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Martin J. Rees

Publications and source records attributed to Martin J. Rees.

At least 19 recordsLinked to original sources
arXiv2019

Titans of the Early Universe: The Prato Statement on the Origin of the First Supermassive Black Holes

In recent years, the discovery of massive quasars at z~7 has provided a striking challenge to our understanding of the origin and growth of supermassive black holes in the early Universe. Mounting observational and theoretical evidence indicates the viability of massive seeds, formed by the collapse of supermassive stars, as a progenitor model for such early, massive accreting black holes. Although considerable progress has been made in our theoretical understanding, many questions remain regarding how (and how often) such objects may form, how they live and die, and how next generation observatories may yield new insight into the origin of these primordial titans. This review focusses on our present understanding of this remarkable formation scenario, based on discussions held at the Monash Prato Centre from November 20--24, 2017, during the workshop "Titans of the Early Universe: The Origin of the First Supermassive Black Holes."

astro-ph.GA↗
arXiv2018

Fine-Tuning, Complexity, and Life in the Multiverse

The physical processes that determine the properties of our everyday world, and of the wider cosmos, are determined by some key numbers: the 'constants' of micro-physics and the parameters that describe the expanding universe in which we have emerged. We identify various steps in the emergence of stars, planets and life that are dependent on these fundamental numbers, and explore how these steps might have been changed, or completely prevented, if the numbers were different. We then outline some cosmological models where physical reality is vastly more extensive than the 'universe' that astronomers observe (perhaps even involving many 'big bangs'), which could perhaps encompass domains governed by different physics. Although the concept of a multiverse is still speculative, we argue that attempts to determine whether it exists constitute a genuinely scientific endeavor. If we indeed inhabit a multiverse, then we may have to accept that there can be no explanation other than anthropic reasoning for some features our world.

physics.hist-ph↗
arXiv2015

Extragalactic star-forming galaxies with hypernovae and supernovae as high-energy neutrino and gamma-ray sources: the case of the 10 TeV neutrino data

In light of the latest IceCube data, we discuss the implications of the cosmic ray energy input from hypernovae and supernovae into the Universe, and their propagation in the hosting galaxy and galaxy clusters or groups. The magnetic confinement in these environments may lead to efficient $pp$ collisions, resulting in a diffuse neutrino spectrum extending from PeV down to 10 TeV energies, with a spectrum and flux level compatible with that recently reported by IceCube. If the diffuse 10 TeV neutrino background largely comes from such the CR reservoirs, the corresponding diffuse gamma-ray background should be compatible with the recent \textit{Fermi} data. In this scenario, the CR energy input from hypernovae should be dominant over that of supernovae, implying that the starburst scenario does not work if the supernova energy budget is a factor of two larger than the hypernova energy budget. Thus, this strong case scenario can be supported or ruled out in near future.

astro-ph.HE↗
arXiv2007

Implications of very rapid TeV variability in blazars

We discuss the implications of rapid (few-minute) variability in the TeV flux of blazars, which has been observed recently with the HESS and MAGIC telescopes. The variability timescales seen in PKS 2155-304 and Mrk 501 are much shorter than inferred light-crossing times at the black hole horizon, suggesting that the variability involves enhanced emission in a small region within an outflowing jet. The enhancement could be triggered by dissipation in part of the black hole's magnetosphere at the base of the outflow, or else by instabilities in the jet itself. By considering the energetics of the observed flares, along with the requirement that TeV photons escape without producing pairs, we deduce that the bulk Lorentz factors in the jets must be >50. The distance of the emission region from the central black hole is less well-constrained. We discuss possible consequences for multi-wavelength observations.

astro-ph↗
arXiv2007

A new method of determining the initial size and Lorentz factor of gamma-ray burst fireballs using a thermal emission component

In recent years increasing evidence has emerged for a thermal component in the gamma- and X-ray spectrum of the prompt emission phase in gamma-ray bursts. The temperature and flux of the thermal component show a characteristic break in the temporal behavior after a few seconds. We show here, that measurements of the temperature and flux of the thermal component at early times (before the break) allow the determination of the values of two of the least restricted fireball model parameters: the size at the base of the flow and the outflow bulk Lorentz factor. Relying on the thermal emission component only, this measurement is insensitive to the inherent uncertainties of previous estimates of the bulk motion Lorentz factor. We give specific examples of the use of this method: for GRB970828 at redshift z=0.9578, we show that the physical size at the base of the flow is r_0 = (2.9+-1.8)*10^8 Y_0^{-3/2} cm and the Lorentz factor of the flow is Gamma = (305\+-28) Y_0^{1/4}, and for GRB990510 at z=1.619, r_0=(1.7+-1.7)*10^8 Y_0^{-3/2} cm and Gamma=(384+-71) Y_0^{1/4}, where Y = 1 Y_0 is the ratio between the total fireball energy and the energy emitted in gamma- rays.

