LIGO/Virgo/KAGRA S230615az

H. Kumar (IITB), V. Swain (IITB), A. Salgundi (IITB), K. Angail (IAO), V. Bhalerao (IITB), G. C. Anupama (IIA), S. Barway (IIA) report on behalf of the GIT team:

We followed up the sky localisation of the gravitational wave trigger S230615az with the 0.7m GROWTH-India telescope. We obtained multiple 300-sec exposures in the r' filter starting at 2023-06-15T17:56:40 UT, 6.55 minutes after the trigger. By targeting galaxies in the region, we covered 0.4% of the source probability on the sky, based on the "bayestar.multiorder.fits,1" map. After performing image subtraction, vetting, and a cross-check with minor planet centre, we obtained the following two transient candidates within the 90% localisation region of the event:

|--------------|----------------|-------------|-------------|--------|----------------|
| Name         | JD             | RA(J2000)   | Dec(J2000)  | Filter | Mag (AB)       |
|--------------|----------------|-------------|-------------|--------|----------------|
| GIT230615aa  | 2460111.271145 | 12:50:03.64 | 20:53:21.77 |   r'   | 20.05 +/- 0.06 |
| GIT230615ab  | 2460111.286931 | 12:42:11.65 | 22:03:25.08 |   r'   | 19.81 +/- 0.05 |
|--------------|----------------|-------------|-------------|--------|----------------|

GIT230615aa was detected in two images separated by 17 minutes. There is no significant fading between the two images. We could not get a second confirmation image for GIT230615ab as the source was set soon after the first image.

The magnitudes are calibrated against PanSTARRS DR1 (Chambers et al., 2016) and not corrected for Galactic extinction.

The GROWTH India Telescope (GIT) is a 70-cm telescope with a 0.7-degree field of view, set up by the Indian Institute of Astrophysics (IIA) and the Indian Institute of Technology Bombay (IITB) with funding from DST-SERB and IUSSTF. It is located at the Indian Astronomical Observatory (Hanle), operated by IIA. We acknowledge funding by the IITB alumni batch of 1994, which partially supports the operations of the telescope. Telescope technical details are available at https://sites.google.com/view/growthindia/.

B. Gompertz, R. Kotak, J. Lyman, D. O'Neill, B. Godson, A. Kumar, P. Clark, K. Wiersema, L. Kelsey, M. Kennedy, G. Ramsay, K. Ackley, K. Ulaczyk, M. Dyer, T. Killestein, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, D. Pollacco, K. Noysena, L. Nuttall, E. Pallé, report on behalf of the GOTO collaboration:


We report on optical observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2021) in response to the LIGO/Virgo candidate BNS event S230615az. Targeted observations covering 686 sqr. degrees or 24.1% of the localisation probability of the BAYESTAR skymap were performed between 21:16 UT June 15 and 00:35 UT June 16 2023 (3.43 to 6.75 hours after trigger).


Exposures were taken in L-band (400-700 nm passband) and the median 5 sigma depth was 20.3 AB magnitudes.


Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using recent survey observations of the same pointings. Source candidates were initially filtered using a classifier (Killestein et al. 2021) and cross-matched against a variety of contextual and minor planet catalogues. Human vetting was carried out in real time on any candidates that passed the above checks.


Within the 90% localisation region of the LVK skymap, we identify two priority candidate counterparts. Both are associated with catalogued galaxies that have photometric redshifts consistent with the GW distance posterior. We find no evidence of these sources prior to the GW alert in previous GOTO observations or the ZTF observations provided by the Lasair broker (Smith et al. 2019). We note possible low significance pre-GW flux in the ATLAS forced photometry server (Shingles et al. 2021) for GOTO23hn.


Name     | RA(J2000)   | Dec(J2000)   | Filter  | Mag(AB)        | D_L(Mpc) | Catalog

GOTO23hn | 12:40:35.55 | +17:02:15.27 | L       | 19.49 +/- 0.10 | 278.0 | WISExSuperCOSMOS

GOTO23hu | 13:22:55.23 | +08:09:49.57 | L       | 19.43 +/- 0.08 | 245.6 | GLADE+


Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and not corrected for Galactic extinction.


Observations are ongoing.

