Lorentz and Diffeomorphism Violations in Linearized Gravity [CL]


Lorentz and diffeomorphism violations are studied in linearized gravity using effective field theory. A classification of all gauge-invariant and gauge-violating terms is given. The exact covariant dispersion relation for gravitational modes involving operators of arbitrary mass dimension is constructed, and various special limits are discussed.

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A. Kostelecky and M. Mewes
Fri, 23 Feb 18

Comments: 17 pages

Searching for propeller-phase ULXs in the XMM-Newton Serendipitous Source Catalogue [HEAP]


We search for transient sources in a sample of ULXs from the 3XMM-DR4 release of the XMM-Newton Serendipitous Source Catalogue in order to find candidate neutron star ULXs alternating between an accreting state and the propeller regime, in which the luminosity drops dramatically. By examining their fluxes and flux upper limits, we identify five ULXs that demonstrate long-term variability of over an order of magnitude. Using Chandra and Swift data to further characterise their light curves, we find that two of these sources are detected only once and could be X-ray binaries in outburst that only briefly reach ULX luminosities. Two others are consistent with being super-Eddington accreting sources with high levels of inter-observation variability. One source, M51 ULX-4, demonstrates apparent bimodal flux behaviour that could indicate the propeller regime. It has a hard X-ray spectrum, but no significant pulsations in its timing data, although with an upper limit of 10% of the signal pulsed at ~1.5 Hz a pulsating ULX cannot be excluded, particularly if the pulsations are transient. By simulating XMM-Newton observations of a population of pulsating ULXs, we predict that there could be approximately 200 other bimodal ULXs that have not been observed sufficiently well by XMM-Newton to be identified as transient.

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H. Earnshaw, T. Roberts and R. Sathyaprakash
Fri, 23 Feb 18

Comments: 6 pages, 3 figures, 1 table. Accepted for publication by MNRAS

Shadows of spherically symmetric black holes and naked singularities [HEAP]


We compare shadows cast by Schwarzschild black holes with those produced by two classes of naked singularities that result from gravitational collapse of spherically symmetric matter. The latter models consist of an interior naked singularity spacetime restricted to radii $r\leq R_b$, matched to Schwarzschild spacetime outside the boundary radius $R_b$. While a black hole always has a photon sphere and always casts a shadow, we find that the naked singularity models have photon spheres only if a certain parameter $M_0$ that characterizes these models satisfies $M_0\geq 2/3$, or equivalently, if $R_b\leq 3M$, where $M$ is the total mass of the object. Such models do produce shadows. However, models with $M_0<2/3$ (or $R_b>3M$) have no photon sphere and do not produce a shadow. Instead, they produce an interesting “full-moon” image. These results imply that the presence of a shadow does not by itself prove that a compact object is necessarily a black hole. The object could be a naked singularity with $M_0\geq 2/3$, and we will need other observational clues to distinguish the two possibilities. On the other hand, the presence of a full-moon image would certainly rule out a black hole and might suggest a naked singularity with $M_0<2/3$. It would be worthwhile to generalize the present study, which is restricted to spherically symmetric models, to rotating black holes and naked singularities.

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R. Shaikh, P. Kocherlakota, R. Narayan, et. al.
Fri, 23 Feb 18

Comments: 24 pages, 9 figures

Broad absorption line symbiotic stars: highly ionized species in the fast outflow from MWC 560 [HEAP]


In symbiotic binaries, jets and disk winds may be integral to the physics of accretion onto white dwarfs from cool giants. The persistent outflow from symbiotic star MWC 560 (=V694 Mon) is known to manifest as low-ionization broad absorption lines (BALs), most prominently at the Balmer transitions, and as high-ionization BALs from metastable He I*. We report the detection of higher-ionization BALs from C IV, Si IV, N V, and He II in International Ultraviolet Explorer spectra obtained on 1990 April 29-30, when an optical outburst temporarily erased the obscuring ‘iron curtain’ of absorption troughs from Fe II and similar ions. The C IV and Si IV BALs reached maximum radial velocities at least 1000 km/s faster than contemporaneous Mg II and He II BALs; the same behaviors occur in the winds of quasars and cataclysmic variables. An iron curtain lifts to unveil high-ionization BALs during the P Cygni phase observed in some novae, suggesting by analogy a temporary switch in MWC 560 from persistent outflow to discrete mass ejection. At least three more symbiotic stars exhibit broad absorption with blue edges faster than 1500 km/s; exclusively high-ionization BALs have been reported in AS 304 (=V4018 Sgr), while instead transient Balmer BALs have been reported in Z And and CH Cyg. These BAL-producing fast outflows can have wider opening angles than has been previously supposed. BAL symbiotics are short-timescale laboratories for their giga-scale analogs, broad absorption line quasars (BALQSOs), which display a similarly wide range of ionization states in their winds.

