Broadband Multi-wavelength Properties of M87 during the
2017 Event Horizon Telescope Campaign
Abstract
In 2017, the Event Horizon Telescope
(
EHT
)
Collaboration succeeded in capturing the
fi
rst direct image of the
center of the M87 galaxy. The asymmetric ring morphology and size are consistent with theoretical expectations
for a weakly accreting supermassive black hole of mass
∼
6.5
×
10
9
M
e
. The EHTC also partnered with several
international facilities in space and on the ground, to arrange an extensive, quasi-simultaneous multi-wavelength
campaign. This Letter presents the results and analysis of this campaign, as well as the multi-wavelength data as a
legacy data repository. We captured M87 in a historically low state, and the core
fl
ux dominates over HST-1 at
high energies, making it possible to combine core
fl
ux constraints with the more spatially precise very long
baseline interferometry data. We present the most complete simultaneous multi-wavelength spectrum of the active
nucleus to date, and discuss the complexity and caveats of combining data from different spatial scales into one
broadband spectrum. We apply two heuristic, isotropic leptonic single-zone models to provide insight into the
basic source properties, but conclude that a structured jet is necessary to explain M87
’
s spectrum. We can exclude
that the simultaneous
γ
-ray emission is produced via inverse Compton emission in the same region producing the
The Astrophysical Journal Letters,
911:L11
(
43pp
)
, 2021 April 10
https:
//
doi.org
/
10.3847
/
2041-8213
/
abef71
© 2021. The Author
(
s
)
. Published by the American Astronomical Society.
J. C. Algaba
1
, J. Anczarski
2
, K. Asada
3
, M. Balokovi
ć
4
,
5
, S. Chandra
6
, Y.-Z. Cui
7
,
8
, A. D. Falcone
9
, M. Giroletti
10
,
C. Goddi
11
,
12
, K. Hada
7
,
8
, D. Haggard
13
,
14
, S. Jorstad
15
,
16
, A. Kaur
9
, T. Kawashima
17
, G. Keating
18
,
J.-Y. Kim
19
,
20
, M. Kino
21
,
22
, S. Komossa
20
, E. V. Kravchenko
10
,
23
,
24
, T. P. Krichbaum
20
, S.-S. Lee
19
, R.-S. Lu
(
路
如
森
)
20
,
25
,
26
, M. Lucchini
27
, S. Markoff
27
,
28
, J. Neilsen
2
, M. A. Nowak
29
, J. Park
30
,
31
,
244
, G. Principe
10
,
32
,
33
,
V. Ramakrishnan
34
, M. T. Reynolds
35
, M. Sasada
21
,
36
, S. S. Savchenko
37
,
38
, K. E. Williamson
15
(
The Event Horizon Telescope Collaboration
)
(
The Fermi Large Area Telescope Collaboration
)
(
H.E.S.S. Collaboration
)
(
MAGIC Collaboration
)
(
VERITAS Collaboration
)
(
EAVN Collaboration
)
(
See the end matter for the full list of authors including corresponding authors for the partner facilities.
)
Received 2020 December 25; revised 2021 March 14; accepted 2021 March 16; published 2021 April 14
239
NASA Hubble Fellowship Program, Einstein Fellow.
240
Now at University of Innsbruck.
241
Also at Port d
’
Informació Cientí
fi
ca
(
PIC
)
E-08193 Bellaterra
(
Barcelona
)
,
Spain.
242
Also at Dipartimento di Fisica, Università di Trieste, I-34127 Trieste, Italy.
243
Also at INAF Trieste and Dept. of Physics and Astronomy, University of
Bologna.
244
EACOA Fellow.
245
UKRI Stephen Hawking Fellow.
246
For questions concerning EHT results contact
ehtcollaboration@
gmail.com
.
*
H.E.S.S. corresponding author. For questions concerning H.E.S.S. results
contact
hess@hess-experiment.eu
.
†
MAGIC corresponding author. For questions concerning MAGIC results
contact
contact.magic@mpp.mpg.de
.
‡
VERITAS corresponding author. For questions concerning VERITAS results
contact
wjin4@crimson.ua.edu
,
jmsantander@ua.edu
.
Original content from this work may be used under the terms
of the
Creative Commons Attribution 3.0 licence
. Any further
distribution of this work must maintain attribution to the author
(
s
)
and the title
of the work, journal citation and DOI.
1
EHT mm-band emission, and further conclude that the
γ
-rays can only be produced in the inner jets
(
inward of HST-1
)
if there are
strongly particle-dominated regions. Direct synchrotron emission from accelerated protons and secondaries cannot yet be excluded.
Key words:
Active galactic nuclei
–
Radio cores
–
Low-luminosity active galactic nuclei
–
High energy
astrophysics
–
Astrophysical black holes
–
Accretion
1. Introduction
M87 is the most prominent elliptical galaxy within the Virgo
Cluster, located just 16.8
±
0.8 Mpc away
(
Blakeslee et al.
2009
;
Bird et al.
2010
; Cantiello et al.
2018
,andseealsoEHT
Collaboration et al.
2019f
)
. As one of our closest active galactic
nuclei
(
AGNs
)
, M87 also harbors the
fi
rst example of an
extragalactic jet to have been noticed by astronomers
(
Curtis
1918
)
,
well before these jets were unders
tood to be a likely signature of
black hole accretion. By now this famous one-sided jet has been
well-studied in almost every wave band from radio
(
down to sub-
parsec scales; e.g., Reid et al.
1989
; Junor et al.
1999
;Hadaetal.
2011
; Mertens et al.
2016
; Kim et al.
2018b
;Walkeretal.
2018
)
,
optical
(
e.g., Biretta et al.
1999
; Perlman et al.
2011
)
, X-ray
(
e.g.,
Marshall et al.
2002
;Sniosetal.
2019
)
,and
γ
-rays
(
e.g., Abdo et al.
2009a
;Abramowskietal.
2012
; MAGIC Collaboration et al.
2020
)
. Extending over 60 kpc in length, the jet shows a system of
multiple knot-like features, incl
uding an active feature HST-1 at a
projected distance of
∼
70 pc from the core, potentially marking the
end of the black hole
’
s sphere of gravitational in
fl
uence
(
Asada &
Nakamura
2012
)
. In contrast, the weakly accreting supermassive
black hole
(
SMBH
)
in the center of our own Milky Way galaxy,
Sgr A
*
, does not show obvious signs of extended jets, although
theory predicts that such out
fl
ows should be formed
(
e.g., Dibi
et al.
2012
;Mo
ś
cibrodzka et al.
2014
; Davelaar et al.