GOTO (https://goto-observatory.org) is a network of telescopes operated at the Roque de los Muchachos Observatory on La Palma, Spain and Siding Spring Observatory in NSW, Australia;  on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), University of Turku, University of Portsmouth, University of Manchester and the Instituto de Astrofisica de Canarias (IAC)

Z.P. Zhu (NAOC, HUST), D. Xu, S.Y. Fu, X. Liu, S.Q. Jiang, T.H. Lu (NAOC) report:

We observed the field of the GW trigger S230615z candidates reported by GIT (Kumar et al., GCN 33974
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 ) with the 2.16m telescope located at Xinglong, Hebei, China, equipped with the BFOSC camera. Observations started at 12:47:19 UT on 2023-06-16 (i.e., 0.8 day after the GW trigger time), and 5x300 s frames were obtained in the R-band for either candidate.

No source is detected in our stacked images at either candidate's position (Kumar et al., GCN 33974
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 ), down to an upper limit of R~22.1 mag for both, calibrated with the nearby Pan-STARRS field.

Gregory S.-H. Paek (SNU ARC/SNU), Myungshin Im (SNU ARC/SNU), and Hyun-Il Sung (KASI), on behalf of the GECKO Team

We report the optical observations conducted with the LOAO telescopes of two optical counterpart candidates, GIT230615aa, and GIT230615ab (H. Kumar, GCN 33974
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 ) of the LIGO/Virgo/KAGRA gravitational-wave event, S230615az. The LOAO is a 1.0-meter class telescope in the USA. The LOAO observation started at 2023-06-16 05:03 (UT) or about 0.46 days after the GW alert in R-band. We calibrated flux with the PANSTARRS catalog and used an AB magnitude system. As a result, we could not detect any significant signals from the candidates. Furthermore, observations taken approximately 0.8 days after the GW alert also reported the non-detection of both candidates (Z.P. Zhu, GCN 33981
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 ). The preliminary 5 sigma depths of these images are below:

OBJECT      DATE-OBS[UTC]       t-t0[days] FILTER EXPTIME DEPTH
GIT230615aa 2023-06-16T05:03:20 0.46       R      180s*10 21.0
GIT230615ab 2023-06-16T05:23:51 0.48       R      180s*10 20.8

Gravitational-wave EM Counterpart Korean Observatory (GECKO) is a network of 10+ 0.5m to 1m class telescopes worldwide. The Korea Astronomy and Space Science Institute operates LOAO. We thank the LOAO operators for their support for the observations.

A. Santos (CBPF/Fermilab), C. D. Kilpatrick (Northwestern), C. R. Bom (CBPF), Eduardo Lacerda (IAG-USP) report on behalf of the STEP-GW collaboration:

We conducted an optical search for candidate counterparts in the localization region of LVK gravitational wave event S230615az with the T80S 0.8-m robotic telescope using the 1.4 x 1.4 field-of-view camera.  The tiling was optimally determined using the latest localization map from the LVK and galaxy catalogs in Teglon (Coulter et al., in prep.).  In total, T80S observed 24 fields (68 individual exposures at total) within the 90th percentile localization region of S230615az.  All of our completed pointings have been uploaded to the GW Treasure Map (Wyatt et al., 2020) at https://treasuremap.space/alerts?graceids=S230615az.

Subtracting DECam and SkyMapper template images from the T80S images using photpipe (Rest et al., 2005), we identified the following candidate counterpart in our imaging:

Name MJD RA(deg) Dec(deg) Filter Mag Magerr 
STEP23a 60111.4340 40.79975 -46.65150667 g 19.806  0.09

We have checked the Minor Planet Center and Transient Name Server, finding no previously reported solar system object or transient. We encourage follow-up observations of this source.

R. Strausbaugh (UMN), O. Pyshna (AO TSNU of Kyiv), T. du Laz (CIT),
S. Pormente (OCA), A. Takey, A. Shokry, A. Fouad, M. Molham, A. Tawfeek
(NRIAG), S. Antier (OCA), M. Coughlin (UMN), P. Hello (IJCLAB),
S. Karpov, M. Prouza, M. Mašek, M. Blazek (FZU), A. Klotz (IRAP),
T. Pradier (Univ. Strasbourg), I. Tosta e Melo (UNICT), D. Turpin (CEA),
O.Burkhonov (UBAI), S.Ehgamberdiev (UBAI, NUUz), Y.Rajabov (UBAI),
T.Sadibekova (UBAI-CEA), Y.Tillayev (UBAI, NUUz), Xinglong ,
F. Guo (THU), X. F. Wang (THU), Y. Zhu (NAOC), J. Zhang (NAOC),
R. Inasaridze, R. Natsvlishvili, N. Kochiashvili, S. Beradze,
V.Aivazyan (AbAO), report on behalf of the GRANDMA collaboration:

The GRANDMA telescope network responded to the GOTO and GIT triggers
from LIGO/Virgo/KAGRA S230615az (Gompertz et al., GCN 33979
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 , Kumar et al.,
GCN 33974
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 , TNS 2023kvr).

