T
HE
A
STROPHYSICAL
J
OURNAL
L
ETTERS
, 826:L13, 2016 J
ULY
20
Preprint typeset using L
A
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X style AASTeX6 v. 1.0
LOCALIZATION AND BROADBAND FOLLOW-UP OF THE GRAVITATIONAL-WAVE TRANSIENT GW150914
T
HE
LIGO S
CIENTIFIC
C
OLLABORATION AND THE
V
IRGO
C
OLLABORATION
,
THE
A
USTRALIAN
S
QUARE
K
ILOMETER
A
RRAY
P
ATHFINDER
(ASKAP) C
OLLABORATION
,
THE
BOOTES C
OLLABORATION
,
THE
D
ARK
E
NERGY
S
URVEY AND THE
D
ARK
E
NERGY
C
AMERA
GW-EM C
OLLABORATIONS
,
THE
Fermi
GBM C
OLLABORATION
,
THE
Fermi
LAT C
OLLABORATION
,
THE
GRA
VITATIONAL
W
AVE
I
NAF
T
E
A
M
(GRAWITA),
THE
INTEGRAL
C
OLLABORATION
,
THE
I
NTERMEDIATE
P
ALOMAR
T
RANSIENT
F
ACTORY
(
I
PTF) C
OLLABORATION
,
THE
I
NTER
P
LANETARY
N
ETWORK
,
THE
J-GEM C
OLLABORATION
,
THE
L
A
S
ILLA
–QUEST S
URVEY
,
THE
L
IVERPOOL
T
ELESCOPE
C
OLLABORATION
,
THE
L
OW
F
REQUENCY
A
RRAY
(LOFAR) C
OLLABORATION
,
THE
MASTER C
OLLABORATION
,
THE
MAXI C
OLLABORATION
,
THE
M
URCHISON
W
IDE
-
FIELD
A
RRAY
(MWA) C
OLLABORATION
,
THE
P
AN
-STARRS C
OLLABORATION
,
THE
PESSTO C
OLLABORATION
,
THE
P
I OF THE
S
KY
C
OLLABORATION
,
THE
S
KY
M
APPER
C
OLLABORATION
,
THE
Swift
C
OLLABORATION
,
THE
TAROT, Z
ADKO
, A
LGERIAN
N
ATIONAL
O
BSERVATORY
,
AND
C2PU C
OLLABORATION
,
THE
TOROS C
OLLABORATION
,
AND THE
VISTA C
OLLABORATION
See the Supplement, Abbott et al. 2016g, for the full list of authors.
(Received 2016 February 29; Accepted 2016 April 26; Published 2016 July 20)
ABSTRACT
A gravitational-wave (GW) transient was identified in data recorded by the Advanced Laser Interferometer
Gravitational-wave Observatory (LIGO) detectors on 2015 September 14. The event, initially designated
G184098 and later given the name GW150914, is described in detail elsewhere. By prior arrangement, prelim-
inary estimates of the time, significance, and sky location of the event were shared with 63 teams of observers
covering radio, optical, near-infrared, X-ray, and gamma-ray wavelengths with ground- and space-based facil-
ities. In this Letter we describe the low-latency analysis of the GW data and present the sky localization of
the first observed compact binary merger. We summarize the follow-up observations reported by 25 teams via
private Gamma-ray Coordinates Network circulars, giving an overview of the participating facilities, the GW
sky localization coverage, the timeline and depth of the observations. As this event turned out to be a binary
black hole merger, there is little expectation of a detectable electromagnetic (EM) signature. Nevertheless, this
first broadband campaign to search for a counterpart of an Advanced LIGO source represents a milestone and
highlights the broad capabilities of the transient astronomy community and the observing strategies that have
been developed to pursue neutron star binary merger events. Detailed investigations of the EM data and results
of the EM follow-up campaign are being disseminated in papers by the individual teams.
Keywords:
gravitational waves; methods: observational
1.
INTRODUCTION
In 2015 September, the Advanced Laser Interferom-
eter Gravitational-wave Observatory (LIGO; Aasi et al.
2015) made the first direct detection of an astrophysical
gravitational-wave (GW) signal that turned out to be from
a binary black hole (BBH) merger.
The LIGO Hanford
and Livingston sites are the first two nodes of a growing
global network of highly sensitive GW facilities, soon to
include Advanced Virgo (Acernese et al. 2015), KAGRA,
and LIGO–India. Some of the most promising astrophysical
sources of GWs are also expected to produce broadband elec-
tromagnetic (EM) emission and neutrinos. This has created
exciting new opportunities for joint broadband EM observa-
tions and multi-messenger astronomy.