2018
)
.The
conditions under which such jet
s are launched is one of the
enduring questions in astrophysics today
(
e.g., Blandford &
Znajek
1977
; Blandford & Payne
1982
; Sikora & Begelman
2013
)
.
In 2019 April, the Event Horizon Telescope
(
EHT
)
Collabora-
tion presented the
fi
rst direct image of an SMBH
“
shadow
”
in the
center of M87
(
EHT Collaboration et al.
2019a
,
2019b
,
2019c
,
2019d
,
2019e
,
2019f
)
. The key result in these papers
was the detection of an asymmetric ring
(
crescent
)
of light around
a darker circle, due to the presence of an event horizon, along with
detailed explanations of all the ingredients necessary to obtain,
analyze, and interpret this rich data set. The ring itself stems from
a convolution of the light produced near the last unstable photon
orbit, as it travels through the geometry of the production region
with radiative transfer in the surrounding plasma, and further
experiences bending and redshifting due to the effects of general
relativity
(
GR
)
. Photons that orbit, sometimes multiple times
before escaping, trace out a sharp feature revealing the shape of
the spacetime metric, the so-called
“
photon ring
”
. From the size of
the measured, blurry ring and three different modeling approaches
(
see EHT Collaboration et al.
2019e
)
, the EHT Collaboration
(
EHTC
)
calibrated for these multiple effects, to derive a mass for
M87
’
s SMBH of
(
6.5
±
0.7
)
×
10
9
M
e
.
One of the primary contributions to the
∼
10% systematic error on
this mass is due to uncertainties in the underlying accretion
properties. As detailed in EHT Collaboration et al.
(
2019d
)
and Porth
et al.
(
2019
)
, the EHTC ran over 45 high-resolution simulations over
a range of possible physical parameters in, e.g., spin, magnetic
fi
eld
con
fi
guration, and electron thermodynamics, using several different
GR magnetohydrodynamic
(
GRMHD
)
codes. These outputs were
then coupled to GR ray-tracing codes, to generate
∼
60,000 images
captured at different times during the simulation runs, which were
veri
fi
ed in Gold et al.
(
2020
)
. While the photon ring remains
relatively robust to changes in spin, in part because of the small
viewing angle
(
see, e.g., Johannsen & Psaltis
2010
)
, the spreading of
the light around this feature strongly depends on the plasma
properties near the event horizon, introducing signi
fi
cant degeneracy.
For instance, a smaller black hole produces a smaller photon ring,
but in some emission models there is extended surrounding emission
leading to a larger
fi
nal blurry ring. Similarly, a larger black hole
with signi
fi
cant emission produced along the line of sight will appear
to have emission within the photon ring, and when convolved
produces the appearance of a smaller blurry ring. The error in
calibrating from a given image to a unique black hole mass is
therefore a combination of image reconstruction limits, as well as our
current level of uncertainty about the plasma properties and emission
geometry very close to the black hole.
However, it is important to note that even in these
fi
rst
analyses, several of the models could already be ruled out using
complementary information from observations with facilities at
other wavelengths. For instance, the estimated minimum power
in the jets,
P
jet
10
42
erg s
−
1
, from prior and recent multi-
wavelength studies
(
e.g., Reynolds et al.
1996
; Stawarz et al.
2006
; de Gasperin et al.
2012
; Prieto et al.
2016
)
, was already
enough to rule out about half of the initial pool of models,
including all models with zero spin. Furthermore, the X-ray
fl
uxes from quasi-simultaneous observations with the Chandra
X-ray Observatory and NuSTAR provided another benchmark
that disfavored several models based on preliminary estimates of
X-ray emission from the simulations. However, detailed
fi
tting
of these and other precision data sets were beyond the focus of
the
fi
rst round of papers and GRMHD model sophistication.
The current cutting edge in modeling accreting black holes,
whether via GRMHD simulations or semi-analytical methods,
focuses on introducing more physically self-consistent, reliable
treatments of the radiating particles
(
electrons or electron-positron
pairs
)
. In particular, key questions remain about how the bulk
plasma properties dictate the ef
fi
ciency of heating, how many
thermal particles are accelerated into a nonthermal population, and
the dependence of nonthermal properties such as spectral index on
plasma properties such as turbulence, magnetization, etc.
(
see,
e.g., treatments in Howes
2010
; Ressler et al.
2015
;Mo
ś
cibrodzka
et al.
2016
;Balletal.
2018
; Anantua et al.
2020
)
.
To test the newest generation of models, it is important to
have extensive, quasi-simultaneous or at least contemporaneous
multi-wavelength monitoring of several AGNs, providing both
spectral and imaging data
(
and ideally polarization where
available
)
over a wide range of physical scales. These types of
campaigns have been limited by the dif
fi
culty in obtaining time
on multiple facilities and by scheduling challenges, so often data
are combined from different epochs. However, the variability
2
The Astrophysical Journal Letters,
911:L11
(
43pp
)
, 2021 April 10
The EHT MWL Science Working Group et al.
timescales for even SMBHs such as M87
’
s are short enough
(
days to months
)
that combining data sets from different
periods can skew modeling results for sensitive quantities such
as radiating particle properties.
This Letter intends to be the
fi
rst of a series, presenting the
substantive multi-wavelength campaigns carried out by the
EHT Multi-Wavelength Science Working Group
(
MWL WG
)
,
including EHTC members and partner facilities, for both our
primary sources M87 and Sgr A
*
, as well as many other targets
and calibrators such as 3C 279, 3C 273, OJ 287, Cen A,
NGC 1052, NRAO 530, and J1924
−
2914. These legacy papers
are meant to be companion papers to the EHT publications, and
will be used for the detailed modeling efforts to come, both
from the EHTC Theory and Simulations WG as well as from
other groups. They serve as a resource for the entire
community, to enable the best possible modeling outcomes
and a benchmark for theory. Here we present the results from
the 2017 EHT campaign on M87, combining very long
baseline interferometry
(
VLBI
)
imaging and spectral index
maps at longer wavelengths, with spectral data from sub-
millimeter
(
submm
)
through TeV
γ
-rays
(
covering more than
17 decades in frequency
)
.
In Section
2
we describe these observations, including
images
(
a compilation of MWL images in one panel is shown
in a later section
)
, spectral energy distributions
(
SEDs
)
and, where relevant, lightcurves and comparisons to prior
observations. In Section
3
we present a compiled SED together
with a table of
fl
uxes. We also
fi
t this SED with a few
phenomenological models and discuss the consequences for the
emission geometry and high-energy properties. In Section
4
we
give our conclusions. All data
fi
les and products are available
for download, as described in Section
3.2
.
