18639894
Nonconservation of energy-momentum tensor in classical Liouville theory
where Ah is the area of the horizon. In this context, the classical Liouville theory [4] has gained considerable importance owing to the work of Solodukhin [5]. He showed that the computation of the black hole entropy can be carried out at the classical level. He observed that the 4-dimensional Einstein-Hilbert action can be reduced to an effective 2-dimensional theory, with fixed boundary conditions on the horizon, by performing a dimensional reduction with the ansatz of spherical symmetry and writing the metric in conformally flat form. This action can then be transformed to a form similar to the classical Liouville theory which in turn admits a Virasoro algebra near the horizon with a nontrivial classical central charge. The computation of this central charge then allows to determine the black hole entropy from the Cardy formula [6]. This approach was then used to study thermodynamic properties of black hole with a bifurcate killing horizon [7]. However, it is a well known fact from general field theoretic considerations [8, 9] that a central charge is related to diffeomorphism anomaly manifested as a nonconservation of the energy-momentum tensor. This relation is discussed from quantum considerations. The central charge and the diffeomorphism anomaly reveal different manifestations of the same phenomenon – a breakdown of classical symmetry upon the process of quantisation. Since the central charge is obtained [5, 7] in a classical Liouville theory, a natural question that arises is whether, in such a theory, there is a diffeomorphism anomaly. The principal objective of this paper is to show that this is indeed true. In this paper, we show explicitly that if we consider terms in the expansion of the metric, near the horizon, beyond leading order, then it leads to a diffeomorphism anomaly which is compatible with the fact that the theory has a nontrivial classical central charge. The computation of this anomaly has in fact been done to all orders. To begin with, we consider a four-dimensional spherically symmetric metric of the form
46389231
Quasilocal conserved charges in a covariant theory of gravity.
In any generally covariant theory of gravity, we show the relationship between the linearized asymptotically conserved current and its nonlinear completion through the identically conserved current. Our formulation for conserved charges is based on the Lagrangian description, and so completely covariant. By using this result, we give a prescription to define quasilocal conserved charges in any higher derivative gravity. As applications of our approach, we demonstrate the angular momentum invariance along the radial direction of black holes and reproduce more efficiently the linearized potential on the asymptotic anti-de Sitter space.
119234273
Hawking radiation and near horizon universality of chiral Virasoro algebra
We show that the diffeomorphism anomaly together with the trace anomaly reveal a chiral Virasoro algebra near the event horizon of a black hole. This algebra is the same irrespective of whether the anomaly is covariant or consistent, thereby manifesting its universal character and the fact that only the outgoing modes are relevant near the horizon. Our analysis therefore clarifies the role of the trace anomaly in the diffeomorphism anomaly approach [Robinson and Wilczek in Phys. Rev. Lett. 95:011303, 2005; Iso et al. in Phys. Rev. Lett. 96:151302, 2006; Banerjee and Kulkarni in Phys. Rev. D 77:024018, 2008; Gangopadhyay and Kulkarni in Phys. Rev. D 77:024038, 2008] to the Hawking radiation.
119246679
Modified dispersion relations and the response of the rotating Unruh-DeWitt detector
We study the response of a rotating monopole detector that is coupled to a massless scalar field which is described by a nonlinear dispersion relation in flat spacetime. Since it does not seem to be possible to evaluate the response of the rotating detector analytically, we resort to numerical computations. Interestingly, unlike the case of the uniformly accelerated detector that has been considered recently, we find that defining the transition probability rate of the rotating detector poses no difficulties. Further, we show that the response of the rotating detector can be computed exactly (albeit, numerically) even when it is coupled to a field that is governed by a nonlinear dispersion relation. We also discuss the response of the rotating detector in the presence of a cylindrical boundary on which the scalar field is constrained to vanish. While superluminal dispersion relations hardly affect the standard results, we find that subluminal dispersion relations can lead to relatively large modifications.
