Astro2020 APC White Paper
Studying black holes on horizon scales
with space-VLBI
Kari Haworth
1
,
∗
, Michael D. Johnson
1
,
2
,
∗
, Dominic W. Pesce
1
,
2
, Daniel C. M.
Palumbo
1
,
2
, Lindy Blackburn
1
,
2
, Kazunori Akiyama
2
,
3
,
4
,
5
, Don Boroson
6
, Kather-
ine L. Bouman
7
, Joseph R. Farah
1
,
2
,
8
, Vincent L. Fish
3
, Mareki Honma
10
,
11
,
Tomohisa Kawashima
5
, Motoki Kino
5
,
9
, Alexander Raymond
1
,
2
, Mark Silver
6
,
Jonathan Weintroub
1
,
2
, Maciek Wielgus
1
,
2
, Sheperd S. Doeleman
1
,
2
, José L.
Gómez
13
, Jens Kauffmann
3
, Garrett K. Keating
1
, Thomas P. Krichbaum
14
,
Laurent Loinard
18
,
19
, Gopal Narayanan
12
, Akihiro Doi
16
, David J. James
1
,
2
,
Daniel P. Marrone
15
, Yosuke Mizuno
17
, Hiroshi Nagai
5
1
Center for Astrophysics
|
Harvard & Smithsonian, 60 Gar-
den Street, Cambridge, MA 02138, USA
2
Black Hole Initiative at Harvard University, 20 Garden
Street, Cambridge, MA 02138, USA
3
Massachusetts Institute of Technology, Haystack Observa-
tory, 99 Millstone Road, Westford, MA 01886, USA
4
National Radio Astronomy Observatory, 520 Edgemont
Road, Charlottesville, VA 22903, USA
5
National Astronomical Observatory of Japan, 2-21-1 Osawa,
Mitaka, Tokyo 181-8588, Japan
6
Massachusetts Institute of Technology, Lincoln Laboratory,
244 Wood St, Lexington, MA 02421
7
California Institute of Technology, 1200 East California
Boulevard, Pasadena, CA 91125, USA
8
University of Massachusetts Boston, 100 William T, Mor-
rissey Blvd, Boston, MA 02125, USA
9
Kogakuin University of Technology & Engineering, Aca-
demic Support Center, 2665-1 Nakano, Hachioji, Tokyo 192-
0015, Japan
10
Mizusawa VLBI Observatory, National Astronomical Ob-
servatory of Japan, 2-12 Hoshigaoka, Mizusawa, Oshu, Iwate
023-0861, Japan
11
Department of Astronomical Science, The Graduate Uni-
versity for Advanced Studies (SOKENDAI), 2-21-1 Osawa,
Mitaka, Tokyo 181-8588, Japan
12
Department of Astronomy, University of Massachusetts,
01003, Amherst, MA, USA
13
Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la
Astronomía s/n, E-18008 Granada, Spain
14
Max-Planck-Institut für Radioastronomie, Auf dem Hügel
69, D-53121 Bonn, Germany
15
Steward Observatory and Department of Astronomy, Uni-
versity of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721,
USA
16
The Institute of Space and Astronautical Science, Japan
Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuou-ku,
Sagamihara, Kanagawa 252-5210, Japan
17
Institut für Theoretische Physik, Goethe Universität, Max-
von-Laue Str. 1, D-60438, Frankfurt am Main, Germany
18
Instituto de Radioastronomía y Astrofísica, Universidad
Nacional Autónoma de México, Morelia 58089, México
19
Instituto de Astronomía, Universidad Nacional Autónoma
de México, CdMx 04510, México
∗
kari.haworth@cfa.harvard.edu, mjohnson@cfa.harvard.edu
arXiv:1909.01405v1 [astro-ph.IM] 3 Sep 2019
1 Introduction
In 2019, after decades of effort by an international team, the Event Horizon Telescope (EHT)
Collaboration presented the first image of a black hole [11, 12, 13, 14, 15, 16]. The impact
of this release, both scientifically and among the public, was extraordinary and felt around
the globe. The capability to image black holes on event horizon scales enables entirely
new tests of General Relativity (GR) near a black hole and opens a direct window into
the astrophysical processes that drive accretion, flaring, and jet genesis. The EHT image,
revealing the supermassive black hole (SMBH) in M87, was captured using a global very-long-
baseline interferometry (VLBI) network operating at 230 GHz [12]. Taking the next steps
towards precise tests of GR and time-domain studies of accretion flows will require sharper
resolution, higher observing frequencies, and faster sampling of interferometric baselines.
The angular resolution of ground-based VLBI is approaching fundamental limits. In-
terferometer baseline lengths are currently limited to the diameter of the Earth, imposing
a corresponding resolution limit for a ground array of
∼
22
μ
as at an observing frequency
of 230 GHz. Observations at higher frequencies can improve the resolution but become in-
creasingly challenging because of strong atmospheric absorption and rapid phase variations,
severely limiting the number of suitable ground sites and the windows of simultaneous good
weather at many global locations.
The extension of the EHT into space with the addition of a single orbiting element would
circumvent these limitations and enable a wealth of new scientific possibilities:
•
The high time resolution afforded by the rapid
(
u,v
)
-filling will enable reconstructed
movies of black hole accretion flows.
•
The improved resolution will increase the number of spatially resolvable black hole
shadows to dozens, yielding a corresponding number of black hole mass measurements.
•
Sharper images will reveal turbulent plasma dynamics, allowing further study of the
crucial role magnetic fields play in black hole feeding and in jet launching.
The highest operating frequency of space-VLBI to date is 25 GHz (with RadioAstron),
and a number of technical challenges must be overcome to access significantly higher frequen-
cies. These challenges would be mitigated by anchoring an orbiting element to the highly
sensitive elements in the EHT such as ALMA, the LMT, and NOEMA, permitting the use
of a modest aperture (of
∼
3
-meter size) in space and reducing costs. In this white paper,
we present a conceptual mission design for a first such submillimeter space mission, which
we expect to fall within the medium cost category of the Astro2020 survey. A companion
white paper details the concurrent expansion of the EHT ground array.
2 Key science goals
We review the key science drivers for submillimeter space-VLBI. For additional details, see
[27], [17], and [33].
1