ABSTRACT: Recent observations show that fast radio bursts (FRBs) are energetic but probably non-catastrophic events occurring at cosmological distances. The properties of their progenitors are largely unknown in spite of many attempts to determine them using the event rate, duration and energetics. Understanding the radiation mechanism for FRBs should provide the missing insights regarding their progenitors, which is investigated in this paper.
The high brightness temperatures (>1035 K) of FRBs mean that the emission process must be coherent. Two general classes of coherent radiation mechanisms are considered — maser and the antenna mechanism. We use the observed properties of the repeater FRB 121102 to constrain the plasma conditions needed for these two mechanisms. We have looked into a wide variety of maser mechanisms operating in either vacuum or plasma and find that none of them can explain the high luminosity of FRBs without invoking unrealistic or fine-tuned plasma conditions. The most favorable mechanism is antenna curvature emission by coherent charge bunches where the burst is powered by magnetic reconnection near the surface of a magnetar (B > 1014 G). We show that the plasma in the twisted magnetosphere of a magnetar may be clumpy due to two-stream instability. When magnetic reconnection occurs, the pre-existing density clumps may provide charge bunches for the antenna mechanism to operate. This model should be applicable to all FRBs that have multiple outbursts like FRB 121102.
ABSTRACT: Achieving relativistic flight to enable extrasolar exploration is one of the dreams of humanity and the long term goal of our NASA Starlight program. We derive a fully relativistic solution for the motion of a spacecraft propelled by radiation pressure from a directed energy system. Depending on the system parameters, low mass spacecraft can achieve relativistic speeds; thereby enabling interstellar exploration. The diffraction of the directed energy system plays an important role and limits the maximum speed of the spacecraft. We consider ‘photon recycling’ as a possible method to achieving higher speeds. We also discuss recent claims that our previous work on this topic is incorrect and show that these claims arise from an improper treatment of causality.
ABSTRACT: We present optical spectroscopy of the recently discovered hyperbolic near-Earth object A/2017 U1, taken on 25 Oct 2017 at Palomar Observatory. Although our data are at a very low signal-to-noise, they indicate a very red surface at optical wavelengths without significant absorption features.
Learning from few examples and generalizing to dramatically different situations are capabilities of human visual intelligence that are yet to be matched by leading machine learning models. By drawing inspiration from systems neuroscience, we introduce a probabilistic generative model for vision in which message-passing based inference handles recognition, segmentation and reasoning in a unified way. The model demonstrates excellent generalization and occlusion-reasoning capabilities, and outperforms deep neural networks on a challenging scene text recognition benchmark while being 300-fold more data efficient. In addition, the model fundamentally breaks the defense of modern text-based CAPTCHAs by generatively segmenting characters without CAPTCHA-specific heuristics. Our model emphasizes aspects like data efficiency and compositionality that may be important in the path toward general artificial intelligence.
The ability to learn and generalize from a few examples is a hallmark of human intelligence. CAPTCHAs, images used by websites to block automated interactions, are examples of problems that are easy for humans but difficult for computers. CAPTCHAs are hard for algorithms because they add clutter and crowd letters together to create a chicken-and-egg problem for character classifiers — the classifiers work well for characters that have been segmented out, but segmenting the individual characters requires an understanding of the characters, each of which might be rendered in a combinatorial number of ways. A recent deep-learning approach for parsing one specific CAPTCHA style required millions of labeled examples from it, and earlier approaches mostly relied on hand-crafted style-specific heuristics to segment out the character; whereas humans can solve new styles without explicit training.
Around the world, telescopes are swiveling to welcome, and then wave farewell to, a new guest to the solar system: a fast-moving asteroid, or potentially a comet. It could be the first interstellar object to visit the solar system that has been detected and observed by astronomers, NASA announced yesterday.
Discovered on 19 October at the University of Hawaii’s Pan-STARRS 1 telescope on Haleakalā, the object, temporarily dubbed “A/2017 U1,” is 400 meters in diameter and moving quickly. It was first detected by Rob Weryk, an astronomer at the University of Hawaii (UH) in Honolulu, and confirmed by the European Space Agency’s telescope on Tenerife in the Canary Islands.
The object’s incoming motion—25.5 kilometers per second—was so extreme that astronomers believe it is not the kind of asteroid or comet typically seen inside the solar system.
“This is the most extreme orbit I have ever seen,” said Davide Farnocchia, a trajectory expert at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, in a release. “It is going extremely fast and on such a trajectory that we can say with confidence that this object is on its way out of the solar system and not coming back.”
