Archil Kobakhidze, Cyril Lagger, Adrian Manning, Jason Yue

We investigate the properties of a stochastic gravitational wave background produced by a first-order electroweak phase transition in the regime of extreme supercooling. We study a scenario whereby the percolation temperature that signifies the completion of the transition, $T_p$, can be as low as a few MeV (nucleosynthesis temperature), while most of the true vacuum bubbles are formed much earlier at the nucleation temperature, $T_n\sim 50$ GeV. This implies that the gravitational wave spectrum is mainly produced by the collisions of large bubbles and characterised by a large amplitude and a peak frequency as low as $f \sim 10^{-9}-10^{-7}$ Hz. We show that such a scenario can occur in (but not limited to) a model based on a non-linear realisation of the electroweak gauge group, such that the Higgs vacuum configuration is altered by a cubic coupling. In order to carefully quantify the evolution of the phase transition of this model over such a wide temperature range, we go beyond the usual fast transition approximation, taking into account the expansion of the universe as well as the behaviour of the nucleation probability at low temperatures. Our computation shows that there exists a range of parameters for which the gravitational wave spectrum lies at the edge between the exclusion limits of current pulsar timing array experiments and the detection band of the future Square Kilometre Array observatory.

André G. C. Guerra, Frederico Francisco, Paulo J. S. Gil, Orfeu Bertolami

Residual accelerations due to thermal effects are estimated through a model of the New Horizons spacecraft and a Monte Carlo simulation. We also discuss and estimate the thermal effects on the attitude of the spacecraft. The work is based on a method previously used for the Pioneer and Cassini probes, which solve the Pioneer anomaly problem. The results indicate that after the encounter with Pluto there is a residual acceleration of the order of $10^{-9}~\mathrm{m/s^2}$, and that rotational effects should be difficult, although not impossible, to detect.

R. Genzel, N.M. Förster Schreiber, H. Übler, P. Lang, T. Naab, R. Bender, L.J. Tacconi, E. Wisnioski, S. Wuyts, T. Alexander, A. Beifiori, S. Belli, G. Brammer, A. Burkert, C.M. Carollo, J. Chan, R. Davies, M. Fossati, A. Galametz, S. Genel, O. Gerhard, D. Lutz, J.T. Mendel, I. Momcheva, E.J. Nelson, A. Renzini, R. Saglia, A. Sternberg, S. Tacchella, K. Tadaki, D. Wilman

In cold dark matter cosmology, the baryonic components of galaxies are thought to be mixed with and embedded in non-baryonic and non-relativistic dark matter, which dominates the total mass of the galaxy and its dark matter halo. In the local Universe, the mass of dark matter within a galactic disk increases with disk radius, becoming appreciable and then dominant in the outer, baryonic regions of the disks of star-forming galaxies. This results in rotation velocities of the visible matter within the disk that are constant or increasing with disk radius. Comparison between the dynamical mass and the sum of stellar and cold gas mass at the peak epoch of galaxy formation, inferred from ancillary data, suggest high baryon factions in the inner, star-forming regions of the disks. Although this implied baryon fraction may be larger than in the local Universe, the systematic uncertainties (stellar initial mass function, calibration of gas masses) render such comparisons inconclusive in terms of the mass of dark matter. Here we report rotation curves for the outer disks of six massive star-forming galaxies, and find that the rotation velocities are not constant, but decrease with radius. We propose that this trend arises because of two main factors: first, a large fraction of the massive, high-redshift galaxy population was strongly baryon dominated, with dark matter playing a smaller part than in the local Universe; and second, the large velocity dispersion in high-redshift disks introduces a substantial pressure term that leads to a decrease in rotation velocity with increasing radius. The effect of both factors appears to increase with redshift. Qualitatively, the observations suggest that baryons in the early Universe efficiently condensed at the centres of dark matter halos when gas fractions were high, and dark matter was less concentrated. [Abridged]

Pawan Kumar, Wenbin Lu, Mukul Bhattacharya

We use the observed properties of fast radio bursts (FRBs) and a number of general physical considerations to provide a broad-brush model for the physical properties of FRB sources and the radiation mechanism. We show that the magnetic field in the source region should be at least 10^{14} Gauss. This strong field is required to ensure that the electrons have sufficiently high ground state Landau energy so that particle collisions, instabilities, and strong electric and magnetic fields associated with the FRB radiation do not perturb electrons' motion in the direction transverse to the magnetic field and destroy their coherent motion; coherence is required by the high observed brightness temperature of FRB radiation. The electric field in the source region required to sustain particle motion for a wave period is estimated to be of order 10^{11} esu. These requirements suggest that FRBs are produced near the surface of magnetars perhaps via forced reconnection of magnetic fields to produce episodic, repeated, outbursts. The beaming-corrected energy release in these bursts is estimated to be ~10^{36} ergs, whereas the total energy in the magnetic field is at least ~10^{45} ergs. We provide a number of predictions for this model which can be tested by future observations. One of which is that short duration FRB-like bursts should exist at much higher frequencies, possibly up to optical.