astro-ph↗
arXiv2007

Massive Black Holes: formation and evolution

Supermassive black holes are nowadays believed to reside in most local galaxies. Observations have revealed us vast information on the population of local and distant black holes, but the detailed physical properties of these dark massive objects are still to be proven. Accretion of gas and black hole mergers play a fundamental role in determining the two parameters defining a black hole: mass and spin. We briefly review here the basic properties of the population of supermassive black holes, focusing on the still mysterious formation of the first massive black holes, and their evolution from early times to now.

astro-ph↗
arXiv2006

Radiation from an expanding cocoon as an explanation of the steep decay observed in GRB early afterglow light curves

Observations of early afterglow emission from gamma-ray bursts (GRB's) with the Swift satellite show steep decay of the X-ray light curve, F_ν(t) ~ t^{-α} with α~ 2.5 - 4 at ~300-500 s after the burst trigger. The spectrum in this time interval is consistent with a spectrum F_ν~ ν^{-β} with β~1. Here, we show that these results can be explained as due to emission from the hot plasma "cocoon" associated with the jet, which expands relativistically after the jet has broken through the stellar envelope, if a substantial fraction of the coccon kinetic energy is dissipated at scattering optical depths τ_T ~ 10^2-10^3. This results in the bulk of the coccon photons being observed at X-ray energies, after a delay of few hundreds of seconds relative to the gamma-ray photons from the jet. Multiple Compton scattering inside the cocoon causes a spread in the arrival times of the X-ray photons. We calculate numerically the observed light curve of photons emerging from the cocoon, and show that it exhibits a steep decay, which resembles that observed in many GRB afterglows. During the adiabatic expansion that follows the dissipation phase, photons lose energy to the expanding plasma, and as a result, the emerging photon energy distribution differs from a black-body spectrum, and can be approximated as a power law in the Swift XRT band. Comparison of the numerical results with the Swift XRT data of GRB050315 and GRB050421 shows good agreement between the light curves and spectra during the initial steep decay phase.

astro-ph↗
arXiv2006

Formation of Supermassive Black Holes by Direct Collapse in Pregalactic Halos

We describe a mechanism by which supermassive black holes can form directly in the nuclei of protogalaxies, without the need for seed black holes left over from early star formation. Self-gravitating gas in dark matter halos can lose angular momentum rapidly via runaway, global dynamical instabilities, the so-called "bars within bars" mechanism. This leads to the rapid buildup of a dense, self-gravitating core supported by gas pressure - surrounded by a radiation pressure-dominated envelope - which gradually contracts and is compressed further by subsequent infall. These conditions lead to such high temperatures in the central region that the gas cools catastrophically by thermal neutrino emission, leading to the formation and rapid growth of a central black hole. We estimate the initial mass and growth rate of the black hole for typical conditions in metal-free halos with T_vir ~ 10^4 K, which are the most likely to be susceptible to runaway infall. The initial black hole should have a mass of <~20 solar masses, but in principle could grow at a super-Eddington rate until it reaches ~ 10^4-10^6 solar masses. Rapid growth may be limited by feedback from the accretion process and/or disruption of the mass supply by star formation or halo mergers. Even if super-Eddington growth stops at \~10^3-10^4 solar masses, this process would give black holes ample time to attain quasar-size masses by a redshift of 6, and could also provide the seeds for all supermassive black holes seen in the present universe.

astro-ph↗
arXiv2006

The observable effects of a photospheric component on GRB's and XRF's prompt emission spectrum

A thermal radiative component is likely to accompany the first stages of the prompt emission of Gamma-ray bursts (GRB's) and X-ray flashes (XRF's). We analyze the effect of such a component on the observable spectrum, assuming that the observable effects are due to a dissipation process occurring below or near the thermal photosphere. We consider both the internal shock model and a 'slow heating' model as possible dissipation mechanisms. For comparable energy densities in the thermal and the leptonic component, the dominant emission mechanism is Compton scattering. This leads to a nearly flat energy spectrum (νF_ν\propto ν^0) above the thermal peak at ~10-100 keV and below 10-100 MeV, for a wide range of optical depths 0.03 <~ τ_{γe} <~ 100, regardless of the details of the dissipation mechanism or the strength of the magnetic field. At lower energies steep slopes are expected, while above 100 MeV the spectrum depends on the details of the dissipation process. For higher values of the optical depth, a Wien peak is formed at 100 keV - 1 MeV, and no higher energy component exists. For any value of τ_{γe}, the number of pairs produced does not exceed the baryon related electrons by a factor larger than a few. We conclude that dissipation near the thermal photosphere can naturally explain both the steep slopes observed at low energies and a flat spectrum above 10 keV, thus providing an alternative scenario to the optically thin synchrotron - SSC model.

