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From

Carlo Ferrigno at ISDC/INTEGRAL <carlo.ferrigno@unige.ch>

Edited By

Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov>

V. Savchenko (APC, Paris, France), S. Mereghetti (IASF-Mi, Italy), C.
Ferrigno, E. Bozzo, T. J.-L. Courvoisier (ISDC, University of Geneva,
CH),  E. Kuulkers (ESAC/ESA, Madrid, Spain), A. Bazzano (IAPS-Roma,
Italy)

The anti-coincidence shield of the spectrometer on board of INTEGRAL
(SPI/ACS) is sensitive to photons above ~50 keV, but without directional
information.

We investigated the SPI/ACS light curve at -1000 +1000 s from the trigger time
(2015-10-22 13:33:19 UTC) on temporal scales from 0.1 to 100 s. The
nearest excess on the constant background is at 2015-10-22 13:36:54, i.e.
215 seconds after the LIGO/Virgo trigger. The excess has a duration of
less than 0.05 seconds and a post-trial significance of 5.1 sigma. This
event is compatible with particle effects in SPI-ACS. We estimate the
chance of a particle effect to happen in the 200 seconds following the
trigger to be 20%. However, with SPI-ACS data alone we can not exclude
a cosmic origin of this excess.

The SPI/ACS light curves, binned at 50 ms, are derived from 91
independent detectors with different lower energy thresholds (mainly
between 50 keV and 150 keV) and an upper threshold at about 100 MeV. The
ACS response varies as a function of the incident angle. The highest
probability sky location of the LIGO trigger is in the direction of the
satellite Z-axis (the direction pointing to the Sun, where the SPI/ACS
has reduced sensitivity). For this direction we estimate a  3-sigma
upper limits corresponding to fluences in the 75-1000 keV band of 4e-6 erg/cm2
for a 100 s duration, 1.8e-6 erg/cm2 for 10 s, 5.8e-7 erg/cm2 for 1 s,
and 2.1e-7 erg/cm2 for 0.1 s. The upper limits assume a typical GRB
spectrum with Band model parameters alpha=1, beta=2.5 and E0 ~ 500 keV.
For the other probability region, the estimated upper limits are roughly
a factor three tighter. Given the same assumptions, if the excess at
2015-10-22 13:36:54 were of cosmic origin, it would have a fluence of 3.2e-7
erg/cm2 in the 75-1000 keV energy range.

We also searched for transient events in time coincidence with the LIGO
trigger in the data of the coded-mask imager IBIS, which at the time of
the trigger was pointed at R.A.=297.1 deg, Dec.=+29.3 deg (i.e., more
than 90 degrees from both LIGO highest probability regions). No
significant events were found on timescales from 2 ms to 100 s.
Typically, the minimum peak flux on 1 s time scale of gamma-ray
bursts detected with IBIS is 0.1 ph/s/cm2 in the 20-200 keV energy band,
in the central 9x9 deg of the field of view, which has zero response at 29x29 degrees.
However, due to the presence of the bright and variable source Cyg X-1  in the
field of view, we estimate that the sensitivity in the current
observation might be at least a factor 3 worse.

[GCN OPS NOTE(26oct15):  Because of a processing error, two circulars
received the same ID Number.  The second was given an "a' suffix.]

GCN OPS NOTE: The new GCN database requires the GCN Circular ID to be a number. 
Therefore, the previous GCN number assigned as 18453a was modified to 18448.5 
during the GCN Circulars migration in April 2023.

GCN Circular 18453

Subject

LIGO/Virgo G194575: RATIR Observations of Nearby Galaxies in the 1sigma Error Region

Date

2015-10-23T18:18:50Z (10 years ago)

From

Nat Butler at Az State U <natbutler@asu.edu>

Nat Butler (ASU), Neil Gehrels (GSFC), V. Zach Golkhou (ASU), Alexander
Kutyrev (GSFC), William H. Lee (UNAM), Eleonora Troja (GSFC), Alan M.
Watson (UNAM) report:

We observed 26 nearby galaxies (D<10 Mpc) within the GWGC catalog (White,
et al. 2011) and also contained with the LIGO/Virgo G194575 1-sigma
confidence interval (Singer, et al. GCN 18442Loading...
 
 ) between RA 23.1 hours and RA
1.5 hours (J2000) with the Reionization and Transients Infrared Camera
(RATIR; www.ratir.org) on the 1.5m Harold Johnson Telescope at the
Observatorio Astron��mico Nacional on Sierra San Pedro M��rtir on the night
of 2015/10/23.

We obtained a total exposure of 8 minutes on each field, reaching typical
depths of r and i = 21 mag, and z = 17 mag (10-sigma).  These magnitudes
are in the AB system and are not corrected for Galactic extinction.  Each
field, centered upon one GWGC galaxy, has a size of approximately 5 by 5
arcmin^2.  Analysis is ongoing.

We thank the staff of the Observatorio Astron��mico Nacional in San Pedro
M��rtir.

Continued observations are planned.