ANTARES Collaboration, IceCube Collaboration, LIGO Scientific Collaboration, Virgo Collaboration

The Advanced LIGO observatories detected gravitational waves from two binary black hole mergers during their first observation run (O1). We present a high-energy neutrino follow-up search for the second gravitational wave event, GW151226, as well as for gravitational wave candidate LVT151012. We find 2 and 4 neutrino candidates detected by IceCube, and 1 and 0 detected by ANTARES, within $\pm500$ s around the respective gravitational wave signals, consistent with the expected background rate. None of these neutrino candidates are found to be directionally coincident with GW151226 or LVT151012. We use non-detection to constrain isotropic-equivalent high-energy neutrino emission from GW151226 adopting the GW event's 3D localization, to less than $2\times 10^{51}-2\times10^{54}$ erg.

Eli Waxman

We derive stringent constraints on the persistent source associated with FRB 121102: Size $10^{17}$ cm $<R<10^{18}$ cm, age $<300$ yr, characteristic electron energy $\varepsilon_e\sim0.3$ GeV, total energy $\sim10^{49}$ erg. The hot radiating plasma is confined by a cold plasma of mass $M_c<0.03 (R/10^{17.5}{\rm cm})^4 M_\odot$. The source is nearly resolved, and may be resolved by 10 GHz observations. The fact that $\varepsilon_e\sim m_p c^2$ suggests that the hot plasma was created by the ejection of a mildly relativistic, $M\sim10^{-5} M_\odot$ shell, which propagated into an extended ambient medium or collided with a pre-ejected shell of mass $M_c$. The inferred plasma properties are inconsistent with those expected for "magnetar wind nebulae". We suggest a physical mechanism for the generation of FRBs (independent of the persistent source model): Ejection from an underlying compact object, $R_s\sim10^{6}$ cm, of highly relativistic shells, with energy $E_s=10^{41}$ erg and Lorentz factor $\gamma_s$~$10^3$, into a surrounding e-p plasma with density $n\sim0.1/cm^3$ (consistent with that inferred for the plasma producing the persistent emission associated with FRB 121102). Such shell ejections with energy typical for FRB events lead to plasma conditions appropriate for strong synchrotron maser emission at the GHz range, $\nu_{ coh.}\sim0.5(E/10^{41}erg)^{1/4}$ GHz. In this model, a significant fraction of the deposited energy is converted to an FRB with duration $R_s/c$, accompanied by ~10 MeV photons carrying less energy than the FRB. The inferred energy and mass associated with the source are low compared to those of typical supernova ejecta. This may suggest some type of a "weak stellar explosion", where a neutron star is formed with relatively low mass and energy ejection. However, the current upper limit on R does not allow one to rule out $M_c\sim1M_\odot$.

R. Gilli, F. Calura, A. D'Ercole, C. Norman

We addressed the so far unexplored issue of outflows induced by exponentially growing power sources, focusing on early supermassive black holes (BHs). We assumed that these objects grow to $10^9\;M_{\odot}$ by z=6 by Eddington-limited accretion and convert 5% of their bolometric output into a wind. We first considered the case of energy-driven and momentum-driven outflows expanding in a region where the gas and total mass densities are uniform and equal to the average values in the Universe at $z>6$. We derived analytic solutions for the evolution of the outflow, finding that, for an exponentially growing power with e-folding time $t_{Sal}$, the late time expansion of the outflow radius is also exponential, with e-folding time of $5t_{Sal}$ and $4t_{Sal}$ in the energy-driven and momentum-driven limit, respectively. We then considered energy-driven outflows produced by QSOs at the center of early dark matter halos of different masses and powered by BHs growing from different seeds. We followed the evolution of the source power and of the gas and dark matter density profiles in the halos from the beginning of the accretion until $z=6$. The final bubble radius and velocity do not depend on the seed BH mass but are instead smaller for larger halo masses. At z=6, bubble radii in the range 50-180 kpc and velocities in the range 400-1000 km s$^{-1}$ are expected for QSOs hosted by halos in the mass range $3\times10^{11}-10^{13}\;M_{\odot}$. By the time the QSO is observed, we found that the total thermal energy injected within the bubble in the case of an energy-driven outflow is $E_{th}\sim5 \times 10^{60}$ erg. This is in excellent agreement with the value of $E_{th}=(6.2\pm 1.7)\times 10^{60}$ erg measured through the detection of the thermal Sunyaev-Zeldovich effect around a large population of luminous QSOs at lower redshift. [abridged]