astro-ph↗
arXiv2005

Neutron-loaded outflows in gamma-ray bursts

Relativistic neutron-loaded outflows in gamma-ray bursts are studied at their early stages, before deceleration by a surrounding medium. The outflow has four components: radiation, electrons, protons and neutrons. The components interact with each other and exchange energy as the outflow expands. The presence of neutrons significantly changes the outflow evolution. Before neutrons decouple from protons, friction between the two components increases their temperatures by many orders of magnitude. After the decoupling, the gradual neutron decay inside the outflow has a drag effect on the protons and reduces their final Lorentz factor.

astro-ph↗
arXiv2005

Possible evidence for the ejection of a supermassive black hole from an ongoing merger of galaxies

Attempts of Magain et al (2005) to detect the host galaxy of the bright QSO HE0450--2958 have not been successful. We suggest that the supermassive black hole powering the QSO was ejected from the observed ULIRG at the same redshift and at 1.5 arcsec distance. Ejection could have either be caused by recoil due to gravitational wave emission from a coalescing binary of supermassive black holes or the gravitational slingshot of three or more supermassive black holes in the ongoing merger of galaxies which triggered the starburst activity in the ULIRG. We discuss implications for the possible hierarchical build-up of supermassive black holes from intermediate and/or stellar mass black holes, and for the detection of coalescing supermassive binary black holes by LISA.

astro-ph↗
arXiv2005

Peak energy clustering and efficiency in compact objects

We study the properties of plasmas containing a low energy thermal photon component at comoving temperature θ\equiv kT'/m_e c^2 \sim 10^{-5} - 10^{-2} interacting with an energetic electron component, characteristic of, e.g., the dissipation phase of relativistic outflows in gamma-ray bursts (GRB's), X-ray flashes, and blazars. We show that, for scattering optical depths larger than a few, balance between Compton and inverse-Compton scattering leads to the accumulation of electrons at values of $γβ~ 0.15 - 0.3$. For optical depths larger than ~ 100, this leads to a peak in the comoving photon spectrum at 1-10 keV, very weakly dependent on the values of the free parameters. In particular, these results are applicable to the internal shock model of GRB, as well as to slow dissipation models, e.g. as might be expected from reconnection, if the dissipation occurs at a sub-photospheric radii. For GRB bulk Lorentz factors ~ 100, this results in observed spectral peaks clustering in the 0.1-1 MeV range, with conversion efficiencies of electron into photon energy in the BATSE range of ~ 30%.

astro-ph↗
arXiv2005

Rapid growth of high redshift black holes

We discuss a model for the early assembly of supermassive black holes (SMBHs) at the center of galaxies that trace their hierarchical build-up far up in the dark halo `merger tree'. Motivated by the observations of luminous quasars around redshift z=6 with SMBH masses of billion solar masses, we assess the possibility of an early phase of stable super-critical quasi-spherical accretion in the BHs hosted by metal free halos with virial temperature larger than 10000 K. We assume that the first `seed' black holes formed with intermediate masses following the collapse of the first generation of stars, in mini-halos collapsing at z=20-30 from high peaks of density fluctuations. In high redshift halos with virial temperature larger than 10000 K, conditions exist for the formation of a fat disc of gas at T_gas=5000-10000 K. Cooling via hydrogen atomic lines is in fact effective in these comparatively massive halos. The cooling and collapse of an initially spherical configuration of gas leads to a rotationally supported disc at the center of the halo if baryons preserve their specific angular momentum during collapse. The conditions for the formation of the gas disc and accretion onto a central black holes out of this supply of gas are investigated, as well as the feedback of the emission onto the host and onto the intergalactic medium. We find that even a short phase of supercritical accretion eases the requirements set by the z=6 quasars.

astro-ph↗
arXiv2005

Cyclotron Maser Emission from Blazar Jets?

