arXiv subjects

Abraham Loeb

Publications and source records attributed to Abraham Loeb.

At least 127 records · Page 7Linked to original sources

arXiv2023

Tidal Wave Breaking in the Eccentric Lead-In to Mass Transfer and Common Envelope Phases

The evolution of many close binary and multiple star systems is defined by phases of mass exchange and interaction. As these systems evolve into contact, tidal dissipation is not always sufficient to bring them into circular, synchronous orbits. In these cases, encounters of increasing strength occur while the orbit remains eccentric. This paper focuses on the outcomes of close tidal passages in eccentric orbits. Close eccentric passages excite dynamical oscillations about the stars' equilibrium configurations. These tidal oscillations arise from the transfer of orbital energy into oscillation mode energy. When these oscillations reach sufficient amplitude, they break near the stellar surface. The surface wave-breaking layer forms a shock-heated atmosphere that surrounds the object. The continuing oscillations in the star's interior launch shocks that dissipate into the this atmosphere, damping the tidal oscillations. We show that the rapid, nonlinear dissipation associated with the wave breaking of fundamental oscillation modes therefore comes with coupled mass loss to the wave breaking atmosphere. The mass ratio is an important characteristic that defines the relative importance of mass loss and energy dissipation and therefore determines the fate of systems evolving under the influence of nonlinear dissipation. The outcome can be rapid tidal circularization ($q\ll1$) or runaway mass exchange ($q\gg1$).

astro-ph.SR↗

arXiv2023

Dusty, Self-obscured Transients from Stellar Coalescence

We discuss the central role that dust condensation plays in shaping the observational appearance of outflows from coalescing binary systems. As binaries begin to coalesce, they shock-heat and expel material into their surroundings. Depending on the properties of the merging system, this material can expand to the point where molecules and dust form, dramatically increasing the gas opacity. We use the existing population of Luminous Red Novae (LRNe) to constrain the thermodynamics of these ejecta, then apply our findings to the progressive obscuration of merging systems in the lead in to their coalescence. Compact progenitor stars near the main sequence or in the Hertzsprung gap along with massive progenitor stars have sufficiently hot circumstellar material to remain unobscured by dust. By contrast, more extended, low-mass giants should become completely optically obscured by dust formation in the circumbinary environment. We predict that 30--50\% of stellar coalescence transients for solar-mass stars will be dusty, infrared-luminous sources. Of these, the optical transients may selectively trace complete merger outcomes while the infrared transients trace common envelope ejection outcomes.

astro-ph.SR↗

arXiv2023

Detection Rate of <50-meter Interstellar Objects with LSST

The previous decade saw the discovery of the first four known interstellar objects due to advances in astronomical viewing equipment. Future sky surveys with greater sensitivity will allow for more frequent detections of such objects, including increasingly small objects. We consider the capabilities of the Legacy Survey of Space and Time (LSST) of the Vera C. Rubin Observatory to detect interstellar objects of small sizes during its period of operation over the next decade. We use LSST's detection capabilities and simulate populations of interstellar objects in the range of 1-50m in diameter to calculate the expected number of small interstellar objects that will be detected. We use previous detections of interstellar objects to calibrate our object density estimates. We also consider the impact of the population's albedo on detection rates by considering populations with two separate albedo distributions: a constant albedo of 0.06 and an albedo distribution that resembles near earth asteroids. We find that the number of detections increases with the diameter over the range of diameters we consider. We estimate a detection rate of up to a small ISO every two years of LSST's operation with an increase by a factor of ten for future surveys that extend a magnitude deeper.

astro-ph.IM↗

arXiv2023

A new limit on intergalactic magnetic fields on sub-kpc scales from fast radio bursts

We use the measured scattering timescales of Fast Radio Bursts (FRBs) from the CHIME catalog to derive an upper limit on the magnetic field on sub-kpc scales in the intergalactic medium (IGM). A nonmagnetized, photoionized IGM is insufficient to explain the turbulent scattering at all redshifts, with a Warm-Hot component being marginally consistent with the data at $z \sim 1$. Accounting for the lower envelope of the temporal smearing distribution with a nonzero magnetic field leads to upper limits $B < 10-30$ nG on scales of 0.07-0.20 kpc in the IGM at $z \sim 1-2$. Our work introduces a novel technique to constrain small-scale magnetic fields in the IGM, in a regime unexplored by the Rotation and Dispersion Measures of FRBs.