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A. Lucy, C. Knigge and J. Sokoloski
Fri, 23 Feb 18

Comments: submitted to MNRAS; comments welcome; 8 pages, 3 figures, 2 tables, centered on A4 paper [for most 8.5×11 printers, set scaling to None/100% and leave auto-centering on]

The PoGO+ view on Crab off-pulse hard X-ray polarisation [HEAP]


The linear polarisation fraction and angle of the hard X-ray emission from the Crab provide unique insight into high energy radiation mechanisms, complementing the usual imaging, timing and spectroscopic approaches. Results have recently been presented by two missions operating in partially overlapping energy bands, PoGO+ (18-160 keV) and AstroSat CZTI (100-380 keV). We previously reported PoGO+ results on the polarisation parameters integrated across the light-curve and for the entire nebula-dominated off-pulse region. We now introduce finer phase binning, in light of the AstroSat CZTI claim that the polarisation fraction varies across the off-pulse region. Since both missions are operating in a regime where errors on the reconstructed polarisation parameters are non-Gaussian, we adopt a Bayesian approach to compare results from each mission. We find no statistically significant variation in off-pulse polarisation parameters, neither when considering the mission data separately nor when they are combined. This supports expectations from standard high-energy emission models.

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M. Chauvin, H. Floren, M. Friis, et. al.
Fri, 23 Feb 18

Comments: 6 pages, 3 figures, 5 tables. Accepted for publication in MNRAS Letters

Effects of Fall-Back Accretion on Proto-Magnetar Outflows in Gamma-Ray Bursts and Superluminous Supernovae [HEAP]


Rapidly spinning, strongly magnetized proto-neutron stars (“millisecond proto-magnetars”) are candidate central engines of long-duration gamma-ray bursts (GRB), superluminous supernovae (SLSNe), and binary neutron star mergers. Magnetar birth may be accompanied by the fall-back of stellar debris, lasting for seconds or longer following the explosion. Accretion alters the magnetar evolution by (1) providing an additional source of rotational energy (or a potential sink, if the propeller mechanism operates); (2) enhancing the spin-down luminosity above the dipole rate by compressing the magnetosphere and expanding the polar cap region of open magnetic field lines; (3) supplying an additional accretion-powered neutrino luminosity that sustains the wind baryon-loading, even after the magnetar’s internal neutrino luminosity has subsided. The more complex evolution of the jet power and magnetization of an accreting magnetar more readily accounts for the high 56Ni yields GRB SNe and irregular time evolution of some GRB light curves (e.g.~bursts with precursors followed by a long quiescent interval before the main emission episode). Additional baryon-loading from accretion-powered neutrino irradiation of the magnetar polar cap lengthens the timeframe over which the jet magnetization is in the requisite range sigma ~< 1e3 for efficient gamma-ray emission, thereby accommodating GRBs with ultra-long durations. Though accretion does not significantly raise the maximum energy budget from the limit of <~ few 1e52 ergs for an isolated magnetar, it greatly expands the range of magnetic field strengths and birth spin periods capable of powering GRB jets, reducing the differences between the magnetar properties normally invoked to explain GRBs versus SLSNe.

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B. Metzger, P. Beniamini and D. Giannios
Fri, 23 Feb 18

Comments: 18 pages, 13 figures, submitted to ApJ

The non-linear onset of neutrino-driven convection in two and three-dimensional core-collapse supernovae [SSA]


A toy model of the post-shock region of core-collapse supernovae is used to study the non-linear development of turbulent motions driven by convection in the presence of advection. Our numerical simulations indicate that buoyant perturbations of density are able to trigger self-sustained convection only when the instability is not linearly stabilized by advection. Large amplitude perturbations produced by strong shock oscillations or combustion inhomogeneities before the collapse of the progenitor are efficiently shredded through phase mixing and generate a turbulent cascade. Our model enables us to investigate several physical arguments that had been proposed to explain the impact of the dimensionality on the onset of explosions in global simulations of core-collapse supernovae. In three-dimensional simulations, we find that turbulent mixing and dissipation of the kinetic energy produce a significant increase of the heating which is barely seen in our two-dimensional models. These results suggest that the three-dimensional nature of convection may ease the onset of the explosion. Increasing the numerical resolution is found to be mostly promising to support explosions in 3D simulations that contain large amplitude perturbations.

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R. Kazeroni, B. Krueger, J. Guilet, et. al.
Fri, 23 Feb 18

Comments: 23 pages, 18 figures, submitted to MNRAS