2. Observations and Data Reduction
In Figure
1
we give a schematic overview of the 2017 MWL
campaign coverage on M87. In the following subsections we
provide detailed descriptions of the observations, data proces-
sing procedures, and band-speci
fi
c analyses. To aid readability,
all tabulated data are collected in Appendix
A
.
2.1. Radio Data
In this subsection we describe the observations and data
reduction of radio
/
mm data obtained with various VLBI
facilities and connected interferometers. Especially regarding
VLBI data, here we introduce the term
“
radio core
”
to represent
the innermost part of the radio jet. A radio core in a VLBI jet
Figure 1.
Instrument coverage summary of the 2017 M87 MWL campaign, covering MJD range 57833
–
57893.
(
Made with the
MWLGen
software by J. Farah.
)
Figure 2.
Radio lightcurves of the M87 core in 2017 at multiple frequencies.
The top and bottom panels are for connected interferometers and VLBI,
respectively. The corresponding beam sizes are indicated in Table
A1
. KVN
data at 22 and 43 GHz are not shown here since KVN captures the data from
the shortest baselines of EAVN.
3
The Astrophysical Journal Letters,
911:L11
(
43pp
)
, 2021 April 10
The EHT MWL Science Working Group et al.
image is conventionally de
fi
ned as the most compact
(
often
unresolved or partially resolved
)
feature seen at the apparent
base of the radio jet in a given map
(
e.g., Lobanov
1998
;
Marscher
2008
)
. For this reason, different angular resolutions
by different VLBI instruments
/
frequencies, together with the
frequency-dependent synchrotron optical depth
(
Marcaide &
Shapiro
1984
; Lobanov
1998
)
, can make the identi
fi
cation of a
radio core not exactly the same for each observation. See also
Section
3.2
for related discussions.
2.1.1. EVN 1.7 GHz
M87 was observed with the European VLBI Network
(
EVN
)
at 1.7 GHz on 2017 May 9. The observations were made as part
of a long-term EVN monitoring program of activity in HST-1,
located at a projected distance of
∼
70 pc from the core
(
Cheung et al.
2007
; Chang et al.
2010
; Giroletti et al.
2012
;
Hada et al.
2015
)
. A total of eight stations joined a 10 hr long
session with baselines ranging from 600 km to 10 200 km,
yielding a maximum angular resolution of
∼
3 mas at 1.7 GHz.
The data were recorded at 1 Gbps with dual-polarization
(
a total
bandwidth of 256 MHz, 16 MHz
×
8 subbands for each
polarization
)
, and the correlation was performed at the Joint
Institute for VLBI ERIC
(
JIVE
)
. Automated data
fl
agging and
initial amplitude and phase calibration were also carried out at
JIVE using dedicated pipeline scripts. This step was followed
by frequency averaging within each spectral band
(
IF
)
and in
time to 8 s. The
fi
nal image was produced using the
Difmap
software
(
Shepherd
1997
)
after several iterations of phase and
amplitude self-calibration
(
see Giroletti et al.
2012
, for more
detail
)
. Here we provide the peak
fl
ux of the radio core
(
see
Figure
2
and Table
A1
)
and a large-scale jet image
(
presented
in Section
3
)
, while a dedicated analysis on the HST-1
kinematics will be presented in a separate paper.
2.1.2. HSA 8, 15, and 24 GHz
M87 was observed with the High Sensitivity Array
(
HSA
)
at
8.4, 15, and 24 GHz on 2017 May 15, 16 and 20, respectively,
which are roughly a month after the EHT-2017 observations.
Each session was made with a 12 hr long continuous track and the
phased Very Large Array and Effelsberg 100 m antenna
participated in the observations along with the 10 stations of the
NRAO Very Long Baseline Array
(
VLBA
)
.Thedatawere
recorded at 2 Gbps with dual-polarization
(
a total bandwidth of
512MHz, 32MHz
×
8 subbands for each polarization
)
,andthe
correlation was done with th
e VLBA correlator in Socorro
(
Deller
et al.
2011
)
. The initial data calibration was performed using the
NRAO Astronomical Image Processing System
(
AIPS
;Grei-
sen
2003
)
based on the standard VLBI data reduction procedures
(
Crossley et al.
2012
;Walker
2014
)
. Similar to other VLBI data,
images were created using the
Difmap
software with iterative
phase
/
amplitude self-calibration.
A detailed study of the parsec-scale structure of the M87 jet
from this HSA program will be discussed in a separate paper.
Here we provide a core peak
fl
ux and VLBI-scale total
fl
ux at
each frequency
(
Table
A1
in Appendix
A
)
. We adopt 10%
errors in
fl
ux estimate, which is typical for HSA.
2.1.3. VERA 22 GHz
The core of M87 was frequently monitored over the entire
year of 2017 at 22 GHz with the VLBI Exploration of Radio
Astrometry
(
VERA; Kobayashi et al.
2003
)
,aspartofaregular
monitoring program of a sample of
γ
-ray bright AGNs
(
Nagai
et al.
2013
)
. A total of 17 epochs were obtained in 2017
(
see
Figure
2
and Table
A1
in Appendix
A
)
.Duringeachsession,M87
was observed for 10
–
30 minutes with an allocated bandwidth of
16 MHz, suf
fi
cient to detect the bright core and create its light
curves. All the data were analyzed in the standard VERA data
reduction procedures
(
see Nagai et al.
2013
;Hadaetal.
2014
,for
more details
)
. Note that VERA can recover only part of the
extended jet emission due to the lack of short baselines, so the
total
fl
uxes of VERA listed in Table
A1
in Appendix
A
signi
fi
cantly underestimate the actual total jet
fl
uxes.
2.1.4. EAVN
/
KaVA 22 and 43 GHz
Since 2013 a joint network of the Korean VLBI Network
(
KVN
)
and VERA
(
KaVA; Niinuma et al.
2014
)
has regularly
been monitoring M87 to trace the structural evolution of the pc-
scale jet
(
Hada et al.
2017
;Parketal.
2019
)
. From 2017, the
network was expanded to the East Asian VLBI Network
(
EAVN; Wajima et al.
2016
; Asada et al.
2017
;Anetal.
2018
)
by adding more stations from East Asia, enhancing the array
sensitivity and angular resolution. Between 2017 March and
May, EAVN densely monitored M87 for a total of 14 epochs
(
fi
ve at 22 GHz, nine at 43 GHz; see Figure
2
and Table
A1
in
Appendix
A
)
. The default array con
fi
gurations were KaVA
+
Tianma
+
Nanshan
+
Hitachi at 22 GHz and KaVA
+
Tianma at
43 GHz, respectively, while occasionally a few more stations
(
Sejong, Kashima, and Nobeyama
)
additionally joined if they
were available. In addition, we also had four more sessions with
KaVA alone
(
2
+
2at22
/
43 GHz
)
in 2017 January
–
February.