In a compact binary coalescence (CBC) event, a tight binary
comprised of two neutron stars (NSs), two black holes (BHs),
lsc-spokesperson@ligo.org
virgo-spokesperson@ego-gw.eu
or a NS and a BH experiences a runaway orbital decay due
to gravitational radiation. In a binary including at least one
NS— a binary neutron star (BNS) or neutron star–black hole
(NSBH) merger—we expect EM signatures due to energetic
outflows at different timescales and wavelengths. If a rela-
tivistic jet forms, we may observe a prompt short gamma-ray
burst (GRB) lasting on the order of one second or less, fol-
lowed by X-ray, optical, and radio afterglows of hours to days
duration (e.g., Eichler et al. 1989; Narayan et al. 1992; Nakar
2007; Berger 2014; Fong et al. 2015). Rapid neutron capture
in the sub-relativistic ejecta (e.g., Lattimer & Schramm 1976)
is hypothesized to produce a kilonova or macronova, an op-
tical and near-infrared signal lasting hours to weeks (e.g., Li
& Paczy
́
nski 1998). Eventually, we may observe a radio blast
wave from this sub-relativistic outflow, detectable for months
to years (e.g., Nakar & Piran 2011). Furthermore, several sec-
onds prior to or tens of minutes after merger, we may see
a coherent radio burst lasting milliseconds (e.g., Hansen &
Lyutikov 2001; Zhang 2014). In short, a NS binary can pro-
arXiv:1602.08492v4 [astro-ph.HE] 21 Jul 2016
2
duce EM radiation over a wide range of wavelengths and time
scales. On the other hand, in the case of a stellar-mass BBH,
the current consensus is that no significant EM counterpart
emission is expected except for those in highly improbable
environments pervaded by large ambient magnetic fields or
baryon densities.
The first campaign to find EM counterparts triggered by
low-latency GW event candidates was carried out with the ini-
tial LIGO and Virgo detectors and several EM astronomy fa-
cilities in 2009 and 2010 (Abadie et al. 2012a,b; Evans et al.
2012; Aasi et al. 2014). In preparing for Advanced detec-
tor operations, the LIGO and Virgo collaborations worked
with the broader astronomy community to set up an evolved
and greatly expanded EM follow-up program.
1
Seventy-
four groups with access to ground- and space-based facili-
ties joined, of which 63 were operational during Advanced
LIGO’s first observing run (O1). Details of the 2009 to 2010
EM follow campaign and changes for O1 are given in Sec-
tion 1 of the Supplement (Abbott et al. 2016g).
After years of construction and commissioning, the Ad-
vanced LIGO detectors at Livingston, Louisiana, and Han-
ford, Washington, began observing in 2015 September with
about
3
.
5
times the distance reach (
>
40
times the sensitive
volume) of the earlier detectors. A strong GW event was iden-
tified shortly after the pre-run calibration process was com-
pleted. Deep analysis of this event, initially called G184098
and later given the name GW150914, is presented in Abbott
et al. (2016c) and companion papers referenced therein. In
this paper we describe the initial low-latency analysis and
event candidate selection (Section 2), the rapid determina-
tion of likely sky localization (Section 3), and the follow-
up EM observations carried out by partner facilities (Sec-
tions 4 and 5). For analyses of those observations, we refer
the reader to the now-public Gamma-ray Coordinates Net-
work (GCN) circulars
2
and to a number of recent papers.
We end with a brief discussion of EM counterpart detection
prospects for future events.
2.
DATA ANALYSIS AND DISCOVERY
As configured for O1, four low-latency pipelines contin-
ually search for transient signals that are coincident in the
two detectors within the
10
ms light travel time separating
them. Coherent WaveBurst (cWB; Klimenko et al. 2016) and
Omicron+LALInference Burst (oLIB; Lynch et al. 2015) both
search for unmodeled GW bursts (Abbott et al. 2016d). GST-
LAL (Cannon et al. 2012; Messick et al. 2016) and Multi-
Band Template Analysis (MBTA; Adams et al. 2015) search
specifically for NS binary mergers using matched filtering.
Because CBC waveforms can be precisely computed from
general relativity, GSTLAL and MBTA are more sensitive to
1
See program description and participation information at
http://
www.ligo.org/scientists/GWEMalerts.php
.
