constraints on the Composition and Initial Speed of CNEOS 2014-01-08

We study the newly released light curve from the fireball of the first interstellar meteor CNEOS 2014-01-08. The measured velocity and three observed flares down to an altitude of $18.7 \mathrm{\; km}$ imply ambient ram pressure in the range of $113-194$ MPa when the meteor disintegrated. The required yield strength is $\gtrsim 20$ times higher than stony meteorites and $\gtrsim 2$ times larger than iron meteorites. The implied slowdown in the atmosphere suggests an initial speed of about $66.5 \; {\rm km~s^{-1}}$, strengthening the case for an interstellar origin of this meteor and making it an outlier relative to the velocity dispersion of local stars.

astro-ph.EP↗

arXiv2022

Where to find over-massive brown dwarfs: new benchmark systems for binary evolution

Under the right conditions brown dwarfs that gain enough mass late in their lives to cross the hydrogen burning limit will not turn into low-mass stars, but rather remain essentially brown dwarf-like. While these objects, called either beige dwarfs or over-massive brown dwarfs, may exist in principle, it remains unclear exactly how they would form astrophysically. We show that accretion from AGB winds, aided by the wind Roche lobe overflow mechanism, is likely to produce a substantial population of observable overmassive brown dwarfs, though other mechanisms are still plausible. Specifically we predict that sun-like stars born with a massive brown dwarf companion on an orbit with a semi-major axis of order 10 AU will likely produce overmassive brown dwarfs, which may be found today as companions to the donor star's remnant white dwarf. The identification and characterization of such an object would produce unique constraints on binary evolution because there is a solid upper limit on the brown dwarf's initial mass.

astro-ph.SR↗

arXiv2022

Self-Interacting Dark Matter from Gravitational Scattering

I show that gravitational scattering of dark-matter objects of 10^4 solar masses and speeds of 10 km/s, provides the cross-section per unit mass required in self-interacting dark matter models that alleviate the small-scale structure challenges to the collisionless cold dark matter model. For primordial objects of mass 10^4*(M_4) solar masses, moving at the velocity dispersion characteristic of dwarf galaxies, 10*(v_1) km/s, the cross-section per unit mass for gravitational scattering is 10*[M_4/(v_1)^4] cm^2/g. The steep decline in interaction with increasing velocity explains why self-interaction is not evident in data on massive galaxies and clusters of galaxies.

astro-ph.CO↗

arXiv2022

Signatures of supermassive black hole binaries on maser systems

We illustrate a novel signature of black hole binaries from their effect on the kinematics of water maser emission in their environments. With the help of simulations, we establish the condition for clumps to mase based on their coherence lengths calibrated to those of the known maser galaxy NGC 4258. This is then used to identify masing clumps around a binary black hole system, and quantify the kinematic and spectral differences relative to the single black hole case. For some generic circumstances, blue-shifted masers around a binary black hole are found to preferentially follow the Keplerian rotation curve observed in the single black hole case. The redshifted ones, however, are found to visibly deviate from this relation, and also display more scatter with a tendency towards lower absolute values of the velocity along the line-of-sight. The spectrum of the masers as a function of line-of-sight velocity also shows a double peaked structure, reminiscent of recent observations of systems such as Mrk 1. Our results motivate future prospects for identifying binary black hole candidates with the help of water maser emissions.

astro-ph.GA↗

arXiv2022

Are the Newly-Discovered $z \sim 13$ Drop-out Sources Starburst Galaxies or Quasars?

The detection of two $z\sim 13$ galaxy candidates has opened a new window on galaxy formation at an era only $330$ Myr after the Big Bang. Here, we investigate the physical nature of these sources: are we witnessing star forming galaxies or quasars at such early epochs? If powered by star formation, the observed ultraviolet (UV) luminosities and number densities can be jointly explained if: (i) these galaxies are extreme star-formers with star formation rates $5-24\times$ higher than those expected from extrapolations of average lower-redshift relations; (ii) the star formation efficiency increases with halo mass and is countered by increasing dust attenuation from $z \sim 10-5$; (iii) they form stars with an extremely top-heavy initial mass function. The quasar hypothesis is also plausible, with the UV luminosity produced by black holes of $\sim 10^8 \, \mathrm{M_{\odot}}$ accreting at or slightly above the Eddington rate ($f_{\rm Edd}\sim 1.0$). This black hole mass at $z\sim13$ would require very challenging, but not implausible, growth parameters. If spectroscopically confirmed, these two sources will represent a remarkable laboratory to study the Universe at previously inaccessible redshifts.

astro-ph.GA↗

arXiv2022

A Statistical Detection of Wide Binary Systems in the Ultra-Faint Dwarf Galaxy Reticulum II

Binary stars can inflate the observed velocity dispersion of stars in dark matter dominated systems such as ultra-faint dwarf galaxies (UFDs). However, the population of binaries in UFDs is poorly constrained by observations, with preferred binary fractions for individual galaxies ranging from a few percent to nearly unity. Searching for wide binaries through nearest neighbor (NN) statistics (or the two-point correlation function) has been suggested in the literature, and we apply this method for the first time to detect wide binaries in a UFD. By analyzing the positions of stars in Reticulum II (Ret II) from Hubble Space Telescope images, we search for angularly resolved wide binaries in Ret II. We find that the distribution of their NN distances shows an enhancement at projected separations of $<1$ arcsec relative to a model containing no binaries. We show that such an enhancement can be explained by a wide~binary fraction of $f_b\approx0.007^{+0.008}_{-0.003}$ at separations of more than 3000 AU. Under the assumption that the binary separation distribution is similar to that in the Milky Way, the total binary fraction in Ret II may be on the order of 50%. We also use the observed magnitude distribution of stars in Ret II to constrain the initial mass function over the mass range $0.34-0.78~M_{\odot}$, finding that a shallow power-law slope of $1.01 \le α\le 1.15$ matches the data.

astro-ph.GA↗

arXiv2022

Spurious Radial Migration from Relativistic Effects in the Milky-Way Disk

The gradient of the gravitational redshift in the potential of the Milky-Way induces an apparent spurious radial migration. I show that this effect is simply related to the local acceleration, which was measured recently by Gaia eDR3, implying a spectroscopic shift of $[-2.4x10^{-2}/(r/8kpc)] km/s/kpc. The transverse Doppler effect yields a comparable contribution. The spurious radial velocity from both relativistic effects amounts to crossing a major portion of the Milky-Way disk during the age of the universe, and must be corrected for in any future measurement of the actual radial migration of stars.

astro-ph.GA↗