Each of the KaVA
/
EAVN sessions was made in a 5
–
7hr
continuous run at a data recording rate of 1 Gbps
(
2-bit
sampling, a total bandwidth of 256 MHz was divided into
32 MHz
×
8 IFs
)
. All the data were correlated at the Daejeon
hardware correlator installed at KASI. All the EAVN data were
calibrated in the standard manner of VLBI data reduction
procedures. We used the
AIPS
software package for the initial
calibration of visibility amplitude, bandpass, and phase
calibration. The imaging
/
CLEAN
(
Högbom
1974
)
and self-
calibration were performed with the
Difmap
software. In
Section
3
we present one of the 22 GHz EAVN images
(
taken
in 2017 March 18
)
where the KaVA, Tianma
—
65 m, Nanshan
—
26 m, and Hitachi
—
32 m radio telescopes participated.
2.1.5. KVN 22, 43, 86, and 129 GHz
The KVN regularly observes M87 at frequencies of 22, 43,
86, and 129 GHz simultaneously via the interferometric
Monitoring of
γ
-ray Bright Active galactic nuclei
(
iMOGABA
)
program, starting in 2012 December and lasting until 2020
January. The total bandwidth of the observations at each
frequency band is 64 MHz and the typical beam sizes of the
observations are 6.1
×
3.1 mas at 22 GHz, 2.8
×
1.6 mas at
43 GHz, 1.5
×
0.8 mas at 86 GHz, and 1.1
×
0.5 mas at
129 GHz. Details of the scheduling, observations, data
reduction including frequency phase transfer technique,
analysis
(
i.e., imaging and model-
fi
tting
)
, and early results for
M87 are shown in Lee et al.
(
2016
)
and Kim et al.
(
2018a
)
.
Despite the limited coverage of baselines and capability of the
array to image the extended jet structure in M87, the
fl
ux
density of the compact core can be rather reliably measured
(
Kim et al.
2018a
)
. We extract the core
fl
ux densities at the four
frequencies and obtain light curves spanning observing periods
4
The Astrophysical Journal Letters,
911:L11
(
43pp
)
, 2021 April 10
The EHT MWL Science Working Group et al.
between 2017 March and December at seven epochs. While
typical
fl
ux density uncertainties at 22
–
86 GHz are of order of
∼
10%, residual phase rotations and larger thermal noise at
129 GHz often lead to uncertainties of
∼
30%. Accordingly, we
adopt these uncertainties for all KVN observing epochs in this
Letter.
2.1.6. VLBA 24 and 43 GHz
M87 was observed with the VLBA at central frequencies of
24 and 43 GHz on 2017 May 5. These observations were
carried out in the framework of the long-term monitoring
program toward M87, which was initiated in 2006
(
Walker
et al.
2018
)
and lasted until 2020. For the 2017 session, the
total on-source time amounts to about 1.7 hr at 24 GHz and 6 hr
at 43 GHz. The sources OJ 287 and 3C 279 were observed to
use as fringe
fi
nders and bandpass calibrators. In each band,
eight 32 MHz wide frequency channels were recorded in both
right and left circular polarization at a rate of 2 Gbps, and
correlated with the VLBA software correlator in Socorro. The
initial data reduction was conducted within
AIPS
following the
standard calibration procedures for VLBI data. Deconvolution
and self-calibration algorithms, implemented in
Difmap
, were
used for phase and amplitude calibration and for constructing
the
fi
nal images. Amplitude calibration accuracy of 10% is
adopted for both frequencies.
The resulting total intensity 43 GHz image of M87 is
presented in Section
3
, with the details given in Table
A1
in
Appendix
A
. The synthesized beam size amounts to
0.76
×
0.40 mas at the position angle
(
PA
)
of the major axis
of
−
8
°
at 24 GHz, and 0.41
×
0.23 mas at PA
=
0
°
at 43 GHz.
We note that these observations were used for the study of a
linear polarization structure toward the M87 core, details of
which can be found in a separate paper
(
Kravchenko et al.
2020
)
.
2.1.7. GMVA 86 GHz
M87 was observed by the Global Millimeter-VLBI-Array
(
GMVA; Boccardi et al.
2017
)
on 2017 March 30
(
project code
MA 009
)
. In total, 14 stations participated in the observations:
VLBA
(
eight stations; without HN and SC
)
, 100 m Green Bank
Telescope, IRAM 30 m, Effelsberg 100 m, Yebes, Onsala, and
Metsahovi. The observation was performed in full-track mode
for a total of 14 hr. Nearby bright sources 3C 279 and 3C 273
were observed as calibrator targets. The raw data were
correlated by using the DiFX correlator
(
Deller et al.
2011
)
.
247
Further post-processing was performed using the
AIPS
software package, following typical VLBI data reduction
procedures
(
see, e.g., Martí-Vidal et al.
2012
)
. After the
calibration, the data were frequency-averaged across the whole
subchannels and IFs, and exported outside AIPS for imaging
with the
Difmap
software. Within
Difmap
, the data were
further time-averaged for 10 s, followed by
fl
agging of outlying
data points
(
e.g., scans with too low amplitudes due to pointing
errors
)
. Afterward, CLEAN and phase self-calibrations were
iteratively performed near the peak of the intensity, but
avoiding CLEANing of the faint counterjet side at the early
stage. When no more signi
fi
cant
fl
ux remained for further
CLEAN steps, a
fi
rst amplitude self-calibration was performed
using the entire time coverage as the solution interval, in order
to
fi
nd average station gain amplitude corrections. A similar
procedure was repeated with progressively shorter self-
calibration solution intervals, and the
fi
nal image was exported
outside
Difmap
when the shortest possible solution interval
was reached and no more signi
fi
cant emission was visible in
the dirty map compared to the off-source rms levels.
Calibrated visibilities are shown in Figure
3
, and the CLEAN
image is given in Section
3
. We note that the
fi
nal image has an
rms noise level of
∼
0.5 mJy beam
−
1
. This noise level is a factor
of a few higher than other 86 GHz images of M87 from previous
observations, which were made with similar array con
fi
guration
as the 2017 session
(
see Hada et al.
2016
; Kim et al.
2018b
)
.