2
All circulars related to GW150914 are collected at
http://gcn.
gsfc.nasa.gov/other/GW150914.gcn3
CBC signals than the burst search pipelines are. All four de-
tection pipelines report candidates within a few minutes of
data acquisition.
LIGO conducted a series of engineering runs throughout
Advanced LIGO’s construction and commissioning to pre-
pare to collect and analyze data in a stable configuration. The
eighth engineering run (ER8) began on 2015 August 17 at
15:00 and critical software was frozen by August 30.
3
The
rest of ER8 was to be used to calibrate the detectors, to carry
out diagnostic studies, to practice maintaining a high coin-
cident duty cycle, and to train and tune the data analysis
pipelines. Calibration was complete by September 12 and O1
was scheduled to begin on September 18. On 2015 Septem-
ber 14, cWB reported a burst candidate to have occurred at
09:50:45 with a network signal-to-noise ratio (S/N) of 23.45
and an estimated false alarm rate (FAR)
<
0
.
371
yr
−
1
based
on the available (limited at that time) data statistics. Also,
oLIB reported a candidate with consistent timing and S/N.
No candidates were reported at this time by the low-latency
GSTLAL and MBTA pipelines, ruling out a BNS or NSBH
merger.
Although the candidate occurred before O1 officially be-
gan, the LIGO and Virgo collaborations decided to send an
alert to partner facilites because the preliminary FAR esti-
mate satisfied our planned alert threshold of 1 month
−
1
. Al-
though we had not planned to disseminate real-time GCN no-
tices before the formal start of O1, most of the computing
infrastructure was in place. Basic data quality checks were
done within hours of GW150914; both interferometers were
stable and the data stream was free of artifacts (Abbott et al.
2016a). A cWB sky map was available 17 minutes after the
data were recorded, and a LALInference Burst (LIB) sky map
was available after 14 hr. After extra data integrity checks and
an update to the GCN server software, these two sky maps
were communicated to observing partners in a GCN circu-
lar nearly two days after the event occurred (GCN 18330).
Mass estimates were not released in this initial circular, and
observers may have assumed the event was associated with a
BNS system or a GW burst (e.g., from a nearby core-collapse
supernova; SN). The knowledge that GW150914 was consis-
tent with a BBH inspiral and merger was only shared later on
October 3 (GCN 18388). Figure 1 shows the chronology of
the GW detection alerts and follow-up observations.
The data were re-analyzed offline with two independent
matched-filter searches using a template bank that includes
both NS binary and BBH mergers. The waveform was con-
firmed to be consistent with a BBH merger and this informa-
tion was shared with observers about three weeks after the
event (GCN 18388). The FAR was evaluated with the data
collected through 20 October, reported to be less than 1 in
100 years (GCN 18851; Abbott et al. 2016b), and ultimately
determined to be much lower. The final results of the offline
3
All dates and times are in UT.
L
OCALIZATION AND BROADBAND FOLLOW
-
UP OF
GW150914
3
10
0
10
1
10
2
t
−
t
merger
(days)
Initial GW
Burst Recovery
Initial
GCN Circular
Updated GCN Circular
(identified as BBH candidate)
Final
sky map
Fermi
GBM, LAT, MAXI,
IPN,
INTEGRAL
(archival)
Swift
XRT
Swift
XRT
Fermi
LAT,
MAXI
BOOTES-3
MASTER
Swift
UVOT, SkyMapper, MASTER, TOROS, TAROT, VST, iPTF,
Keck
,
Pan-STARRS1, KWFC, QUEST, DECam,
LT
,
P200
, Pi of the Sky,
PESSTO
,
UH
Pan-STARRS1
VST
TOROS
VISTA
MWA
ASKAP,
LOFAR
ASKAP,
MWA
VLA
,
LOFAR
VLA
,
LOFAR
VLA
Figure 1
. Timeline of observations of GW150914, separated by band and relative to the time of the GW trigger. The top row shows
GW information releases. The bottom four rows show high-energy, optical, near-infrared, and radio observations, respectively.
Optical spectroscopy and narrow-field radio observations are indicated with darker tick marks and boldface text. Table 1 reports
more detailed information on the times of observations made with each instrument.
search are reported in Abbott et al. (2016c).
3.
SKY MAPS
We produce and disseminate probability sky maps using a
sequence of algorithms with increasing accuracy and compu-
tational cost. Here, we compare four location estimates: the
prompt cWB and LIB localizations that were initially shared
with observing partners plus the rapid BAYESTAR localiza-
tion and the final localization from LALInference. All four
are shown in Fig. 2.
cWB performs a constrained maximum likelihood estimate
of the reconstructed signal on a sky grid (Klimenko et al.