astro-ph.HE↗

arXiv2023

Triple and Quadruple Black Holes in the ASTRID Simulation at $z \sim 2$

We use the ASTRID cosmological hydrodynamic simulation to investigate the properties and evolution of triple and quadruple Massive Black Hole (MBH) systems at $z = 2-3$. Only a handful of MBH tuple systems have been detected to date. In ASTRID, we find $4\%$ of the $M_{\rm BH}>10^7\,M_\odot$ are in tuples with $Δr_{\rm max} < 200\,{\rm kpc}$. The tuple systems span a range of separations with the majority of the observable AGN systems at $Δr \sim 50-100$ kpc. They include some of the most massive BHs (up to $10^{10} \,M_\odot$) but with at least one of the components of $M_{\rm BH} \sim 10^7 \,M_\odot$. Tuples' host galaxies are typically massive with $M_* \sim 10^{10-11} \,M_\odot$. We find that $>10\%$ massive halos with $M_{\rm halo} > 10^{13} M_\odot$ host MBH tuples. Following the subsequent interactions between MBHs in tuples, we found that in $\sim 5\%$ of the triplets all three MBHs merge within a Gyr, and $15\%$ go through one merger. As a by-product of the complex multi-galaxy interaction of these systems, we also find that up to $\sim 5\%$ of tuples lead to runaway MBHs. In ASTRID, virtually all of the ultramassive black holes ($>10^{10} \,M_\odot $) have undergone a triple quasar phase while for BHs with $M_{\rm BH} \sim 10^9 \,M_\odot$ this fraction drops to $50\%$.

astro-ph.GA↗

arXiv2023

Quantum Tunneling of Ultralight Dark Matter Out of Satellite Galaxies

The idea of ultralight scalar (axion) dark matter is theoretically appealing and may resolve some small-scale problems of cold dark matter; so it deserves careful attention. In this work we carefully analyze tunneling of the scalar field in dwarf satellites due to the tidal gravitational force from the host halo. The tidal force is far from spherically symmetric; causing tunneling along the axis from the halo center to the dwarf, while confining in the orthogonal plane. We decompose the wave function into a spherical term plus higher harmonics, integrate out angles, and then numerically solve a residual radial Schrödinger-Poisson system. By demanding that the core of the Fornax dwarf halo can survive for at least the age of the universe places a bound on the dark matter particle mass $2\times 10^{-22}\,\mbox{eV}\lesssim m\lesssim 6\times 10^{-22}\,$eV. Interestingly, we show that if another very low density halo is seen, then it rules out the ultralight scalar as core proposal completely. Furthermore, the non-condensed particles likely impose an even sharper lower bound. We also determine how the residual satellites could be distributed as a function of radius.

astro-ph.CO↗

arXiv2023

Interstellar Objects From Broken Dyson Spheres

Without extensive maintenance, Dyson spheres inevitably disintegrate by asteroid impacts over billions of years. The resulting fragments would appear as anomalous interstellar objects, potentially sharing the unusual shape and motion of 1I/Oumuamua or the unusual material strength of the first two interstellar meteors, IM1 and IM2. If the Dyson sphere tiles are light sails, the number of fragments could exceed that of interstellar asteroids because of their reduced escape speed from the star and the increase in stellar luminosity during the red giant phase.

physics.pop-ph↗

arXiv2023

Constraining the cosmic merger history of intermediate-mass black holes with gravitational wave detectors

Intermediate-mass black holes (IMBHs) have not been detected beyond any reasonable doubt through either dynamical or accretion signatures. Gravitational waves (GWs) represent an unparalleled opportunity to survey the sky and detect mergers of IMBHs, making it possible for the first time to constrain their formation, growth, and merger history across cosmic time. While the current network LIGO-Virgo-KAGRA is significantly limited in detecting mergers of IMBH binaries, the next generation of ground-based observatories and space-based missions promise to shed light on the IMBH population through the detection of several events per year. Here, we asses this possibility by determining the optimal network of next-generation of GW observatories to reconstruct the IMBH merger history across cosmic time. We show that Voyager, the Einstein Telescope, and Cosmic Explorer will be able to constrain the distribution of the primary masses of merging IMBHs up to $\sim 10^3\ M_\odot$ and with mass ratio $\gtrsim 0.1$, while LISA will complementary do so at higher mass and smaller mass ratios. Therefore, a network of next-generation ground-based and space-based observatories will potentially reconstruct the merger history of IMBHs. Moreover, IMBHs with masses $\lesssim 5\times 10^3\ M_\odot$ could be observed in multiband up to a redshift of $z\approx 4$, ushering in a new of era GW astronomy.

astro-ph.HE↗

arXiv2022

A new approach to constrain the Hubble expansion rate at high redshifts by gravitational waves