Therefore, we refer to this image as tentative, and it only reveals
the compact core and faint base of the jet, mainly due to poor
weather conditions during the observations. We consider an
error budget of 30% for the
fl
ux estimate. The peak
fl
ux density
on the resultant image amounts to
∼
0.52 Jy beam
−
1
for the
synthesized beam of 0.243
×
0.066 mas at PA
=
−
9
°
.3
(
see
Table
A1
in Appendix
A
for other details
)
.Wenotethatthepeak
fl
ux density as well as the
fl
ux, integrated over the core region,
are comparable to their historical values
(
Hada et al.
2016
;Kim
et al.
2018b
)
, except for the 2009 May epoch, when the GMVA
observations revealed about two times brighter core region in
both intensity and integrated
fl
ux
(
Kim et al.
2018b
)
.
2.1.8. Atacama Large Millimeter
/
submillimeter Array
(
ALMA
)
221 GHz
The observations at Band 6 with phased-ALMA
(
Matthews
et al.
2018
)
were conducted as part of the 2017 EHT campaign
(
Goddi et al.
2019
)
. The VLBI observations were carried out
while the array was in its most compact con
fi
gurations
(
with
longest baselines
<
0.5 km
)
. The spectral setup at Band 6
includes four spectral windows
(
SPWs
)
of 1875 MHz, two in
the lower and two in the upper sideband, correlated with 240
channels per SPW
(
corresponding to a spectral resolution of
7.8125 MHz
)
. The central frequencies at this band are 213.1,
215.1, 227.1, and 229.1 GHz. Details about the ALMA
observations and a full description of the data processing and
calibration can be found in Goddi et al.
(
2019
)
.
Imaging was performed with the Common Astronomy
Software Applications
(
CASA
; McMullin et al.
2007
)
package
using the task
tclean
. Only phased antennas were used to
produce the
fi
nal images
(
with baselines
<
360 m
)
, yielding
Figure 3.
Visibility amplitude vs.
uv
-distance plot of GMVA data on 2017
March 30. In this display the visibilities are binned in 30 s intervals for clarity.
247
We use the correlator at the Max-Planck-Institut für Radioastronomie
(
MPIfR
)
in Bonn, Germany.
5
The Astrophysical Journal Letters,
911:L11
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)
, 2021 April 10
The EHT MWL Science Working Group et al.
synthesized beam sizes in the range 1
0
–
2
4
(
depending on
the observing band and date
)
. We produced 256
×
256 pixel
maps, with a cell size of 0
2 yielding a
fi
eld of view
of 51
′′
×
51
′′
.
The main observational and imaging parameters are
summarized in Table
A1
in Appendix
A
. For each data set
and corresponding image, the table reports the
fl
ux-density
values of the central compact core and the overall
fl
ux,
including the extended jet. In order to isolate the core emission
from the jet, we compute the sum of the central nine pixels of
the model
(
CLEAN component
)
map
(
an area of 3
×
3
pixels
)
;
248
the contribution from the jet is accounted for by
also summing the clean components along the jet. The
extended emission accounts for less than 20% of the total
emission at 1.3 mm. The large-scale jet image is presented in
Section
3
. Details about the imaging and
fl
ux extraction
methods can be found in Goddi et al.
(
2021
)
.
2.1.9. Submillimeter Array
(
SMA
)
230 GHz
The long term 1.3 mm band
(
230 GHz
)
fl
ux density light-
curve for M87 shown in Figure
2
was obtained at the SMA
near the summit of Maunakea
(
Hawaii
)
. M87 is included in an
ongoing monitoring program at the SMA to determine
fl
ux
densities for compact extragalactic radio sources that can be
used as calibrators at mm wavelengths
(
Gurwell et al.
2007
)
.
Available potential calibrators are occasionally observed for
3
–
5 minutes, and the measured source signal strength
calibrated against known standards, typically solar system
objects
(
Titan, Uranus, Neptune, or Callisto
)
. Data from this
program are updated regularly and are available at the SMA
Observer Center website
(
SMAOC
249
)
. Data were primarily
obtained in a compact con
fi
guration
(
with baselines extending
from 10 to 75 m
)
though a small number were obtained at
longer baselines up to 210 m. The effective spatial resolution,
therefore, was generally around 3
′′
. The
fl
ux density was
obtained by vector averaging of the calibrated visibilities from
each observation.
Observations of M87 were additionally conducted as part of
the 2017 EHT campaign, with the SMA running in phased-
array mode, operating at 230 GHz. All observations were
conducted while the array was in compact con
fi
guration, with
the interferometer operating in dual-polarization mode. The
SMA correlator produces four separate but contiguous 2 GHz
spectral windows per sideband, resulting in frequency coverage
of 208
–
216 and 224
–
232 GHz. Data were both bandpass and
amplitude calibrated using 3C 279, with
fl
ux calibration
performed using either Callisto, Ganymede, or Titan. Due in
part to poor phase stability at the time of observations, phase
calibration is done through self-calibration of the M87 data
itself, assuming a point-source model. Data are then imaged
and deconvolved using the CLEAN algorithm.
A summary of the measurements made from these data are
shown in Table
A1
in Appendix
A
, along with measurements
taken within a month of these observations from the SMAOC
data mentioned above. The reported core
fl
ux for M87 is the
fl
ux measured in the center of the cleaned map. Combined
images of all data show the same jet-like structure seen in the
ALMA image in Figure
13
, although recovery of the
fl
ux for
individual days through imaging is limited by a lack of
(
u
,
v
)
-
coverage within individual tracks. Therefore, we estimate the
total
fl
ux by measuring the mean
fl
ux density of all baselines
within a
(
u
,
v
)
-angle of 110
°±
5
°
, as these baselines are not
expected to resolve the jet and central region.
2.2. Optical and Ultraviolet
(
UV
)
Data
We performed optical and UV observations of M87 with the
Neil Gehrels Swift Observatory during the EHT campaign, and
have also analyzed contemporaneous archival observations
from the Hubble Space Telescope
(
HST
)
.
2.2.1. UV
/
Optical Telescope
(
UVOT
)
Observations
The Neil Gehrels Swift Observatory
(
Gehrels et al.
2004
)
is
equipped with UVOT
(
Roming et al.
2005
)
, as well as with
X-ray imaging optics
(
see Section
2.3.4
below
)
. We retrieved
UVOT optical and UV data from the NASA High-Energy
Astrophysics Archive Research Center
(
HEASARC
)
and
reduced them with v6.26.1 of the HEASOFT software
250
and
CALDB v20170922. The observations were performed from
2017 March 22 to April 20 in six bands,
v
(
5458
Å
)
,
b
(
4392
Å
)
,
u
(
3465
Å
)
,
uvw1
(
2600
Å
)
,
uvm2
(
2246
Å
)
, and
uvw2
(
1928
Å
)
,
with 24 measurements in each band, and effective
fi
lter
wavelengths as given in Poole et al.