Based on its current trajectory, the visitor came from the constellation Lyra and approached our solar system from “above,” perpendicular to the plane that most planets orbit the sun. (NASA has prepared this nice visualization of the object’s path.) On 9 September, it made its closest approach to the sun, with gravity then tugging it on a route “under” the solar system. On 14 October, it made its closest pass by Earth, at 60 times the distance to the moon. It is now looping back above the planetary plane and, traveling at 44 kilometers per second, is shooting toward the constellation Pegasus.
It’s no surprise that such a space rock, or comet, exists—scientists expect such grist to be wobbling around the galaxy, the ejected remnants of planetary formation. More observations are needed, and coming, to confirm its origins. Ultimately, the visitor will need a name, and rules do not yet exist for naming such extra–solar system guests.
“We have long suspected that these objects should exist, because during the process of planet formation a lot of material should be ejected from planetary systems. What’s most surprising is that we’ve never seen interstellar objects pass through before,” said Karen Meech, an astronomer at UH’s Institute for Astronomy in Honolulu specializing in small bodies and their connection to solar system formation.
Because this is the first object of its type ever discovered, rules for naming this type of object will need to be established by the International Astronomical Union.
“We have been waiting for this day for decades,” said JPL’s Center for Near-Earth Object Studies Manager Paul Chodas. “It’s long been theorized that such objects exist—asteroids or comets moving around between the stars and occasionally passing through our solar system—but this is the first such detection. So far, everything indicates this is likely an interstellar object, but more data would help to confirm it.”
When it comes to measuring how round the electron is, physicists hate uncertainty. Much depends on the most precise measurement possible, including a potential answer to a major scientific puzzle: why the universe contains any matter at all.
In a series of ever-more-sensitive experiments over the past 30 years, researchers have established that if the shape of the electron has any distortion at all, the bulge must be smaller than 1 thousand trillion trillionths of a millimeter (10-27 mm). Now, a group at the JILA research institute in Boulder, Colorado, has demonstrated what it describes as a “radically different” approach that probes electrons inside larger charged particles. Ed Hinds of Imperial College London calls the approach “brilliant” for the field, because it promises to help reduce the uncertainty still further—and perhaps reveal an actual distortion.
The electron’s egg shape, if real, would be quantified by what is known as the electric dipole moment (EDM). Whereas scientists usually think of the electron as an exceedingly, if not infinitely, small and uniform sphere of negative charge, a nonzero EDM would mean that charge is distributed unevenly—forming one region fractionally more negative than the particle’s average charge and one slightly less negative.
This tiny spatial asymmetry would have far-reaching implications, because it would contradict the idea that all physical processes look the same whether time runs forward or backward. Whereas time reversal would flip the direction of another property of the electron, its magnetic spin, it would leave any EDM unaffected, changing the relationship between the two. This breakdown in time-reversal symmetry would, in turn, “blow a hole” in particle physicists’ simplest model of particles and forces, Hinds says. Instead, he adds, it would require a model in which nature contains many more fundamental particles than have been seen to date. It would also imply a fundamental asymmetry between matter and antimatter that would go some way toward explaining why the universe today contains far more matter than antimatter, even though equal amounts of each should have been made in the big bang.
According to David Weiss, an atomic physicist at Pennsylvania State University (Penn State) in State College, the cosmic matter excess implies that the electron’s EDM is “very likely” to exist. And although the size of the EDM remains unknown, the most popular theories predict it is big enough to detect.
Because an EDM would cause an electron—or, more precisely, its spin axis—to rotate when placed in an electric field, simply sticking an electron between positive and negative electrodes should reveal it, in principle. But the resulting rotational force would be extremely weak—so weak that the electron would barely begin to turn before it crashed into the positive electrode. Scientists usually get around this problem by studying electrons within certain neutral atoms and molecules, in which internal fields far stronger than any external field can be induced. Researchers probe beams of these atoms or molecules for signs that certain electrons wobble, or precess—evidence of an EDM. But the motion of a beam limits the measurement time.
In the latest work, reported in Physical Review Letters this month, Eric Cornell and colleagues at JILA opt for an audacious alternative. Instead of probing a beam of neutral particles, they confine molecular ions of hafnium fluoride in a rotating electric field, which causes the ions to trace out little circles rather than flying away. After overcoming a few technical hurdles related to this circular motion, they tracked electrons’ spin precession over the course of 0.7 seconds—about 1000 times longer than was previously possible with beams, which should open the way to greater sensitivity.
Cornell’s group hasn’t yet improved on the best existing measurement of the electron’s sphericity, because the grouped ions disturb each other’s spin and limit the number the trap can contain. The team’s upper limit of 1.3 × 10-28 centimeters is some 1.5 times higher than the current best limit set with molecular beams, from the ACME collaboration at Harvard and Yale universities in 2014. (Harvard’s Gerald Gabrielse says that by next year, the ACME team could reduce the uncertainty by a further factor of 20.)