[GCN OPS NOTE(26oct15):  In the FROM-line the SB was replaced with NB.
SB manually processed the submission due to problems with NB's entry
in the vetting list.  The record has now been corrected.]

[GCN OPS NOTE(27oct15):  The Trigger number was changed from "G19457"
to "G194575" in the Subject-line and the first paragraph.]

[GCN OPS NOTE(15jun16):  If you are looking for the V.Savchenko circ 18453
("LIGO/Virgo G194575: INTEGRAL search of temporally coincident prompt hard X-ray emission"),
please note that the ID for that circular has been changed to "18453a".]

GCN Circular 18451

Subject

LIGO/Virgo G194575: more details on transients from Pan-STARRS in the RA=1hr localisation region

Date

2015-10-23T15:27:09Z (10 years ago)

From

S. J. Smartt at Queens U Belfast <s.smartt@qub.ac.uk>

K. Chambers, M. Huber, (IfA, University of Hawaii), K. W. Smith, 
D. Young, D. Wright, S.J. Smartt (Queen���s University Belfast), (QUB) E. Magnier,
H. Flewelling, C. Waters, J. Tonry, A. Schultz, N. Primak 
A. Heinze, B. Stalder, L. Denneau, A. Sherstyuk (IfA), C. Stubbs, M. Coughlin (Harvard), A. Rest (STScI)

Further to GCN 18445Loading...
 
 , we report that 7 transients were discovered within 
the 45% probability contour of the RA=1hr blob between the dates 
2015-10-16 and 2015-10-21. G194575 was reported with a time stamp of
2015-10-22 13:33:20 UTC, hence these first detections are a few
days before. They should be useful to other surveys to weed out any freshly 
appearing transients from existing ones. 

These are all likely supernova with identifications :  
PS15ckv, PS15clm, PS15cmc, PS15cll, PS15ckt, PS15cmj and PS15cng. 

All details for these transients are on 
http://star.pst.qub.ac.uk/ps1threepi/psdb/
We note the bright transient PS15cng (i=18.6) in MCG -01-59-024 (145 Mpc)

In addition, two new transients were detected at 2015-10-22 07:34 UTC and 
2015-10-22 07:32 UTC (6hrs before the GW event) 

PS15coc	00:28:48.57 -02:42:35.8 18.18(i)
PS15coa	00:16:47.00 -02:25:10.0 20.28(i)

They are both offset from obvious SDSS host galaxies by 7��� and 5.5���, but
no catalogued redshifts are available. Again, these are likely to be unrelated field supernovae
given their discovery before the GW event, and unrestrictive limits on their explosion epochs. 
Further analysis and observations are ongoing.

GCN Circular 18448

Subject

LIGO/Virgo G194575: Fermi GBM Observations

Date

2015-10-23T01:18:43Z (10 years ago)

From

Valerie Connaughton at NASA/MSFC/USRA <valerie@nasa.gov>

LIGO/Virgo G194575:  Fermi GBM Observations

Lindy Blackburn (CfA), Michael S. Briggs (UAH), Eric Burns (UAH), Jordan Camp (NASA/GSFC), 
Nelson Christensen (Carleton College), Valerie Connaughton(USRA), Tito Dal Canton (MPG), 
Adam Goldstein (NASA/MSFC), Peter Jenke (UAH), Tyson Littenberg (USRA/UAH), 
Judith Racusin (NASA/GSFC), Peter Shawhan (UMD), Leo Singer (NASA/GSFC), 
John Veitch (Birmingham), Binbin Zhang (UAH)

The region around 13h in Right Ascension, containing most of the probability for the LIGO CBC 
candidate G194575 reported by Singer et al. GCN 18442Loading...
 
 , was observed by Fermi GBM during 
the GW event.  The region around 01h was occulted by the Earth.

A search of the GBM Time-Tagged Event data between 8 keV and 40 MeV from 64 s before to 
82 s after the CBC candidate event revealed no significant emission on search timescales in 
factors of 2 from 64 ms to 2.048 s.

[GCN OPS NOTE(26oct15):  In the FROM-line the SB was replaced with VC.
SB manually processed the submission due to problems with VC's entry
in the vetting list.  The record has now been corrected.]

[GCN OPS NOTE(15jun16):  If you are looking for the P.Evans circ 18448
("GRB 151022A: Swift-XRT refined Analysis"), please note that the ID
for that circular has been changed to "18448a".]

GCN Circular 18446

Subject

LIGO/Virgo G194575: Mini-MegaTORTORA observations

Date

2015-10-23T00:50:00Z (10 years ago)

From

Sergey Karpov at Special Astrophysical Obs <karpov.sv@gmail.com>

S.Karpov, G.Beskin (SAO RAS and Kazan Federal University,
Russia), S.Bondar, E.Ivanov, E.Katkova, A.Perkov (OJS RPC PSI,
Russia), A.Biryukov (SAI MSU and Kazan Federal University,
Russia), V.Sasyuk (Kazan Federal University, Russia)

Following GCN #18442Loading...
 
  on the possible GW event G194575, we
observed its secondary probability density region with
Mini-MegaTORTORA nine-channel wide-field monitoring system
(located at Special Astrophysical Observatory near Russian 6-m
telescope) starting on 2015-10-22 22:33:24 UT under bad weather
conditions.