15809457
Hawking radiation, effective actions and covariant boundary conditions
From an appropriate expression for the effective action, the Hawking radiation from charged black holes is derived, using only covariant boundary conditions at the event horizon. The connection of our approach with the Unruh vacuum and the recent analysis [S.P. Robinson, F. Wilczek, Phys. Rev. Lett. 95 (2005) 011303, gr-qc/0502074; S. Iso, H. Umetsu, F. Wilczek, Phys. Rev. Lett. 96 (2006) 151302, hep-th/0602146; R. Banerjee, S. Kulkarni, arXiv: 0707.2449 [hep-th]] of Hawking radiation using anomalies is established.
13913623
Hawking fluxes, back reaction and covariant anomalies
Starting from the chiral covariant effective action approach of Banerjee and Kulkarni (2008 Phys. Lett. B 659 827), we provide a derivation of the Hawking radiation from a charged black hole in the presence of gravitational back reaction. The modified expressions for charge and energy flux, due to the effect of one-loop back reaction are obtained.
211130151
A VLA SEARCH FOR 5GHZ RADIO TRANSIENTS AND VARIABLES AT LOW GALACTIC LATITUDES
We present the results of a 5GHz survey with the Very Large Array (VLA) and the expanded VLA, designed to search for short-lived (<∼ 1 day) transients and to characterize the variability of radio sources at milli-Jansky levels. A total sky area of 2.66 deg, spread over 141 fields at low Galactic latitudes (b ∼= 6–8deg) was observed 16 times with a cadence that was chosen to sample timescales of days, months and years. Most of the data were reduced, analyzed and searched for transients in near real time. Interesting candidates were followed up using visible light telescopes (typical delays of 1–2hr) and the X-Ray Telescope on board the Swift satellite. The final processing of the data revealed a single possible transient with a flux density of fν ∼= 2.4mJy. This implies a transients sky surface density of κ(fν > 1.8mJy) = 0.039 +0.13,+0.18 −0.032,−0.038 deg −2 (1, 2-σ confidence errors). This areal density is consistent with the sky surface density of transients from the Bower et al. survey extrapolated to 1.8mJy. Our observed transient areal density is consistent with a Neutron Stars (NSs) origin for these events. Furthermore, we use the data to measure the sources variability on days to years time scales, and we present the variability structure function of 5GHz sources. The mean structure function shows a fast increase on ≈ 1 day time scale, followed by a slower increase on time scales of up to 10 days. On time scales between 10–60days the structure function is roughly constant. We find that >∼ 30% of the unresolved sources brighter than 1.8mJy are variable at the > 4-σ confidence level, presumably due mainly to refractive scintillation. Subject headings: radio continuum: general — stars: neutron — techniques: photometric
17818272
Dynamical Masses of the Binary Brown Dwarf GJ 569 Bab
We have obtained new images and high-resolution (R ~ 22,400) near-infrared (1.2400-1.2575 μm) spectra of each component of the brown dwarf binary GJ 569 Bab using the adaptive optics facility of the Keck II telescope and the NIRSPEC spectrometer. These data have allowed us to improve the determination of the astrometric orbit and to measure radial velocities of the components. We have used the astrometric and spectroscopic measurements to derive the dynamical mass of each brown dwarf and the systemic velocity of the pair by means of a χ2 fitting technique. From various considerations, the mass of each component is likely in the range 0.034-0.070 M☉ (GJ 569 Bb) and 0.055-0.087 M☉ (GJ 569 Ba). This implies that the mass ratio q of the binary is greater than 0.4, the most likely value being q = 0.75-0.85. Adopting 0.072 M☉ as the most conservative location of the substellar limit for solar metallicity, our analysis confirms GJ 569 Bb as the first genuine brown dwarf known without any theoretical assumptions. We have compared the dynamical masses of GJ 569 Ba and Bb, and their effective temperatures and luminosities, to the predictions of state-of-the-art theoretical evolutionary isochrones, finding that models exhibit good performance in the regime of high substellar masses if the binary is about a few hundred million years old. However, the surface gravities of GJ 569 Ba (M8.5 V) and Bb (M9 V) derived from our spectral analysis (the observed data have been compared to the latest synthetic spectra) appear to be smaller than the values provided by the evolutionary models.