(
2008
)
. The reduction of
the data followed the standard prescriptions of the instrument
team at the University of Leicester.
251
We checked the UVOT
observations for small-scale sensitivity issues and found that
none of our observations are affected by bad charge-coupled
device
(
CCD
)
pixels. In 2020 November the Swift team
released new UVOT calibration
fi
les, along with coef
fi
cients of
the
fl
ux density correction as a function of time, for data
reduced with the previous version of CALDB. For our
observations the coef
fi
cients of the
fl
ux density correction
(
multiplicative factors
)
are very close to 1: 0.974
±
0.001
(
for
the optical
fi
lters
)
, 0.947
±
0.001
(
uvw1
)
, 0.964
±
0.001
(
uvm2
)
, and 0.958
±
0.001
(
uvw2
)
. We have used these
coef
fi
cients to correct our measurements for the UVOT
sensitivity change.
252
We performed aperture photometry for each individual
observation using the tool
UVOTSOURCE
, with a circular
aperture of a radius of 5
′′
centered on the sky coordinates of
M87 and detection signi
fi
cance
σ
5. This aperture includes
the M87 core, the knot HST-1, and some emission from the
extended jet, which we cannot separate due to the size of the
UVOT point-spread function
(
PSF;
∼
2
5
)
. Since there is
contamination from the bright host galaxy surrounding the core
of M87, we measure the background level in three circular
regions, each of 30
′′
radius in a source-free area located outside
the host galaxy radius
(
see below
)
. We used the count-rate to
magnitude and
fl
ux density conversion provided by Breeveld
et al.
(
2011
)
and retained only those measurements with
magnitude errors of
σ
mag
<
0.2. This calibration of the UVOT
broadband
fi
lters
(
Breeveld et al.
2011
)
includes additional
calibration sources with a wider range of colors with respect to
the one reported by Poole et al.
(
2008
)
.
248
An alternative method is provided by taking the integrated
fl
ux within the
synthesized full width at half maximum
(
FWHM
)
size centered at the phase-
center in each cleaned image, which provides consistent values with those
obtained from the model cleaning components.
249
http:
//
sma1.sma.hawaii.edu
/
callist
/
callist.html
250
NASA High-Energy Astrophysics Archive Research Center HEASOFT
package
https:
//
heasarc.gsfc.nasa.gov
/
docs
/
software
/
heasoft
/
.
251
https:
//
www.swift.ac.uk
/
analysis
/
uvot
/
252
https:
//
www.swift.ac.uk
/
analysis
/
uvot
/
index.php
6
The Astrophysical Journal Letters,
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, 2021 April 10
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To estimate the contributions of the host galaxy to the
derived
fl
ux densities, we modeled the UVOT images. First we
combined individual images using the tool
UVOTIMSUM
to
produce a stacked image in each
fi
lter. Ordinarily, this would
have allowed us to obtain the best signal-to-noise ratio
(
S
/
N
)
for modeling. However, since the source was not centered in
the different images at the same CCD position, the area of
intersection between the images was too small. Hence, even in
the outer regions the
fl
ux from the galaxy was still signi
fi
cant.
Therefore, we made a set of models using individual images
and then found a mean model for each band after a mask of
background objects was used to exclude all objects
(
except for
M87 itself
)
. Since a uni
fi
ed mask was used, the images have the
same pixels included and excluded from the analysis, so no
bias is introduced.
After this procedure, the decomposition process was run for
each image using a model consisting of three components: the core
region, jet, and host galaxy. The core region includes the core and
knot HST-1, which was modeled by a point source, while the jet
was
fi
tted by a highly elliptical Gaussian. The galaxy was modeled
by a Sérsic function:
()
(
[(
)
]
)
=- -
I
RI bRR
exp
1
n
n
ee
1
,
where
n
is the Sérsic parameter,
b
n
=
2
n
−
1
/
3,
R
e
is the half-
light radius, and
I
e
is the intensity at
R
e
. All three structural features
can be seen in the combined image in the
uvw1
band presented in
Figure
13
. A point source model for the core
+
HST-1 region was
convolved with the PSF determined for each
fi
lter using several
isolated non-saturated stars in a
number of images. The parameters
of each PSF were averaged over stars and images. Before the
decomposition, the images were normalized to a one-second
exposure to make them uniform. The program
im
fi
t
(
Erwin
2015
)
was employed to perform decomposition for each image. Each
model parameter for a given
fi
lter corresponds to the median value
averaged over all images using the best-
fi
t image parameters
(
according to
χ
2
)
,with
>
2
σ
outliers removed
(
number of outliers
for each
fi
lter
4
)
.
Table
A2
in Appendix
A
lists the derived median values of
the Sérsic model
n
-parameter and its uncertainty for different
bands. The uncertainty corresponds to a scatter of models
among the individual images. The derived values of the Sérsic
n
-parameter show a dependence on wavelength that is caused
by a difference in the stellar population and, probably, the
scattered light from the core, which is blue, leading to higher
n
-
values for blue
fi
lters. Note that there is signi
fi
cant scatter
among values of the
n
-parameter reported in the literature, from
n
=
2.4
(
Vika et al.
2012
)
to
n
=
3.0
(
D
’
Onofrio
2001
)
,
n
=
6.1
(
Ferrarese et al.
2006
)
,
n
=
6.9
(
Graham & Driver
2007
)
, and
n
=
11.8
(
Kormendy et al.
2009
)
.
Table
A2
in Appendix
A
also gives the derived values of the
effective radius of the galaxy in different bands,
R
e
, ellipticity,
ò
, and PA,
Φ
, of the major axis of the galaxy calculated counter
clockwise from north. Figure
4
shows an example of a
comparison between the mean observed surface brightness
pro
fi
le along the semimajor axis of the host galaxy and the
results of the modeling. The mean observed pro
fi
le is obtained
by azimuthally averaging along a set of concentric ellipses with
the ellipticity and PA equal to those of the host galaxy.
According to our X-ray data analysis
(
see Section
2.3.3
)
, the
hydrogen column density corresponding to absorption in both
our Galaxy and near M87 is equal to
N
H
=
́
-
+
0
.050
10
0.002
0.003
22
cm
−
2
, while there is evidence from our X-ray data for
additional X-ray absorption within the central 1
′′
around M87
with a column density of
́
-
+
0
.12
10
0.04
0.05
22
cm
−
2
. However, this
latter value is most likely variable since much lower values of
N
H
for M87 were detected previously, e.g.,
N
H
≈
0.01
×
10
22
cm
−
2
(
Sabra et al.