The first observations started 20 hours after the reported detection
time from the sources candidates.

As time reference (T0), we choose:
for GOTO23hn 2023-06-15T21:27:14.688 UT (60110.89392 MJD)
for GOTO23hu 2023-06-15T21:35:15.936 UT (60110.89949 MJD)
for GIT230615aa 2023-06-15T18:30:26.928 UT (60110.77114 MJD)
for GIT230615ab 2023-06-15T18:53:10.838 UT (60110.78693 MJD)

In the following table we report a subset of the preliminary photometry
of our observations. Upper limits are reported as the faintest
source detected in frame; magnitudes and upper limits are reported
in the AB system.

GOTO23hn:
T-T0 (day) |MJD   | Obser.   |Exposure| Filter | Mag +/- err  |Upp.Lim. (AB)
________________________________________________________________________
0.847|60111.74163|AbAO-T70|14x60s|R| - | > 20.6
0.956|60111.84993|KAO|7x120s|sdssr| 18.9 +/- 0.05 |     -
1.038|60111.93207|FRAM-CTA-N|28x120s|R| - |     > 18.7


GOTO23hu:
T-T0 (day) |MJD   | Obser.   |Exposure| Filter | Mag +/- err  |Upp.Lim. (AB)
________________________________________________________________________
0.862|60111.76180|AbAO-T70|15×60s|R| - | > 20.9
0.984|60111.88393|FRAM-CTA-N|29x120s|R| - |     > 18.6


GIT230615aa:
T-T0 (day) |MJD   | Obser.   |Exposure| Filter | Mag +/- err  |Upp.Lim. (AB)
________________________________________________________________________
0.939|60111.71026|UBAI-NT60|10x180s|R| - |     > 20.3
0.942|60111.72810|SNOVA|2x180s|C| - |     > 19.2
1.099|60111.87106|KAO|7x120s|sdssr| - |     > 20.3


GIT230615ab:
T-T0 (day) |MJD   | Obser.   |Exposure| Filter | Mag +/- err  |Upp.Lim. (AB)
________________________________________________________________________
0.954|60111.72906|SNOVA|1x180s|C| - |     > 18.5
0.997|60111.78488|AbAO-T70|12×60s|R| - | > 21
1.037|60111.82444|KAO| 7x120s |sdssr| - |     > 21.3

KAO data has been calibrated with respect to the PS1 catalog.
FRAM-CTA-N data has been calibrated with respect to the APASS catalog.
Abastumani-T70 data has been calibrated with respect to the PS1 catalog.

This single detection of GOTO23hn indicates that the source is either flat
or increasing in brightness, but we note the difficulty of comparing
this narrow-band value to the original wide-band detection by GOTO.

GRANDMA is a worldwide coordinated telescope network
(grandma.ijclab.in2p3.fr)  devoted to the observation of transients
in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS
497, 5518). Kilonova-Catcher (KNC) is the citizen science program of
GRANDMA (http://kilonovacatcher.in2p3.fr/).

Anisha Anisha (LMU/University of Hamburg), Julius Gassert (LMU), Daniel Gruen (LMU), Lei Hu (Purple Mountain Observatory/Carnegie Mellon U.), Antonella Palmese (Carnegie Mellon U.), Arno Riffeser (LMU/MPE), Ananya Shankar (LMU)

With the 3kk imager at the 2m Fraunhofer telescope at Wendelstein Observatory, Germany, we have observed the highest probability candidate host galaxies based on their sky position and DESI spectroscopic redshift with respect to the 3D localization of event S230615az. Observations took place between MJD 60110.8 and MJD 60110.9, 3-4 hours after the gravitational wave candidate event. Using Legacy Survey DR10 imaging as a template where available, we perform difference imaging and do not detect any viable candidate counterparts. We report the following upper limits within a ~7’x7’ field around the given coordinates:

RA(J2000) [deg] | dec(J2000) [deg] | date_obs [UTC] | filters | lim_mag(AB, 5sigma) 
167.3605 | 29.5734 | 2023-06-15T21:27:32.858393 | r,i | 22.9, 22.7
167.6634 | 28.3207 | 2023-06-15T21:48:52.763590 | r,i | 22.6, 22.6
167.7216 | 30.1690 | 2023-06-15T21:38:13.808422 | r,i | 22.8, 22.6
168.0171 |  27.5943 | 2023-06-15T21:16:55.039852 | r,i | 22.7, 22.5
169.5725, 28.2641 | 2023-06-15T23:10:35.119420 | r,i | 22.6, 22.3
174.2707 | 24.1003 | 2023-06-15T21:59:32.521545 | r,i | 22.3, 22.3
175.6390 | 26.4931 | 2023-06-15T20:48:24.528448 | r,i | 22.6, 22.3
149.4729| 36.3526 | 2023-06-15T21:06:05.721774 | r,i | 22.3, 22.3

We have also followed up the GROWTH-India candidate counterparts reported by GCN 33974
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  consistent with the gravitational wave event S230615az. We report the following upper limits:

Name | date_obs [UTC] | filters | lim_mag(AB, 5sigma) 
GIT230615aa | 2023-06-15T22:29:56.229398 | r | 22.7
GIT230615ab | 2023-06-15T22:38:41.867278 | g | 21.9
GIT230615ab |2023-06-15T22:47:20.897141| r | 22.7

Magnitudes are not corrected for Milky Way extinction.

A. Rossi, E. Maiorano, E. Palazzi, F. Cusano (INAF-OAS Bologna), A. Melandri (INAF-OAR), P. D’Avanzo (INAF-OAB), V. D’Elia (SSDC and INAF-OAR), and L. Izzo (INAF-OAC, DARK/NBI) on behalf of the on behalf of GRAvitational Wave Inaf TeAm (GRAWITA) collaboration report:

We report the results of spectroscopic observations of GOTO23hn (Gompertz et al., GCN #33979
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 ;  Strausbaugh et al., GCN 34020
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 ), one of the GOTO candidate counterparts of the LIGO/Virgo event S230615az.  

The spectra were obtained with the Multi-Object Double Spectrographs (MODS) instrument mounted on the 2x8.4-m LBT telescope (Mt. Graham, AZ, USA) at the mid-time 04:33 UT on 2023-06-21, 5.7 days after the GW trigger, for a total on-source time of 1200 s, covering the wavelength range 3200-10000 AA.

The MODS slit of 1.2 arcsec and its inclination was adjusted to a parallactic angle of 4.8 degrees to cover the positions of the transient and its host, which is very elongated as visible also in Pan-STARRS and SDSS archival images. Preliminary analysis of the red part (>5800 AA) of the host spectrum shows several well detected emission lines which we interpret as H-alpha, [NII] 6548,6583AA, and [SII] 6716,6731AA, at a common redshift of z=0.0929, suggesting that this is a star-forming galaxy.

Analysis of the spectrum of the GOTO candidate is ongoing.

We acknowledge the excellent support from the LBTO and LBT-INAF staff, particularly D. Thompson, D. G. Huerta , and D. Paris, in obtaining these observations.

E. Maiorano, A. Rossi,  E. Palazzi,  F. Cusano (INAF-OAS Bologna), L. Izzo (INAF-OAC, DARK/NBI), S.Benetti (INAF OAPd), A. Melandri (INAF-OAR), P. D’Avanzo (INAF-OAB), and V. D’Elia (SSDC and INAF-OAR)  report on behalf of GRAvitational Wave Inaf TeAm (GRAWITA):

We report the classification of spectroscopic observations of GOTO23hn (Gompertz et al., GCN #33979
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 ;  Strausbaugh et al., GCN 34020
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 ), one of the GOTO candidate counterparts of the LIGO/Virgo event S230615az. The LBT/MODS spectrum has been obtained at the mid-time 04:33 UT on 2023-06-21, 5.7 days after the GW trigger, for a total on-source time of 1200 s, covering the wavelength range 3200-10000 AA as described in  Rossi et al. (GCN #34061
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 ), where we measure a spectroscopic redshift of z=0.0929 for the host galaxy. 

The spectra display a strong SII line, indicative of a SN Ia. Using the SNID software (Blondin & Tonry 2007), the best match is found with SN 1993ac at about 7 days after maximum. Multiband photometry was also collected and a preliminary analysis gives  r=19.015 +-0.014 and g=18.856+- 0.008,  AB mag.

We acknowledge the excellent support from the LBTO and LBT-INAF staff, particularly D. Thompson, D. G. Huerta , and D. Paris, in obtaining these observations.