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Register a new userThe Next-Generation Very Large Array: Supermassive Black Hole Pairs and Binaries
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Sarah Burke-Spolaor
Publication date
2018
fields
Physics
and research's language is
English
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The Next-Generation Very Large Array (ngVLA) has the potential to be a workhorse for the discovery and study of paired supermassive black holes either at large separations (dual) or in tightly bound systems (binary). In this chapter, we outline the science case for the study of these supermassive pairs, and summarize discovery methods that can be used at radio wavelengths to discover them: including morphological, spectral, and time-domain identifications. One critical aspect of this work is that multi-messenger binary black hole studies may be possible with the ngVLA when combined with gravitational-wave searches using pulsar timing array techniques. However, long-baseline interferometery (>>1000 km) will make this possibility more likely by expanding the redshift range at which radio emission arising from two separate black holes may be resolved and studied.
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The next generation Very Large Array (ngVLA) is a transformational radio observatory being designed by the U.S. National Radio Astronomy Observatory (NRAO). It will provide order of magnitude improvements in sensitivity, resolution, and uv coverage over the current Jansky Very Large Array (VLA) at ~1.2-50 GHz and extend the frequency range up to 70-115 GHz. This document is a white paper written by members of the Canadian community for the 2020 Long Range Plan panel, which will be making recommendations on Canadas future directions in astronomy. Since Canadians have been historically major users of the VLA and have been valued partners with NRAO for ALMA, Canadas participation in ngVLA is welcome. Canadians have been actually involved in ngVLA discussions for the past five years, and have played leadership roles in the ngVLA Science and Technical Advisory Councils. Canadian technologies are also very attractive for the ngVLA, in particular our designs for radio antennas, receivers, correlates, and data archives, and our industrial capacities to realize them. Indeed, the Canadian designs for the ngVLA antennas and correlator/beamformer are presently the baseline models for the project. Given the size of Canadas radio community and earlier use of the VLA (and ALMA), we recommend Canadian participation in the ngVLA at the 7% level. Such participation would be significant enough to allow Canadian leadership in gVLAs construction and usage. Canadas participation in ngVLA should not preclude its participation in SKA; access to both facilities is necessary to meet Canadas radio astronomy needs. Indeed, ngVLA will fill the gap between those radio frequencies observable with the SKA and ALMA at high sensitivities and resolutions. Canadas partnership in ngVLA will give it access to cutting-edge facilities together covering approximately three orders of magnitude in frequency.
In this proceeding, we summarize the key science goals and reference design for a next-generation Very Large Array (ngVLA) that is envisaged to operate in the 2030s. The ngVLA is an interferometric array with more than 10 times the sensitivity and spatial resolution of the current VLA and ALMA, that will operate at frequencies spanning $sim 1.2 -116$ GHz, thus lending itself to be highly complementary to ALMA and the SKA1. As such, the ngVLA will tackle a broad range of outstanding questions in modern astronomy by simultaneously delivering the capability to: unveil the formation of Solar System analogues; probe the initial conditions for planetary systems and life with astrochemistry; characterize the assembly, structure, and evolution of galaxies from the first billion years to the present; use pulsars in the Galactic center as fundamental tests of gravity; and understand the formation and evolution of stellar and supermassive blackholes in the era of multi-messenger astronomy.
We have searched for continuous gravitational wave (CGW) signals produced by individually resolvable, circular supermassive black hole binaries (SMBHBs) in the latest EPTA dataset, which consists of ultra-precise timing data on 41 millisecond pulsars. We develop frequentist and Bayesian detection algorithms to search both for monochromatic and frequency-evolving systems. None of the adopted algorithms show evidence for the presence of such a CGW signal, indicating that the data are best described by pulsar and radiometer noise only. Depending on the adopted detection algorithm, the 95% upper limit on the sky-averaged strain amplitude lies in the range $6times 10^{-15}<A<1.5times10^{-14}$ at $5{rm nHz}<f<7{rm nHz}$. This limit varies by a factor of five, depending on the assumed source position, and the most constraining limit is achieved towards the positions of the most sensitive pulsars in the timing array. The most robust upper limit -- obtained via a full Bayesian analysis searching simultaneously over the signal and pulsar noise on the subset of ours six best pulsars -- is $Aapprox10^{-14}$. These limits, the most stringent to date at $f<10{rm nHz}$, exclude the presence of sub-centiparsec binaries with chirp mass $cal{M}_c>10^9$M$_odot$ out to a distance of about 25Mpc, and with $cal{M}_c>10^{10}$M$_odot$ out to a distance of about 1Gpc ($zapprox0.2$). We show that state-of-the-art SMBHB population models predict $<1%$ probability of detecting a CGW with the current EPTA dataset, consistent with the reported non-detection. We stress, however, that PTA limits on individual CGW have improved by almost an order of magnitude in the last five years. The continuing advances in pulsar timing data acquisition and analysis techniques will allow for strong astrophysical constraints on the population of nearby SMBHBs in the coming years.
The next-generation Very Large Array (ngVLA) is an astronomical observatory planned to operate at centimeter wavelengths (25 to 0.26 centimeters, corresponding to a frequency range extending from 1.2 GHz to 116 GHz). The observatory will be a synthesis radio telescope constituted of approximately 214 reflector antennas each of 18 meters diameter, operating in a phased or interferometric mode. We provide an overview of the current system design of the ngVLA. The concepts for major system elements such as the antenna, receiving electronics, and central signal processing are presented. We also describe the major development activities that are presently underway to advance the design.
We perform simulations of the capabilities of the next generation Very Large Array to image stellar radio photospheres. For very large (in angle) stars, such as red supergiants within a few hundred parsecs, good imaging fidelity results can be obtained on radio photospheric structures at 38 GHz employing standard techniques, such as disk model fitting and subtraction, with hundreds of resolution elements over the star, even with just the ngVLA-classic baselines to 1000 km. Using the ngVLA Rev B plus long baseline configuration (with baselines out to 9000 km, August 2018), we find for main sequence stars within $sim$ 10 pc, the photospheres can be easily resolved at 85 GHz, with accurate measures of the mean brightness and size, and possibly imaging large surface structures, as might occur on e.g., active M dwarf stars. For more distant main sequence stars, we find that measurements of sizes and brightnesses can be made using disk model fitting to the u,v-data down to stellar diameters $sim$ 0.4 mas in a few hours. This size would include M0 V stars to a distance of 15 pc, A0 V stars to 60 pc, and Red Giants to 2.4 kpc. Based on the Hipparcos catalog, we estimate that there are at least 10,000 stars that will be resolved by the ngVLA. While the vast majority of these (95%) are giants or supergiants, there are still over 500 main sequence stars that can be resolved, with $sim$ 50 to 150 in each spectral type (besides O stars). Note that these are lower limits, since radio photospheres can be larger than optical, and the Hipparcos catalog might not be complete. Our initial look into the Gaia catalog suggests these numbers might be pessimistic by a factor few.
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