4 research outputs found
Immigration and the US farm labour supply
This paper uses unique data from rural Mexico to examine the supply of immigrant hired labour to US farms. Econometric evidence indicates that immigration policy reforms had unintended consequences for farm labour supply. The long-term trend in migration from rural Mexico to US farms is decreasing, and in recent years, US farms have drawn more labour from remote and less developed areas of rural Mexico. Other high income countries, as well as some developing nations, mirror the US in reliance on foreign agricultural workers. Our analysis questions the sustainability of an agricultural system that depends on foreign sources of labour, and highlights the importance of labour productivity-enhancing technological change.Farm labour, Mexico-US migration, immigration reform.
Narrow-band search of continuous gravitational-wave signals from Crab and Vela pulsars in Virgo VSR4 data
In this paper we present the results of a coherent narrow-band search for continuous gravitational-wave signals from the Crab and Vela pulsars conducted on Virgo VSR4 data. In order to take into account a possible small mismatch between the gravitational-wave frequency and two times the star rotation frequency, inferred from measurement of the electromagnetic pulse rate, a range of 0.02 Hz around two times the star rotational frequency has been searched for both the pulsars. No evidence for a signal has been found and 95% confidence level upper limits have been computed assuming both that polarization parameters are completely unknown and that they are known with some uncertainty, as derived from x-ray observations of the pulsar wind torii. For Vela the upper limits are comparable to the spin-down limit, computed assuming that all the observed spin-down is due to the emission of gravitational waves. For Crab the upper limits are about a factor of 2 below the spin-down limit, and represent a significant improvement with respect to past analysis. This is the first time the spin-down limit is significantly overcome in a narrow-band search.by Anand Sengupta et al
Low-latency Gravitational-wave Alerts for Multimessenger Astronomy during the Second Advanced LIGO and Virgo Observing Run
Advanced LIGO's second observing run (O2), conducted from November 30, 2016
to August 25, 2017, combined with Advanced Virgo's first observations in August
2017 witnessed the birth of gravitational-wave multi-messenger astronomy. The
first ever gravitational-wave detection from the coalescence of two neutron
stars, GW170817, and its gamma-ray counterpart, GRB 170817A, led to an
electromagnetic follow-up of the event at an unprecedented scale. Several teams
from across the world searched for EM/neutrino counterparts to GW170817, paving
the way for the discovery of optical, X-ray, and radio counterparts. In this
article, we describe the online identification of gravitational-wave transients
and the distribution of gravitational-wave alerts by the LIGO and Virgo
collaborations during O2. We also describe the gravitational-wave observables
which were sent in the alerts to enable searches for their counterparts.
Finally, we give an overview of the online candidate alerts shared with
observing partners during O2. Alerts were issued for 14 candidates, six of
which have been confirmed as gravitational-wave events associated with the
merger of black holes or neutron stars. Eight of the 14 alerts were issued less
than an hour after data acquisition.Comment: 27 pages, 5 figures, 3 tables, accepted for publication in the
Astrophysical Journa