We consider the production of electron cyclotron maser emission by low-density, highly magnetized plasmas in relativistic jets. The population inversion required to drive cyclotron maser instability could occur in localized, transient sites where hydromagnetic instabilities, shocks, and/or turbulence lead to magnetic mirroring along current-carrying flux tubes. The maser is pumped as electrons are accelerated by the parallel electric field that develops as a result of the mirror. We estimate the maximum brightness temperatures that can be obtained in a single maser site and in an array of many masers operating simultaneously, under conditions likely to apply in blazar jets. Synchrotron absorption, by relativistic electrons within the jet, presents the largest obstacle to the escape of the maser radiation, and may render most of it invisible. However, we argue that a high brightness temperature could be produced in a thin boundary layer outside the synchrotron photosphere, perhaps in the shear layer along the wall of the jet. Induced Compton scattering provides additional constraints on the maximum brightness temperature of a masing jet. We suggest that recent observations of diffractive scintillation in the blazar J1819+3845, indicating intrinsic brightness temperatures greater than 10^{14} K at 5 GHz, may be explained in terms of cyclotron maser emission. High brightness temperature maser emission from blazar jets may extend to frequencies as high as ~100 GHz, with the maximum possible T_B scaling roughly as 1/frequency. Less massive relativistic jet sources, such as microquasars, are even better candidates for producing cyclotron maser emission, primarily in the infrared and optical bands.

astro-ph↗
arXiv2004

Have we detected one of the sources responsible for an early reionisation of the Universe?

In a recent paper Pell'o et al. have reported a candidate z=10 galaxy, A1835#1916, which was found in a near-infrared survey of the central regions of the gravitational lensing cluster A1835. If this detection is confirmed and the detection rate turns out to be typical, then the volume averaged ultraviolet emissivity must be rising rapidly with increasing redshift. For a magnification due to gravitational lensing by a factor M>25 estimated by Pell'o et al., the inferred star formation rate density at z=10 would be about one order of magnitude higher than estimates of the star formation rate density at z=6. Objects at z=10 would contribute substantially to the total source counts at 1.6 micronm and the estimated space density of sources may exceed the space density of dark matter haloes in a LCDM model. We therefore argue that if A1835#1916 is indeed at z=10 then either the magnification factor may have been overestimated or the galaxy has a top-heavy initial mass function. Sources with the ultraviolet flux and space density of A1835#1916 may be sufficient to reionise most of the diffuse hydrogen in the Universe at z=10. We further use a correlation between the equivalent width and the redshift of the Ly-alpha emission line with respect to the systemic redshift observed in Lyman break galaxies to obtain constraints on the ionisation state of the surrounding intergalactic medium (IGM) from the Gunn-Peterson absorption. These constraints also argue in favour of the surrounding IGM being partially or fully ionised.

astro-ph↗
arXiv2004

Dark Matter: Introduction

This short review was prepared as an introduction to the Royal Society's 'Dark Matter' conference. It addresses the embarrassing fact that 95% of the universe is unaccounted for. Favoured dark matter candidates are axions or weakly-interacting particles that have survived from the very early universe, but more exotic options cannot be excluded. Experimental searches are being made for the 'dark' particles but we have indirect clues to their nature too. Comparisons of data (from, eg, gravitational lensing) with numerical simulations of galaxy formation can constrain (eg) the particle velocities and collision cross sections. The mean cosmic density of dark matter (plus baryons) is now pinned down to be only about 30% of the critical density However, other recent evidence -- microwave background anisotropies, complemented by data on distant supernovae -- reveals that our universe actually is 'flat', and that its dominant ingredient (about 70% of the total mass-energy) is something quite unexpected -- 'dark energy' pervading all space, with negative pressure. We now confront two mysteries: (i) Why does the universe have three quite distinct basic ingredients -- baryons, dark matter and dark energy -- in the proportions (roughly) 5%, 25% and 70%? (ii) What are the (almost certainly profound) implications of the 'dark energy' for fundamental physics?

astro-ph↗
arXiv2004

Constraining Alternate Models of Black Holes: Type I X-ray Bursts on Accreting Fermion-Fermion and Boson-Fermion Stars

The existence of black holes remains open to doubt until other conceivable options are excluded. With this motivation, we consider a model of a compact star in which most of the mass consists of dark particles of some kind, and a small fraction of the mass is in the form of ordinary nucleonic gas. The gas does not interact with the dark matter other than via gravity, but collects at the center as a separate fermionic fluid component. Depending on whether the dark mass is made of fermions or bosons, the objects may be called fermion-fermion stars or boson-fermion stars, respectively. For appropriate choices of the mass of the dark matter particles, these objects are viable models of black hole candidates in X-ray binaries. We consider models with a dark mass of 10 solar masses and a range of gas mass from 10^{-6} to nearly one solar mass, and analyse the bursting properties of the models when they accrete gas. We show that all the models would experience thermonuclear Type I X-ray bursts at appropriate mass accretion rates. Since no Type I bursts have been reported from black hole candidates, the models are ruled out. The case for identifying black hole candidates in X-ray binaries as true black holes is thus strengthened.

astro-ph↗