J/ApJS/245/15 Swift XRT follow-up of LIGO/Virgo GW triggers (Klingler+, 2019)
Swift-XRT follow-up of gravitational-wave triggers in the second advanced
LIGO/Virgo observing run.
Klingler N.J., Kennea J.A., Evans P.A., Tohuvavohu A., Cenko S.B.,
Barthelmy S.D., Beardmore A.P., Breeveld A.A., Brown P.J., Burrows D.N.,
Campana S., Cusumano G., D'Ai A., D'Avanzo P., D'Elia V., de Pasquale M.,
Emery S.W.K., Garcia J., Giommi P., Gronwall C., Hartmann D.H., Krimm H.A.,
Kuin N.P.M., Lien A., Malesani D.B., Marshall F.E., Melandri A.,
Nousek J.A., Oates S.R., O'Brien P.T., Osborne J.P., Page K.L., Palmer D.M.,
Perri M., Racusin J.L., Siegel M.H., Sakamoto T., Sbarufatti B.,
Tagliaferri G., Troja E.
<Astrophys. J. Suppl. Ser., 245, 15 (2019)>
=2019ApJS..245...15K 2019ApJS..245...15K
ADC_Keywords: Gravitational wave; X-ray sources
Keywords: Gravitational waves ; Astronomy data analysis ; Gamma-ray bursts ;
X-ray sources ; Surveys ; X-ray surveys ; Catalogs ;
Gravitational wave sources
Abstract:
The Neil Gehrels Swift Observatory carried out prompt searches for
gravitational-wave (GW) events detected by the LIGO/Virgo
Collaboration (LVC) during the second observing run ("O2"). Swift
performed extensive tiling of eight LVC triggers, two of which had
very low false-alarm rates (GW170814 and the epochal GW170817),
indicating a high confidence of being astrophysical in origin; the
latter was the first GW event to have an electromagnetic counterpart
detected. In this paper we describe the follow-up performed during O2
and the results of our searches. No GW electromagnetic counterparts
were detected; this result is expected, as GW170817 remained the only
astrophysical event containing at least one neutron star after LVC's
later retraction of some events. A number of X-ray sources were
detected, with the majority of identified sources being active
galactic nuclei. We discuss the detection rate of transient X-ray
sources and their implications in the O2 tiling searches. Finally, we
describe the lessons learned during O2 and how these are being used to
improve the Swift follow-up of GW events. In particular, we simulate a
population of gamma-ray burst afterglows to evaluate our source
ranking system's ability to differentiate them from unrelated and
uncataloged X-ray sources. We find that ∼60%-70% of afterglows whose
jets are oriented toward Earth will be given high rank (i.e.,
"interesting" designation) by the completion of our second follow-up
phase (assuming that their location in the sky was observed), but that
this fraction can be increased to nearly 100% by performing a third
follow-up observation of sources exhibiting fading behavior.
Description:
The observing strategy employed in this campaign is the same as that
which has been described in detail by Evans+ (2016MNRAS.462.1591E 2016MNRAS.462.1591E and
2016MNRAS.455.1522E 2016MNRAS.455.1522E).
The overall observing strategy was to carry out three phases of
observations. First (phase 1), a series of short (60s) exposures were
taken covering as much of the galaxy-map-convolved gravitational-wave
(GW) error region as possible. This phase was designed with the
intention of detecting an on-axis sGRB afterglow, if present, and
continued for ∼2 days. Next (phase 2), 2-3 days after the trigger, the
GW error region was reobserved for 500s per tile. These observations,
which continued for up to 4 days, were optimized to search for the
rising X-ray afterglow from an sGRB observed off-axis. Collectively,
phases 1 and 2 are referred to as the "wide-area search" phase.
Finally (phase 3), if no confirmed counterpart was found, any
potentially interesting (i.e., unidentified) X-ray source was
reobserved with exposures >1ks. Any such source found to be fading was
reobserved repeatedly over the following days until it could be
confirmed to be the counterpart, or ruled out as such.