The system field of view has been centered on RA, Dec = 16.8, 5.7
covering roughly 900 square degrees simultaneously. Coverage map
of all nine channels is shown at
http://mmt.favor2.info/scheduler/761/lvc

Every channel acquired 10 x 60s exposure images in white light
between 2015-10-22 22:33:24 UT and 2015-10-22 22:42:31 UT,
further observations interrupted due to heavy clouds. The
footprints of acquired images are uploaded to GraceDB.

Quick-look analysis did not reveal any transient brighter than
V~13.5 magnitude in the data.

GCN Circular 18445

Subject

LIGO/Virgo G194575: transients from Pan-STARRS in the RA=1hr localisation region

Date

2015-10-23T00:15:21Z (10 years ago)

From

S. J. Smartt at Queens U Belfast <s.smartt@qub.ac.uk>

S.J. Smartt (Queen���s University Belfast), K. Chambers 
(IfA, University of Hawaii), K. W. Smith,  (QUB), M. Huber, E. Magnier,
H. Flewelling, C. Waters, J. Tonry, A. Schultz, N. Primak 
A. Heinze, B. Stalder, L. Denneau, A. Sherstyuk (IfA), D. Young, 
D. Wright, (QUB), C. Stubbs, M. Coughlin (Harvard), A. Rest (STScI)

Report several transients discovered in and around the 
RA=1hr DEC=+4deg error region as defined in the BAYESTAR 
skymap.fits file. 

This area has been covered in the w and i-bands 
during the routine course of the Pan-STARRS Survey for Transients
(see Huber et al. 2015, ATel 7153 and http://star.pst.qub.ac.uk/ps1threepi/psdb/)
and around 10 supernova candidates are known from the last 
7 days of observations in this region. These are almost certainly all 
unrelated to the GW trigger, as they were discovered days before the 
LIGO/Virgo detection. Details are available on the public webpage above. 

For example, the most recent discovery is a i=18.6 supernova candidate at 
7.2��� from the core of MCG -01-59-024 at z=0.033. This was discovered on 57315.31, 
54h before the GW trigger (at position 23:28:24.29 -02:47:57.8)

Pan-STARRS was observing the RA=1hr localisation region (south of dec=0), 
during the night of 2015-10-22. However the observations were taken 5-6hrs
before the LIGO/Virgo detection. Difference imaging and transient searches
are underway and further observations are planned for the 
coming nights. We plan to cover this RA=1hr region with the PS1 1.8m
telescope, 7 sq degree camera, in i and z bands.

GCN Circular 18442

Subject

LIGO/Virgo G194575: Identification of a GW CBC Candidate

Date

2015-10-22T20:03:45Z (10 years ago)

From

Leo Singer at NASA/GSFC <leo.p.singer@nasa.gov>

Leo Singer (NASA/GSFC), Peter Shawhan (UMD), Nipuni Palliyaguru (TTU), B.
S. Sathyaprakash (Cardiff), Alex Urban (UWM), Jeff Bartlett (LIGO
Hanford), Travis Sadecki (LIGO Hanford), Mike Landry (LIGO Hanford), Joe
Hanson (LIGO Livingston), Bryan Smith (LIGO Livingston), Brian O'Reilly
(LIGO Livingston), Kipp Cannon (CITA), Gianluca Guidi (Urbino), Andy
Lundgren (AEI/Hannover), Laura Nuttall (Syracuse), T. J. Massinger
(Syracuse), Jessica McIver (Caltech), and Joshua Smith (Fullerton) report
on behalf of the LIGO Scientific Collaboration and Virgo:

The gstlal CBC analysis, which is sensitive to binary coalescence events
from systems containing at least one neutron star, identified candidate
G194575 during real-time processing of data from LIGO Hanford Observatory
(H1) and LIGO Livingston Observatory (L1) at 2015-10-22 13:33:19.942 UTC
(GPS time: 1129556016.942). Both the H1 and L1 detectors were in
observation mode and operating normally.

G194575 is an event of interest because its false alarm rate, as
determined by the online analysis, is 9.65e-08 Hz or about one in 4
months, passing our stated alert threshold of ~1/month for CBC candidates.
The event's properties can be found at this URL:

https://gracedb.ligo.org/events/G194575

Fermi triggered (bn151022577) about 1 ks after the LIGO candidate, and
Swift detected GRB 151022A (D���Avanzo et al., GCN 18436Loading...
 
 ) about 2 ks after.
Neither gamma-ray trigger���s localization is consistent with the rapid
position reconstruction of the LIGO candidate. Based on the separation in
time and sky position, we infer that these high-energy triggers are
unrelated.

One sky map is available at this time and can be retrieved from the
GraceDB event page: skymap.fits.gz, an initial localization generated by
BAYESTAR. The probability is concentrated in two main equatorial regions
around right ascensions of 1h and 13h. About 30% of the probability is
confined to the top 500 deg2.

Updates on our analysis of this event will be sent as they become
available.