2003
)
based on UV spectroscopy.
Therefore, as discussed in Prieto et al.
(
2016
)
we assume that
the additional X-ray absorber is dust-free and employ the
extinction curve
(
R
V
=
3.1
)
and the extinction value
(
E
B
−
V
=
0.022
)
given by Schlegel et al.
(
1998
)
along with
the formalism provided by Cardelli et al.
(
1989
)
to derive the
Figure 4.
Mean surface brightness in
uvw1
band as a function of the semimajor
axes of concentric ellipses with ellipticity and PA equal to those of the host
galaxy; the red, blue, and green curves show the core
+
HST-1, jet, and host
galaxy pro
fi
les, respectively; the solid black curve represents the observed
fl
ux,
and the black dotted curve gives the total
fl
ux of the model; all of the curves
show the average pro
fi
les over all images in this band.
Figure 5.
Optical and UV light curves of the core region of M87 in all six
UVOT
fi
lters; the red lines show the average
fl
ux density in each
fi
lter during
the EHT campaign and their standard deviations
(
red dotted lines
)
along with
the time and duration of the campaign.
7
The Astrophysical Journal Letters,
911:L11
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)
, 2021 April 10
The EHT MWL Science Working Group et al.
extinction values in different bands, which are listed in
Table
A2
in Appendix
A
.
Tables
A3
and
A4
in Appendix
A
give
fl
ux densities of the
core region of M87 corrected for the host galaxy contamination
and extinction in optical and UV bands, respectively. Figure
5
shows light curves of the core region during the campaign.
According to Table
A2
in Appendix
A
and as can be seen from
Figure
5
the standard deviation of
F
core
averaged over the EHT
campaign is signi
fi
cantly less than the uncertainty of an
individual measurement in all
fi
lters. The uncertainty is
dominated by that of the host galaxy decomposition, which is
added
(
in quadrature
)
to the photometric uncertainty of each
measurement. Therefore, based on the UVOT observations we
cannot detect variability in the core region of M87 during the
EHT campaign, which is consistent with apparently a low
activity state of both the core and HST-1 knot. Based on these
results we have calculated the UVOT spectral index of the core
region during the EHT campaign using the average values of
the
fl
ux density in the UVOT bands as
S
∝
ν
−
α
, which results
in
α
=
1.88
±
0.55. Although
α
has a signi
fi
cant error, most
likely connected with large uncertainties in the host galaxy
decomposition in different bands, the spectral index is
consistent with the optical
/
UV spectral index of the core of
M87 reported by Perlman et al.
(
2011
)
. This indicates that the
core dominates the innermost region of M87 at UV
/
optical
wavelengths during the campaign.
2.2.2. HST Observations
We have downloaded HST images of M87 from the HST
archive obtained on 2017 April 7, 12, and 17 with the Wide
Field Camera 3
(
WFC3
)
camera in two wide bands, F275W
and F606W
(
ID5o30010, ID5o30020, ID5o31010, ID5o31020,
ID5o32010, and ID5o32020
)
. We used fully calibrated and
dither combined images. The image obtained on 2017 April 12
in the F606W
fi
lter is a part of a composite of M87 multi-
wavelength images presented in Section
3
. The decomposition
of the HST images was made using the same
im
fi
t
package as
for the images obtained with the UVOT
(
Section
2.2.1
)
. The
substantially higher spatial resolution of the HST images
(
1
pixel is 0
04
)
, however, led to a somewhat different approach.
First, we masked out the jet on the images: the HST images
reveal its very complex structure, which cannot be approxi-
mated with a simple analytical model, as was the case for the
low-resolution UVOT images. We also masked out knot HST-1
and the core, which are clearly resolved in the HST images
(
Figure
6
)
. Second, we used a more complex model for the host
galaxy: a sum of two Sérsic models
(
instead of one as in the
case of the UVOT images
)
, which gave us signi
fi
cantly better
residual
(
observations
—
model
)
maps. Finally, we used the
HST library of PSF images provided at the HST website
253
to
model the PSF in each
fi
lter. Figure
6
shows the resulting map
of M87 on 2017 April 11 in F606W
fi
lter, with the host galaxy
subtracted.
The decomposition analysis was limited to the central region
of the images
(
16
′′
×
16
′′
)
, since the main goal of the process
was to subtract the host galaxy
fl
ux before the aperture
photometry of the core and HST-1. Fitting the full image of the
galaxy would have required a more complex model to have
comparable quality of the
fi
t for the central regions
(
see, e.g.,
Huang et al.
2013
)
. After subtracting the best-
fi
t model from
the images, we performed the aperture photometry with a
radius of 0
4to
fi
nd the
fl
ux densities from the core and HST-1
in each band.
im
fi
t
allows one to perform a bootstrap method
to derive estimates of uncertainties of the decomposition
parameters. For each image, the program was run 100 times,
each with same number of random pixels involved in the
decomposition process. For each decomposition, a residual
FITS
fi
le was constructed containing only the core and HST-1
(
100 cases for each band and date
)
. For each such residual
fi
le,
we performed the aperture photometry with a radius of 0
4 and
calculated the standard deviation of
fl
ux density measurements
of the core and HST-1 over different decompositions. This
standard deviation was added in quadrature to the uncertainty
of the photometry from the decomposition of the original image
in each band and date. Table
A5
in Appendix
A
gives
fl
ux
densities of the core and HST-1, measured in two different
bands and at epochs contemporaneous with the EHT observa-
tions. The
fl
ux densities are corrected for the extinction in the
same manner as described in Section
2.2.1
.
According to Table
A5
in Appendix
A
the core shows a slight
increase in
fl
ux density over 10 days, while knot HST-1 has a
constant
fl
ux density. We have estimated optical
/
UV spectral
indices of the core and HST-1, which are 1.44
±
0.09 and
0.60
±
0.02, respectively
(
the spectral index is de
fi
ned in the
same way as in Section
2.2.1
)
. These are in good agreement with
those given by Perlman et al.
(
2011
)
,
∼
1.5 for the core and
∼
0.5
for HST-1, which con
fi
rm a
fl
atter spectral index of HST-1 with
respect to that of the core at UV
/
optical wavelengths.