119342715
An X-Ray Search for Compact Central Sources in Supernova Remnants. I. SNRS G093.3+6.9, G315.4–2.3, G084.2+0.8, and G127.1+0.5
Most astronomers now accept that stars more massive than about 9?M? explode as supernovae and leave stellar remnants, either neutron stars or black holes, with neutron stars being more prevalent. Recent modeling of the explosions suggests a significant diversity in the key natal properties?rotation rate, velocity, and magnetic field strength?of the resulting neutron stars that account for the association of active radio pulsars, pulsar wind nebulae, and magnetars with supernova remnants (SNRs). The discovery of a central X-ray source in Cas?A, the youngest known Galactic SNR, dramatized the expected diversity. However, less than half of the SNRs within 5 kpc have identified central sources, and only three are identified as the remnants of Type?Ia SNe. Here we report a systematic effort to search for compact central sources in the remaining 23 SNRs of this distance limited sample. Our search was inspired, on empirical considerations, by the enigmatic faint X-ray source in Cas?A; motivated, on theoretical grounds, by the expectation that young neutron stars emit cooling X-ray emission; and made possible by the superb angular resolution offered by the Chandra X-ray mission and the sensitivity of the XMM-Newton mission.?????In this first paper we report Chandra observations of four SNRs (G093.3+6.9, G315.4-2.3, G084.2+0.8, and G127.1+0.5). We have undertaken a systematic optical/IR identification program of the X-ray sources detected in the field of each SNR. Foreground (flare stars, active stars) and background (active galactic nuclei) sources have identifiable IR/optical counterparts. In contrast, the counterparts of neutron stars (or black holes) are expected to be very faint. We are able to account for all the well-detected X-ray sources and thus able to state with some confidence that there are no associated central sources down to a level of one-tenth of that of the Cas?A central source, LX 1031 ergs s-1. We compare our limits with cooling curves for neutron stars and find that any putative neutron stars in these SNRs must be cooling faster than expected for traditional 1.35 M? neutron stars and that any putative pulsar must have low spin-down luminosities 1034 ergs s-1. However, our limits are unable to constrain the presence or absence of more unusual options, such as relatively more massive neutron stars with M 1.45 M?, neutron stars with exotic interiors, or quiescent black holes. In subsequent papers, we will report on the X-ray and optical/IR observations of the remaining members of the 5 kpc sample.
15887815
Optical Astrometry of Accreting Black Holes and Neutron Stars: Scientific Opportunities
The extreme conditions found near black holes and neutron stars provide a unique opportunity for testing physical theories. Observations of both types of compact objects can be used to probe regions of strong gravity, allowing for tests of General Relativity. Furthermore, a determination of the properties of matter at the remarkably high densities that exist within neutron stars would have important implications for nuclear and particle physics. While many of these objects are in binary systems ("X-ray binaries"), where accreting matter from the stellar companion provides a probe of the compact object, a main difficulty in making measurements that lead to definitive tests has been uncertainty about basic information such as distances to sources, orientation of their binary orbits, and masses of the compact objects. Optical astrometry at the microarcsecond level will allow for accurate determinations of distances and proper motions with precisions of a few percent and will provide an opportunity to directly map X-ray binary orbits, leading to measurements of orbital inclination and compact object masses accurate to better than 4%. Using astrometry at this unprecedented accuracy, which would be enabled by the Space Interferometry Mission ("SIM Lite"), will lead to breakthroughs in the investigations of the physics of black holes and neutron stars
14455435
EVIDENCE FOR A COMPACT WOLF-RAYET PROGENITOR FOR THE TYPE Ic SUPERNOVA PTF 10vgv
We present the discovery of PTF 10vgv, a Type Ic supernova (SN) detected by the Palomar Transient Factory, using the Palomar 48 inch telescope (P48). R-band observations of the PTF 10vgv field with P48 probe the SN emission from its very early phases (about two weeks before R-band maximum) and set limits on its flux in the week prior to the discovery. Our sensitive upper limits and early detections constrain the post-shock-breakout luminosity of this event. Via comparison to numerical (analytical) models, we derive an upper-limit of R ≾ 4.5 R_☉ (R ≾ 1 R_☉) on the radius of the progenitor star, a direct indication in favor of a compact Wolf-Rayet star. Applying a similar analysis to the historical observations of SN 1994I yields R ≾ 1/4 R_☉ for the progenitor radius of this SN.