107 research outputs found
A first search of transients in the Galactic Center from 230 GHz ALMA observations
The Galactic Center (GC) presents one of the highest stellar densities in our
Galaxy, making its surroundings an environment potentially rich in radio
transients, such as pulsars and different kinds of flaring activity. In this
paper, we present the first study of transient activity in the region of the GC
based on Atacama Large Millimeter/submillimeter (mm/submm) Array (ALMA)
continuum observations at 230 GHz. This search is based on a new
self-calibration algorithm, especially designed for variability detection in
the GC field. Using this method, we have performed a search of radio transients
in the effective field of view of~arcseconds of the GC central
supermassive black hole Sagittarius A* (SgrA*) using ALMA 230 GHz observations
taken during the 2017 Event Horizon Telescope (EHT) campaign, which span
several observing hours (5-10) on 2017 April 6, 7, and 11. This calibration
method allows one to disentangle the variability of unresolved SgrA* from any
potential transient emission in the wider field of view and residual effects of
the imperfect data calibration. Hence, a robust statistical criterion to
identify real transients can be established: the event should survive at least
three times the correlation time and it must have a peak excursion of at least
seven times the instantaneous root-mean-square between consecutive images. Our
algorithms are successfully tested against realistic synthetic simulations of
transient sources in the GC field. Having checked the validity of the
statistical criterion, we provide upper limits for transient activity in the
effective field of view of the GC at 230 GHz.Comment: Accepted for publication in Astronomy and Astrophysic
Orbital configurations of spaceborne interferometers for studying photon rings of supermassive black holes
Recent advances in technology coupled with the progress of observational
radio astronomy methods resulted in achieving a major milestone of astrophysics
- a direct image of the shadow of a supermassive black hole, taken by the
Earth-based Event Horizon Telescope (EHT). The EHT was able to achieve a
resolution of 20 as, enabling it to resolve the shadows of the black
holes in the centres of two celestial objects: the supergiant elliptical galaxy
M87 and the Milky Way Galaxy. The EHT results mark the start of a new round of
development of next generation Very Long Baseline Interferometers (VLBI) which
will be able to operate at millimetre and sub-millimetre wavelengths. The
inclusion of baselines exceeding the diameter of the Earth and observation at
as short a wavelength as possible is imperative for further development of high
resolution astronomical observations. This can be achieved by a spaceborne VLBI
system. We consider the preliminary mission design of such a system,
specifically focused on the detection and analysis of photon rings, an
intrinsic feature of supermassive black holes. Optimised Earth, Sun-Earth L2
and Earth-Moon L2 orbit configurations for the space interferometer system are
presented, all of which provide an order of magnitude improvement in resolution
compared to the EHT. Such a space-borne interferometer would be able to conduct
a comprehensive survey of supermassive black holes in active galactic nuclei
and enable uniquely robust and accurate tests of strong gravity, through
detection of the photon ring features.Comment: Accepted for publication in Acta Astronautica. 40 pages, 13 figure
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