Detection of massive binary black hole (BBH) mergers at high redshifts is a target for LISA space mission. While the individual masses of a BBH merger are redshifted, the mass ratio of BBH mergers is independent of their redshift. Therefore, if there is an independent correlation between the mass ratio and redshift, such a relationship can be used to i) infer the redshift of the merging binaries, and together with the luminosity distance measurement ($D_L$), constrain the expansion rate of the universe at high redshifts $H(z)$, and ii) constrain models of supermassive black hole seed formation in the universe assuming a fixed cosmology. We discuss why there is an expected relation between the mass ratio of the massive BBHs with their redshift and show the clues for this relation by analyzing cosmological hydrodynamical simulations of BBH mergers. This approach opens up the possibility of directly measuring the expansion rate at redshift $z \approx 2$ with LISA for the first time. Moreover, we discover a trend between seed mass and mass ratio of massive BBHs which by itself is a major result that could be exploited to constrain the formation scenarios of supermassive BH seeds.

astro-ph.CO↗

arXiv2022

The Two $z\sim 13$ Galaxy Candidates HD1 and HD2 Are Likely Not Lensed

The discovery of two UV-bright galaxy candidates at $z\sim 13$, HD1 and HD2, laid the foundation for a new race to study the early Universe. Previous investigations suggested that they are either powered by a supermassive black hole or by an extreme, transient burst of star formation. Given their uncertain nature, we investigate whether these sources could be lensed by a hitherto undetected, faint foreground galaxy. We find that at the current limiting magnitude with which HD1 and HD2 were imaged, there is only a $7.39\%$ probability they are strongly lensed by spherical deflectors and that the hypothetical lensing galaxy was too faint to be detected. Meanwhile, with the limiting magnitudes of the Hubble Space Telescope (HST) and James Webb Space Telescope (JWST), the theoretical probability would drop precipitously to $0.058\%$ and $0.0012\%$, respectively. We further find it unlikely that the luminosities of both sources can be accounted for by lensing that produces a single, resolved image with sufficiently high magnification. Alternatively, in the unlikely event that their brightness results from lensing by an elliptical isothermal galaxy, there is a $30.9 \%$ probability that the lensing galaxy is too faint to be observable at the current limiting magnitude. Future HST (JWST) imaging will drop this probability to $0.245 \%$ ($0.0025 \%$). In summary, while deep imaging with HST and JWST is required to discard the lensing hypothesis entirely, it is unlikely that the exceptional luminosity of the two $z \sim 13$ sources can be accounted for by gravitational lensing.

astro-ph.GA↗

arXiv2022

Life on Titan May Signal Early Life in the Universe

The temperature of the cosmic microwave background (CMB) was equal to the surface temperature of Saturn's moon Titan, 94K, at a redshift z=33.5, after the first galaxies formed. Titan-like objects would have maintained this surface temperature for tens of Myr irrespective of their distance from a star. Titan has the potential for the chemistry of familiar life in its subsurface water ocean, as well new forms of life in the rivers, lakes and seas of liquid methane and ethane on its surface. The potential future discovery of life on Titan would open the possibility that the earliest lifeforms emerged in metal-rich environments of the earliest galaxies in the universe, merely 100 Myr after the Big Bang.

astro-ph.CO↗

arXiv2022

Interstellar Meteors are Outliers in Material Strength

The first interstellar meteor larger than dust was detected by US government sensors in 2014, identified as an interstellar object candidate in 2019, and confirmed by the Department of Defense in 2022. Here, we describe an additional interstellar object candidate in the CNEOS fireball catalog, and compare the implied material strength of the two objects, referred to here as IM1 and IM2, respectively. IM1 and IM2 are ranked 1 and 3 in terms of material strength out of all 273 fireballs in the CNEOS catalog. Fitting a log-normal distribution to material strengths of objects in the CNEOS catalog, IM1 and IM2 are outliers at the levels of $3.5 σ$ and $2.6 σ$, respectively. The random sampling and Gaussian probabilities, respectively, of picking two objects with such high material strength from the CNEOS catalog, are $\sim 10^{-4}$ and $\sim 10^{-6}$. If IM2 is confirmed, this implies that interstellar meteors come from a population with material strength characteristically higher than meteors originating from within the solar system. Additionally, we find that if the two objects are representative of a background population on random trajectories, their combined detections imply that $\sim 40\%$ of all refractory elements are locked in meter-scale interstellar objects. Such a high abundance seemingly defies a planetary system origin.

