National Autonomous University of Mexico

About: National Autonomous University of Mexico is a education organization based out in Mexico City, Distrito Federal, Mexico. It is known for research contribution in the topics: Population & Galaxy. The organization has 72868 authors who have published 127797 publications receiving 2285543 citations. The organization is also known as: UNAM & Universidad Nacional Autónoma de México.

Localization and broadband follow-up of the gravitational-wave transient GW150914

Abstract: A gravitational-wave (GW) transient was identified in data recorded by the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO) detectors on 2015 September 14. The event, initially designated G184098 and later given the name GW150914, is described in detail elsewhere. By prior arrangement, preliminary estimates of the time, significance, and sky location of the event were shared with 63 teams of observers covering radio, optical, near-infrared, X-ray, and gamma-ray wavelengths with ground- and space-based facilities. In this Letter we describe the low-latency analysis of the GW data and present the sky localization of the first observed compact binary merger. We summarize the follow-up observations reported by 25 teams via private Gamma-ray Coordinates Network circulars, giving an overview of the participating facilities, the GW sky localization coverage, the timeline, and depth of the observations. As this event turned out to be a binary black hole merger, there is little expectation of a detectable electromagnetic (EM) signature. Nevertheless, this first broadband campaign to search for a counterpart of an Advanced LIGO source represents a milestone and highlights the broad capabilities of the transient astronomy community and the observing strategies that have been developed to pursue neutron star binary merger events. Detailed investigations of the EM data and results of the EM follow-up campaign are being disseminated in papers by the individual teams.

The Karl G. Jansky Very Large Array Sky Survey (VLASS). Science Case and Survey Design

Abstract: The Very Large Array Sky Survey (VLASS) is a synoptic, all-sky radio sky survey with a unique combination of high angular resolution (≈2.5\"), sensitivity (a 1σ goal of 70 μJy/beam in the coadded data), full linear Stokes polarimetry, time domain coverage, and wide bandwidth (2-4 GHz). The first observations began in September 2017, and observing for the survey will finish in 2024. VLASS will use approximately 5500 hours of time on the Karl G. Jansky Very Large Array (VLA) to cover the whole sky visible to the VLA (Declination > −40∘), a total of 33,885 deg^2. The data will be taken in three epochs to allow the discovery of variable and transient radio sources. The survey is designed to engage radio astronomy experts, multi-wavelength astronomers, and citizen scientists alike. By utilizing an \"on the fly\" interferometry mode, the observing overheads are much reduced compared to a conventional pointed survey. In this paper, we present the science case and observational strategy for the survey, and also results from early survey observations.

AU-Scale Synchrotron Jets and Superluminal Ejecta in GRS 1915+105

Abstract: Radio imaging of the microquasar GRS 1915+105 with the Very Long Baseline Array (VLBA) over a range of wavelengths (13, 3.6, 2.0, and 0.7 cm), in different states of the black hole binary, always resolves the nucleus as a compact jet of length ~10λcm AU. The nucleus is best imaged at the shorter wavelengths, on scales of 2.5-7 AU (0.2-0.6 mas resolution). The brightness temperature of the core is TB ≥ 109 K, and its properties are better fitted by a conically expanding synchrotron jet model rather than a thermal jet. The nuclear jet varies in ~30 minutes during minor X-ray/radio outbursts and reestablishes within ~18 hr of a major outburst, indicating the robustness of the X-ray/radio (or disk/jet) system to disruption. At lower resolution (80-240 AU), more extended ejecta are imaged at ~500 AU separation from the stationary core. Time-lapse images clearly detect the superluminal motion of the ejecta in a few hours. The measured velocity is 1.5c ± 0.1c (D/12 kpc) for the approaching component and is consistent with ballistic motion of the ejecta from 500 AU outward, perhaps even since birth. The axis of the ejecta differs by ≤12° clockwise from the axis of the AU-scale jet, measured in the same observation. Both axes are stable in time (±5°), the AU scale for 2 yr and the large scale for over 4 yr. Astrometry over 2 yr relative to an extragalactic reference locates the black hole to ±1.5 mas, and its secular parallax due to Galactic rotation is 5.8 ± 1.5 mas yr-1, consistent with a distance of 12 kpc. Finally, a limit of ≤100 km s-1 is placed on its proper motion with respect to its neighborhood. Some accreting black holes of stellar mass (e.g., Cyg X-1, 1E 1740-2942, GRS 1758-258, GX 339-4) and supermassive black holes at the center of galaxies (e.g., Sgr A*) lack evidence of large flares and discrete transient ejecta but have compact radio cores with steady, flat-spectrum "plateau" states, like GRS 1915+105. To the present day GRS 1915+105 is the only system where both AU-scale steady jets and large-scale superluminal ejections have been unambiguously observed. Our observations suggest that the unresolved flat-spectrum radio cores of accreting black holes are compact quasi-continuous synchrotron jets.

Cryo-EM structure of the ribosome–SecYE complex in the membrane environment

Abstract: The ubiquitous SecY-Sec61 complex translocates nascent secretory proteins across cellular membranes and integrates membrane proteins into lipid bilayers. Several structures of mostly detergent-solubilized Sec complexes have been reported. Here we present a single-particle cryo-EM structure of the SecYEG complex in a membrane environment, bound to a translating ribosome, at subnanometer resolution. Using the SecYEG complex reconstituted in a so-called Nanodisc, we could trace the nascent polypeptide chain from the peptidyltransferase center into the membrane. The reconstruction allowed for the identification of ribosome-lipid interactions. The rRNA helix 59 (H59) directly contacts the lipid surface and appears to modulate the membrane in immediate vicinity to the proposed lateral gate of the protein-conducting channel (PCC). On the basis of our map and molecular dynamics simulations, we present a model of a signal anchor-gated PCC in the membrane.