2016) weighted by the detectors’ antenna patterns (Essick
et al. 2015) and makes minimal assumptions about the wave-
form morphology. With two detectors, this amounts to re-
stricting the signal to only one of two orthogonal GW polar-
izations throughout most of the sky. LIB performs Bayesian
inference assuming the signal is a sinusoidally modulated
Gaussian (Lynch et al. 2015). While this assumption may not
perfectly match the data, it is flexible enough to produce reli-
able localizations for a wide variety of waveforms, including
BBH inspiral-merger-ringdown signals (Essick et al. 2015).
BAYESTAR produces sky maps by triangulating the times,
amplitudes, and phases on arrival supplied by all the CBC
pipelines (Singer & Price 2016). BAYESTAR was not avail-
able promptly because the low-latency CBC searches were
not configured for BBHs; the localization presented here is
derived from the offline CBC search. LALInference performs
full forward modeling of the data using a parameterized CBC
waveform which allows for BH spins and detector calibra-
tion uncertainties (Veitch et al. 2015). It is the most accurate
method for CBC signals but takes the most time due to the
high dimensionality. We present the same LALInference map
as Abbott et al. (2016e), with a spline interpolation proce-
dure to include the potential effects of calibration uncertain-
ties. The BAYESTAR and LALInference maps were shared
with observers on 2016 January 13 (GCN 18858), at the con-
clusion of the O1 run. Since GW150914 is a CBC event, we
consider the LALInference map to be the most accurate, au-
thoritative, and
final
localization for this event. This map has
a 90% credible region with area 630 deg
2
.
All of the sky maps agree qualitatively, favoring a broad,
long section of arc in the southern hemisphere and to a lesser
extent a shorter section of nearly the same arc near the equa-
tor. While the majority of LIB’s probability is concentrated
in the southern hemisphere, a non-trivial fraction of the 90%
confidence region extends into the northern hemisphere. The
LALInference sky map shows much less support in the north-
ern hemisphere which is likely associated with the stronger
constraints available with full CBC waveforms. The cWB lo-
calization also supports an isolated hot spot near
α
∼
9
h
,δ
∼
5
◦
, where the detector responses make it possible to indepen-
dently measure two polarization components. In this region,
cWB considers signals not constrained to have the elliptical
polarization expected from a compact binary merger.
Quantitative comparisons of the four sky maps can be found
in section 2 of the Supplement (Abbott et al. 2016g). The
main feature in all of the maps is an annulus with polar an-
gle
θ
HL
determined by the arrival time difference
∆
t
HL
be-
tween the two detectors. However, refinements are possible
due to phase as well as amplitude consistency and the mildly
directional antenna patterns of the LIGO detectors (Kasli-
wal & Nissanke 2014; Singer et al. 2014). In particular, the
detectors’ antenna patterns dominate the modulation around
the ring for un-modeled reconstructions through a correlation
with the inferred distance of the source (Essick et al. 2015).
As shown in Fig. 2, the algorithms all infer polar angles that
are consistent at the
1
σ
level.
4
∆
t
HL
,
θ
HL
cWB
LIB
BAYESTAR
LALInference
4h
8h
12h
16h
20h
24h
7.7
ms
7.1
ms
6.4
ms
40°
45°
50°
Figure 2
.
Comparison of different GW sky maps, showing the 90% credible level contours for each algorithm. This is an
orthographic projection centered on the centroid of the LIB localization. The inset shows the distribution of the polar angle
θ
HL
(equivalently, the arrival time difference
∆
t
HL
).
4.
FOLLOW-UP OBSERVATIONS
Twenty-five participating teams of observers responded to
the GW alert to mobilize satellites and ground-based tele-
scopes spanning 19 orders of magnitude in EM wavelength.
Observations and archival analysis started shortly after the
candidate was announced, two days after the event was
recorded. Most facilities followed tiling strategies based on
the cWB and LIB sky maps. Some groups, considering the
possibility of a NS merger or core-collapse SN, selected fields
based on the areal density of nearby galaxies or targeted the
Large Magellanic Cloud (LMC) (e.g., Annis et al. 2016). Had
the BBH nature of the signal been promptly available, most
groups would not have favored local galaxies because LIGO’s
range for BBH mergers is many times larger than that for
BNSs. Fig. 3 displays the footprints of all reported obser-
vations. The campaign is summarized in Table 1 in terms of