See Section 2 for further details.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 89 14 Overview of gravitational-wave (GW) Triggers and
Swift Follow-up in LIGO/Virgo Collaboration (LVC)
second observing run ("O2")
table2.dat 90 157 Catalog of X-ray sources detected in the follow-up
searches for the O2 GW triggers
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See also:
IX/10 : ROSAT All-Sky Bright Source Catalogue (1RXS) (Voges+ 1999)
VII/267 : Gravitational Wave Galaxy Catalogue (White+, 2011)
VII/273 : The Half Million Quasars (HMQ) catalogue (Flesch, 2015)
VII/277 : The Million Quasars (Milliquas) catalog (Flesch, 2016)
VII/280 : The Million Quasars (Milliquas) catalog (V5.2) (Flesch, 2017)
VII/283 : The Million Quasars (Milliquas) catalog (6.3) (Flesch, 2019)
J/A+A/480/611 : XMM-Newton slew survey catalogue, XMMSL1 (Saxton+, 2008)
J/MNRAS/397/1177 : Swift-XRT observations of GRBs (Evans+, 2009)
J/MNRAS/448/2210 : Swift follow-up of IceCube triggers (Evans+, 2015)
J/ApJS/219/39 : QSOs selection from SDSS and WISE (Richards+, 2015)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 2 I2 --- GW [4/17] GW number
4- 10 A7 --- Trigger Trigger ID (1)
12 A1 --- f_Trigger [ac] Flag on Trigger (2)
14- 32 A19 "datime" Date Trigger date
34- 35 A2 --- l_Dist [≲] Limit flag on Dist
36- 39 I4 Mpc Dist Estimated distance to the GW
41- 43 I3 Mpc e_Dist [7/477]? Uncertainty on Dist
45- 47 I3 % Pns ? Probability that the event included
at least one neutron star
48 A1 --- f_Pns b: Flag on compact binary coalescence (CBC)
triggers (2)
50- 53 F4.1 ks Delay [3.3/62.8]? Delay (3)
55- 58 I4 ks Dur [29/9206]? Duration (4)
60- 62 I3 ks Exp [9/269]? Exposure time
64- 67 I4 --- Nf [4/1408]? Number of fields
69- 73 F5.1 deg2 Area [0.5/171]? Area corrected for the overlaps
between adjacent tiles
75- 78 F4.1 % Praw [0/24]? Fraction of the LVC sky map
that was enclosed by XRT observations
80- 83 F4.1 % Pconv [0/94]? Fraction of the galaxy-convolved sky
map that was covered by XRT observations
85- 86 I2 --- Nxrt [0/58]? Number of sources detected by the
XRT in each follow-up search
(only phases 1 and 2)
88- 89 I2 --- Nun [0/19]? Number (of NXRT) that are uncataloged
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Note (1): For completeness, it is worth noting that there were two additional
GW triggers that were only detected in post-O2 analyses and
consequently could not have been followed up within a reasonable
amount of time: GW170729 and GW170818 (see Abbott+ 2019PhRvX...9c1040A 2019PhRvX...9c1040A).
Note (2): Flag as follows:
a = Trigger G275697 was retracted after the follow-up search was performed.
b = The event was a burst GW event, in which case (since little is known about
their origin) the probability that they involve an NS cannot be determined.
c = Triggers G275404 and G299232 (although originally marked with PNS=100%)
were later determined not to be real astronomical events
(see Abbott+ 2019ApJ...875..161A 2019ApJ...875..161A).
Note (3): The delay is the interval between the GW trigger time and the time at
which the first follow-up observation began.
Note (4): The duration is the time from the start of the first observation with
Swift-XRT to the end of the last one. Swift-XRT was not observing the
GW region for the entirety of this time, so the total exposure is
given in column "Exp".
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Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 2 I2 --- GW [4/17] GW event number
4 I1 --- Rank [2/4] Rank (1)
6- 7 I2 h RAh Hour of Right Ascension (J2000)
9- 10 I2 min RAm Minute of Right Ascension (J2000)
12- 16 F5.2 s RAs Second of Right Ascension (J2000)
18 A1 --- DE- Sign of the Declination (J2000)
19- 20 I2 deg DEd Degree of Declination (J2000)
22- 23 I2 arcmin DEm Arcminute of Declination (J2000)
25- 28 F4.1 arcsec DEs Arcsecond of Declination (J2000)
30- 32 F3.1 arcsec PosErr [3.6/9.8] The 90% positional uncertainty
34- 38 F5.3 ct/s PRate [0.01/2.7] Peak source XRT count rate
40- 44 F5.3 ct/s e_PRate [0.004/0.5] Uncertainty in PRate
46- 53 E8.2 mW/m2 PFlux [4.1e-13/1.2e-10] Peak flux (2)
55- 62 E8.2 mW/m2 e_PFlux [1.8e-13/2.3e-11] Uncertainty in PFlux (2)
64- 78 A15 --- OType Simbad source type (3)
80 A1 --- Known Source has been previously detected in X-rays
(Y: 103 occurrences)
82- 83 I2 --- Near [0/10] Number of known "nearby" galaxies;
Section 4.1
85 I1 --- N2m [0/6] Number of nearby/positionally coincident
2MASS sources
87- 90 F4.1 --- Fade [0/5.2] Statistical significance of fading
behavior, if present
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Note (1): Rank code as follows:
2 = interesting source;
3 = uncataloged X-ray source;
4 = catalogued X-ray source.
Note (2): See section 3 for details. In units of erg/s/cm2.
Note (3): "AGN; MQ" and "AGN; S/W" denote that there is a positionally
coincident entry in the Million Quasars catalog or the QSO selection
from SDSS and WISE; see Flesch 2015PASA...32...10F 2015PASA...32...10F (Cat. VII/283) and
Richards+ 2015, J/ApJS/219/39
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History:
From electronic version of the journal
(End) Prepared by [AAS], Emmanuelle Perret [CDS] 23-Mar-2020