Last month, however, the JILA group started up a new version of its experiment with higher electric fields in order to trap more ions simultaneously. Combined with other “nickel and dime” improvements, Cornell says this could boost sensitivity by about a factor of 10 over the next couple years. Eventually, he adds, the group plans to start using thorium fluoride, which is harder to measure than hafnium fluoride, but whose greater stability offers even longer precession times.
Other groups are deploying new measurement strategies that could also crank up the sensitivity. Weiss and colleagues at Penn State aim for a 30-fold improvement over the ACME result by creating a trap with lasers rather than electric fields. They hope to confine and measure cold neutral cesium atoms for several seconds. Physicists at Imperial College, meanwhile, hope to study a fountain of laser-cooled ytterbium fluoride molecules, which could yield a 1000-fold gain in the next 5 years. If no asymmetry appears at that sensitivity level, says group leader Hinds, the team should be able to “rule out a whole range of theories” predicting an electron EDM. But that, he adds, “won’t stop theorists from coming up with new ideas.”
ABSTRACT: We describe the first precision measurement of the electron’s electric dipole moment (eEDM, de) using trapped molecular ions, demonstrating the application of spin interrogation times over 700 ms to achieve high sensitivity and stringent rejection of systematic errors. Through electron spin resonance spectroscopy on 180Hf19F+ in its metastable 3Δ1 electronic state, we obtain de = (0.9 ± 7.7stat ± 1.7syst) × 10-29 e cm, resulting in an upper bound of |de| < 1.3 × 10-28 e cm (90% confidence). Our result provides independent confirmation of the current upper bound of |de| < 9.3 × 10-29 e cm [J. Baron et al., Science 343, 269 (2014)], and offers the potential to improve on this limit in the near future.
ABSTRACT: IsolateIsolated Neutron Stars are some of the most exciting stellar objects known to astronomers: they have the most extreme magnetic fields, with values up to 1015 G, and, with the exception of stellar-mass black holes, they are the most dense stars, with densities of ≈ 1014 g/cm3. As such, they are perfect laboratories to test theories of electromagnetism and nuclear physics under conditions of magnetic field and density unattainable on Earth. In particular, the interaction of radiation with strong magnetic fields is the cause of the vacuum birefringence, an effect predicted by quantum electrodynamics in 1936 but that lacked an observational evidence until now. Here, we show how the study of the polarisation of the optical radiation from the surface of an isolated neutron star yielded such an observational evidence, opening exciting perspectives for similar studies at other wavelengths. Here, we show how the study of the polarisation of the optical radiation from the surface of an isolated neutron star yielded such an observational evidence, opening exciting perspectives for similar studies at other wavelengths.
ABSTRACT: We undertook observations with the Green Bank Telescope, simultaneously with the 300m telescope in Arecibo, as a follow-up of a possible flare of radio emission from Ross 128. We report here the non-detections from the GBT observations in C band (4-8 GHz), as well as non-detections in archival data at L band (1.1-1.9 GHz). We suggest that a likely scenario is that the emission comes from one or more satellites passing through the same region of the sky.
“At a size of 1000 – 10000 AU, the globule is far larger than known size of cometary knots in planetary nebulae. Regardless, the proposed analogy to cometary knots is only suggestive, and we advocate for a search for molecular gas associated with the globule via imaging in ro-vibrational lines.”
Note, that the intersteller asteroid A/2017 U1 comes from the same general direction. Sky & Telescope notes: “More intriguing is the fact that A/2017 U1 is coming from a spot only 6° from the solar apex, the direction that our Sun is moving (at about 20 km/s) through its interstellar neighborhood and thus, statistically, the most likely incoming direction for an interstellar visitor.“
ABSTRACT: The radio source J1819+3845 underwent a period of extreme interstellar scintillation between circa 1999 and 2007. The plasma structure responsible for this scintillation was determined to be just 1-3 pc from the solar system and to posses a density of ne ∼ 100 cm-3 that is three orders of magnitude higher than the ambient interstellar density (de Bruyn & Macquart 2015). Here we present radio-polarimetric images of the field towards J1819+3845 at wavelengths of 0.2, 0.92 and 2 m. We detect an elliptical plasma globule of approximate size 1∘ × ≳ 2∘ (major-axis position angle of ≈ -40∘), via its Faraday-rotation imprint (≈15 rad/m2) on the diffuse Galactic synchrotron emission. The extreme scintillation of J1819+3845 was most likely caused at the turbulent boundary of the globule (J1819+3845 is currently occulted by the globule). The origin and precise nature of the globule remain unknown. Our observations are the first time plasma structures that likely cause extreme scintillation have been directly imaged.