astro-ph.EP↗

arXiv2022

A Lunar Backup Record of Humanity

The risk of a catastrophic or existential disaster for our civilization is increasing this century. A significant motivation for a near-term space settlement is the opportunity to safeguard civilization in the event of a planetary-scale disaster. A catastrophic event could destroy the significant cultural, scientific, and technological progress on Earth. However, early space settlements can preserve records of human activity by maintaining a backup data storage system. The backup can also store information about the events leading up to the disaster. The system would improve the ability of early space settlers to recover our civilization after collapse. We show that advances in laser communications and data storage enable the development of a data storage system on the lunar surface with a sufficient uplink data rate and storage capacity to preserve valuable information about the achievements of our civilization and the chronology of the disaster.

physics.soc-ph↗

arXiv2022

Physical Considerations for an Intercept Mission to a 1I/'Oumuamua-like Interstellar Object

In this paper, we review some of the extant literature on the study of interstellar objects (ISOs). With the forthcoming Vera C. Rubin Telescope and Legacy Survey of Space and Time (LSST), we find that $0.38 - 84$ `Oumuamua-like interstellar objects are expected to be detected in the next 10 years, with 95\% confidence. The feasibility of a rendezvous trajectory has been demonstrated in previous work. In this paper, we investigate the requirements for a rendezvous mission with the primary objective of producing a resolved image of an interstellar object. We outline the rendezvous distances necessary as a function of resolution elements and object size. We expand upon current population synthesis models to account for the size dependency on the detection rates for reachable interstellar objects. We assess the trade-off between object diameter and occurrence rate, and conclude that objects with the size range between a third of the size and the size of `Oumuamua will be optimal targets for an imaging rendezvous. We also discuss expectations for surface properties and spectral features of interstellar objects, as well as the benefits of various spacecraft storage locations.

astro-ph.EP↗

arXiv2022

Accretion from Winds of Red Giant Branch Stars May Reveal the Supermassive Black Hole in Leo I

A supermassive black hole (SMBH) of $\sim 3\times 10^6 \, \rm M_\odot$ was recently detected via dynamical measurements at the center of the dwarf galaxy Leo I. Standing $\sim 2$ orders of magnitude above standard scaling relations, this SMBH is hosted by a galaxy devoid of gas and with no significant star formation in the last $\sim 1$ Gyr. This detection can profoundly impact the formation models for black holes and their hosts. We propose that winds from a population of $\sim 100$ evolved stars within the Bondi radius of the SMBH produce a sizable accretion rate, with Eddington ratios between $9\times10^{-8}$ and $9\times10^{-7}$, depending on the value of the stellar mass loss. These rates are typical of SMBHs accreting in advection-dominated accretion flow (ADAF) mode. The predicted spectrum peaks in the microwaves at $\sim 0.1-1$ THz ($300-3000 \, \mathrm{μm}$) and exhibits significant variations at higher energies depending on the accretion rate. We predict a radio flux of $\sim 0.1$ mJy at $6$ GHz, mildly dependent on the accretion properties. Deep imaging with Chandra, VLA, and ALMA can confirm the presence of this SMBH and constrain its accretion flow.

astro-ph.GA↗

arXiv2022

The challenge of ruling out inflation via the primordial graviton background

Recent debates around the testability of the inflationary paradigm raise the question of how to model-independently discriminate it from competing scenarios. We argue that a detection of the Cosmic Graviton Background (CGB), the relic radiation from gravitons decoupling around the Planck time, would rule out the inflationary paradigm, as realistic inflationary models would dilute the CGB to an unobservable level. The CGB contribution to the effective number of relativistic species, $ΔN_{{\rm eff},g} \approx 0.054$, is well within the reach of next-generation cosmological probes. We argue that detecting the high-frequency stochastic gravitational wave background associated to the CGB will be challenging but potentially feasible. We briefly discuss expectations within alternatives to inflation, focusing on bouncing cosmologies and emergent scenarios.

astro-ph.CO↗

arXiv2022

"Down to Earth" Limits on Unidentified Aerial Phenomena in Ukraine (Comment on arXiv:2208.11215)

A recent report by astronomers about Unidentified Aerial Phenomena (UAP) in Ukraine (arXiv:2208.11215) suggests dark phantom objects of size 3-12 meters, moving at speeds of up to 15 km/s at a distance of up to 10-12 km with no optical emission. I show that the friction of such objects with the surrounding air would have generated a bright optical fireball. Reducing their inferred distance by a factor of ten is fully consistent with the size and speed of artillery shells.

astro-ph.IM↗

arXiv2022

Ultra Long-Term Cosmology and Astrophysics

We examine astronomical observations that would be achievable over a future timeline corresponding to the documented history of human civilization so far, $\sim 10^4$ years. We examine implications for measurements of the redshift drift, evolution of the CMB, and cosmic parallax. A number of events that are rare on the scale of centuries will become easily observable on a timescale $\sim 10^4$ years. Implications for several measurements related to gravity are discussed.

astro-ph.CO↗