To determine the activity state of M87 during the 2017
campaign, we have constructed light curves of the core and
HST-1 in two bands, F275W
(
from 1999 to 2017
)
and F606W
(
from 2002 to 2017
)
. During the period 1999 to 2010 different
instruments were used at HST: UV observations were per-
formed with STIS
/
F250QTZ
λ
eff
=
2365
Å
, ACS
/
F220W
λ
eff
=
2255.5
Å
, and ACS
/
F250W
λ
eff
=
2716
Å
(
e.g., Madrid
2009
)
. We have used the UV measurements presented in
Madrid
(
2009
)
and translated them into WFC3
/
F275W using
spectral indices reported by Perlman et al.
(
2011
)
, who
observed M87 in four UV
/
optical bands during the same
period. In addition, Madrid
(
2009
)
performed photometry with
an aperture of radius 0
25, so that to construct a uniform UV
lightcurve, we have recalculated our measurements in F275W
band using the same aperture. For the optical lightcurve in
F606W band, we have used measurements provided by
Figure 6.
HST image of M87 in F606W
fi
lter with host galaxy subtracted; the
core and HST-1 are designated, the distance between the features is
0
86
±
0
04.
253
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//
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/
hst
/
instrumentation
/
wfc3
/
data-analysis
/
psf
8
The Astrophysical Journal Letters,
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(
43pp
)
, 2021 April 10
The EHT MWL Science Working Group et al.
Perlman et al.
(
2011
)
obtained with Advanced Camera for
Surveys
(
ACS
)
/
F606W and Wide Field Camera 2
(
WFC2
)
/
F606W. Since
λ
eff
of these instruments in F606W band is the
same as for WFC3
/
F606W and photometry was performed
with the same aperture of radius 0
4, no corrections to the
measurements have been applied. Figure
7
shows the historical
UV
/
optical light curves of M87. Independent of the
fi
lter used,
HST-1 was in its lowest brightness state ever observed during
the EHT campaign. Although the core was in its lowest
brightness state at optical wavelengths in 2017, the better-
sampled UV lightcurve suggests that the core was in an average
quiescent state.
2.3. X-Ray Observations
2.3.1. Chandra
We requested Director
’
s Discretionary Time
(
DDT
)
obser-
vations of M87 with the Chandra X-ray Observatory to
coordinate with the EHT campaign. The source was observed
with Advanced CCD Imaging Spectrometer
(
ACIS
)
-S for 13.1
ks starting on 2017 April 11 23:46:58 UT
(
ObsID 20034
)
and
again starting on 2017 April 14 02:00:28 UT
(
ObsID 20035
)
.
In order to perform spatially resolved spectral and variability
analysis
(
see Section
2.3.3
)
, we also analyzed several archival
Chandra observations. Following Wong et al.
(
2017
)
, for
constraints on the intra-cluster medium
(
ICM
)
, we included
ObsIDs 352, 3717, and 2707, which were acquired on 2000
July 29, 2002 July 5, and 2004 July 6 and have good exposures
of 37.7 ks, 98.7 ks, and 20.6 ks, respectively.
The Chandra observations were processed using standard
data reduction procedures in
CIAO
v4.9.
254
We focused on
extracting spectra from the core, the knot HST-1, and the outer
jet, along with instrumental response
fi
les for spectral analysis.
We took positions for the core and HST-1 from Perlman &
Wilson
(
2005
)
. For the core, we used a circular source
extraction region with a radius of 0
4 centered on M87
(
the
approximate FWHM of 0
8 is quoted in Table
A8
in
Appendix
A
)
. The core background region is a half annulus
centered on M87 with inner and outer radii of 2
′′
and 3
5,
respectively; we excluded the half of the annulus that is
on the same side of the core as the extended X-ray jet. For
HST-1, we used a similar circular source region, but for the
background annulus we excluded
∼
90
°
wedges containing
the core on one side and the extended jet on the other. For the
jet itself, we used a 19
5
×
3
′′
rectangular source region
centered on the jet, with 19
5
×
1
5 rectangular regions on
either side. To illustrate the relative brightness and vari-
ability of the core and HST-1, we show their lightcurves
(
1ks
bins
)
in Figure
8
. Sun et al.
(
2018
)
presented a Chandra study
spanning
∼
8 yr from 2002 to 2010 with coverage of the core
and HST-1 in low and high states
(
see their Figure 3
)
. In their
study, the core
fl
ux drops as low as
∼
10
−
12
erg s
−
1
cm
−
2
(
the
average is
∼
4
×
10
−
12
erg s
−
1
cm
−
2
)
. In the 2017 Chandra
observations, the unabsorbed core
fl
ux in the 0.3
–
7 keV
band is 3
×
10
−
12
erg s
−
1
cm
−
2
, and the absorbed
fl
ux is
2
×
10
−
12
erg s
−
1
cm
−
2
—
hence our observations show the core
below the historical mean.
Because the ICM contributes signi
fi
cantly to the NuSTAR
background, we used a single set of extraction regions to
produce the Chandra ICM spectrum and the NuSTAR spectra
(
circular regions of radius 45
′′
, see Section
2.3.2
for more
details
)
. In extracting the Chandra ICM spectrum, we excluded
the source regions for the core, HST-1, and the extended jet, all
of which
fi
t well within the NuSTAR PSF
(
Section
2.3.2
)
.
2.3.2. NuSTAR
We also requested two DDT observations of M87 with
NuSTAR
(
Harrison et al.
2013
)
to coordinate with the EHT
campaign in 2017 April. These observations are contempora-
neous with the Chandra observations described in Section
2.3.1
,
and they are summarized in Table
A6
in Appendix
A
. NuSTAR
observations of M87 in 2017 February and April have been
presented in Wong et al.
(
2017
)
.
Raw data from NuSTAR observations were processed using
standard procedures outlined in the NuSTAR data analysis
software guide
(
Perri et al.
2017
)
. We used data analysis
software
(
NuSTARDAS, version 1.8.0
)
, distributed by HEA-
SARC
/
HEASOFT, version 6.23. Instrumental responses were
calculated based on HEASARC CALDB version 20180312.
Figure 8.
Chandra X-ray lightcurve of the core of M87
(
black
)
and HST-1
(
blue
)
in 2017 April, showing a small amount of variability between
observations 20034
(
left panel
)
and 20035
(
right panel
)
.
Figure 7.
Historical optical and UV light curves of M87
’
s core and the HST-
1 knot
(
see also Madrid
2009
; Perlman et al.
2011
)
. Horizontal dashed lines
indicate the lowest
fl
ux density level of the core in the F606W
(
red
)
and
F275W
(
black
)
fi
lters, the vertical green dotted line marks the time of the EHT
campaign.
254
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//
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/
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/
threads
/
index.html
9
The Astrophysical Journal Letters,
911:L11
(
43pp
)
, 2021 April 10
The EHT MWL Science Working Group et al.