ABSTRACT: We examine the long-term evolution of the intra-hour variable quasar, J1819+3845, whose variations have been attributed to interstellar scintillation by extremely local turbulent plasma, located only 1-3 pc from Earth. The variations in this source ceased some time between June 2006 and February 2007. The evolution of the source spectrum and the long-term lightcurve, and the persistent compactness of the source VLBI structure indicates that the cessation of rapid variability was associated with the passage of the scattering material out of the line of sight to the quasar. We present an analysis of the linear polarization variations and their relation to total intensity variations. The proper motion of polarized features in the quasar jet is found to be subluminal. Systematic time delays between Stokes I, Q and U, in combination with the structure of the source obtained from 8.4 GHz VLBI data, confirm the estimate of the screen distance: 1-2 pc, making the screen one of the nearest objects to the Solar System.
We determine the physical properties of this scattering material. The electron density in the scattering region is extremely high with respect to the warm ionized ISM, with an estimated density of ne ∼ 97 l01/3 ΔL100-1/2 cm-3, where l0 is the outer scale of the turbulence in AU and ΔL = 100 ΔL100 AU is the depth of the scattering region. If this plasma is in pressure balance with the local magnetic field, one expects a ~2 rad/m2 rotation measure change associated with the passage of this material past the quasar. We examine the rotation measures of sources and the diffuse polarized emission in the surrounding region. We place a limit of 10 rad/m2 on the RM change. The variability of sources near J1819+3845 is used to deduce that the screen must therefore be either very small (~100 AU) or patchy.
ABSTRACT: Quasars shine brightly due to the liberation of gravitational energy as matter falls onto a supermassive black hole in the centre of a galaxy. Variations in the radiation received from active galactic nuclei (AGN) are studied at all wavelengths, revealing the tiny dimensions of the region and the processes of fuelling the black hole. Some AGN are variable at optical and shorter wavelengths, and display radio outbursts over years and decades. These AGN often also show faster variations at radio wavelengths (intraday variability, IDV) which have been the subject of much debate. The simplest explanation, supported by a correlation in some sources between the optical (intrinsic) and faster radio variations, is that the rapid radio variations are intrinsic. However, this explanation implies physically difficult brightness temperatures, suggesting that the variations may be due to scattering of the incident radiation in the interstellar medium of our Galaxy. Here we present results which show unambiguously that the variations in one extreme case are due to interstellar scintillation. We also measure the transverse velocity of the scattering material, revealing a surprising high velocity plasma close to the Solar System.
Hvad er massen af denne plasma globule? 1 parsec (pc) = 3.09×1018 cm. Hvis afstanden sættes til d = 2 pc, svarer 1.5∘ til afstanden (1.5/180)*pi*2*3.09e18 cm = 1.6×1017 cm vinkelret på retningen til globulen. Jeg vil nu antage, at globulen kan approximeres med en terning med denne sidelængde. Terningens rumfang er R = 1.6e17^3 = 4.1×1051 cm3. Der er lige så mange protoner som elektroner, idet jeg ser bort fra helium og tungere grundstoffer. Protonens masse er m = 1.67×10-27 kg. Globulens masse bliver tilnærmelsesvis 4.1e51*100*1.67e-27 = 6.8×1026 kg. Jordens masse er 5.97×1024 kg, så globulens masse er 6.8e26/5.97e24 = 114 jordmasser. Plasma globulens masse er altså ca. 100 jordmasser.
ABSTRACT: Gravitational-wave observation together with a large number of electromagnetic observations shows that the source of the latest gravitational-wave event, GW170817, detected primarily by advanced LIGO, is the merger of a binary neutron star. We attempt to interpret this observational event based on our results of numerical-relativity simulations performed so far paying particular attention to the optical and infra-red observations. We finally reach a conclusion that this event is described consistently by the presence of a long-lived massive neutron star as the merger remnant, because (i) significant contamination by lanthanide elements along our line of sight to this source can be avoided by the strong neutrino irradiation from it and (ii) it could play a crucial role to produce an ejecta component of appreciable mass with fast motion in the post-merger phase. We also point out that (I) the neutron-star equation of state has to be sufficiently stiff (i.e., the maximum mass of cold spherical neutron stars, Mmax, has to be appreciably higher than 2M⊙ in order that a long-lived massive neutron star can be formed as the merger remnant for the binary systems of GW170817, for which the initial total mass is >~ 2.73M⊙ and (II) no detection of relativistic optical counterpart suggests a not-extremely high value of Mmax approximately as 2.15-2.25M⊙.