LIGO/Virgo/KAGRA S230917af

H. Negoro, M. Nakajima (Nihon U.),
T. Mihara, N. Kawai (RIKEN),
S. Sugita, M. Serino, H. Hiramatsu, H. Nishikawa, Y. Kondo (AGU),
Y. Kawakubo (LSU)
report on behalf of the MAXI team:

We examined MAXI/GSC all-sky X-ray images (2-20 keV)
after compact binary merger candidate S230917af at 2023-09-17 10:04:17 UTC .

At the trigger time of S230917af, the high-voltage of MAXI/GSC was on,
but the FOV was out of the 90% credible region of the cwb skymap.
The first one-orbit (92 min) scan observation with GSC after the event covered 100%
of the 90% credible region of the cwb skymap from 10:41:56 to 10:53:01 UTC (T0+2259 to T0+2924 sec).

There was a low significance burst-like X-ray event at 
(R.A., Dec) = (333.784 deg, 39.435 deg) = (22 15 08, +39 26 06) (J2000) 
with a statistical 90% C.L. error region of ~0.6 deg.
There is an additional systematic uncertainty of 0.1 deg (90% containment radius).
The sources region  was observed around 10:50:13 UTC.
The X-ray flux averaged over the scan was 48 +- 13 mCrab
(4.0-10.0keV, 1 sigma error).

If you require information about X-ray flux by MAXI/GSC at specific coordinates,
please contact the submitter of this circular by email. 

Tomas Ahumada (CIT), Vishwajeet Swain (IITB), Shreya Anand (CIT), Robert Stein (CIT), Viraj Karambelkar (CIT), Akash Anumarlapudi (UWM),  Mansi Kasliwal (CIT), Anirudh Salgundi (IITB), Gaurav Waratkar (IITB), Avery Wold (IPAC), Theophile du Laz (CIT), Simeon Reusch (DESY), Igor Andreoni (SURA),  Eric Bellm (UW), Varun Bhalerao (IITB), Brad Cenko (UMD), Michael Coughlin (UMN), Brian Healy (UMN), David Kaplan (UWM),  Jannis Necker (DESY), D. Perley (LJMU) report on behalf of the ZTF and GROWTH collaborations:

We observed the localization region of the LVC trigger S230917af as part of routine Zwicky Transient Facility (ZTF; Graham et al., 2019; Bellm et al., 2019) survey operations. We obtained images in the r and g bands beginning at 2023-09-17T10:05:51 UT (8 seconds after the LVC trigger time), covering ~83% of the probability enclosed in the localization region. Our observations covered ~92% of the overlapping region between the MAXI low significance burst (GCN 34723
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 ) and the GW event. 

We queried the ZTF alert stream using Kowalski (Duev et al. 2019) through Fritz (Coughlin et al. 2023), AMPEL (Nordin et al. 2019), and ZTFReST (Andreoni & Coughlin et al., 2021). We required at least 2 detections separated by at least 15 minutes to select against moving objects. Furthermore, we cross-match our candidates with the Minor Planet Center to flag known asteroids, reject stellar sources (Tachibana and Miller 2018), and apply machine learning algorithms (Mahabal et al. 2019). We require that no spatially coincident ZTF alerts were issued before the detection time of the LVC trigger. We also run forced photometry on ZTF images (Masci et al. 2019) and require no detections before the LVC trigger.

No candidates passed our filtering criteria. Furthermore, we run forced photometry over two years of ZTF data at the center of the Swift sources circulated in Evans et al. (GCN 34736
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 ) with a 1 arcsec radius, and we queried ZTF alerts within their 90% error region. Three of the six Swift sources have previous detections and can be associated to ZTF objects. These sources are unlikely to be associated with the GW trigger: 

Swift name | ZTF name | JD discovery | magnitude at discovery
---------------------------------------------------------------
S230917af_X9  | ZTF18abvbsom | 2460152.82912 | g = 20.24
S230917af_X1  | ZTF18abvucic | 2460206.80333 | g = 18.93
S230917af_X10 | ZTF18abvzkzr | 2459817.80154 | g = 20.44

No predetections or alerts in the ZTF archive were found for the Swift sources S230917af_X3, S230917af_X4, and S230917af_X6. However, we note that (1) S230917af_X3 is 2.3 arcsec from a GALEX point-like UV source, at nuv = 20.03 mag, (2) S230917af_X4 is 2.5 arcsec from a Gaia source at g = 20.62 mag with a proper motion ~5.16 mas/year, and (3) S230917af_X6 is 9 arcsec from a WISE galaxy and its colors (W1-W2 = 0.9 mag, and W2-W3 =  2.6 mag) are indicative of a QSO/Seyfert galaxy. 


ZTF and GROWTH are worldwide collaborations comprising Caltech, USA; IPAC, USA, WIS, Israel; OKC, Sweden; JSI/UMd, USA; U Washington, USA; DESY, Germany; MOST, Taiwan; UW Milwaukee, USA; LANL USA; Tokyo Tech, Japan; IITB, India; IIA, India; LJMU, UK; TTU, USA; SDSU, USA and USyd, Australia. ZTF acknowledges the generous support of the NSF under AST MSIP Grant No 1440341. GROWTH acknowledges the generous support of the NSF under PIRE Grant No 1545949. Alert distribution service provided by DIRAC@UW (Patterson et al. 2019). Alert database searches are done by AMPEL (Nordin et al. 2019) and Kowalski (Duev et al. 2019). GROWTH India telescope is located at the Indian Astronomical Observatory (Hanle), operated by the Indian Institute of Astrophysics (IIA). GROWTH-India project is supported by SERB and administered by IUSSTF, under grant number IUSSTF/PIRE Program/GROWTH/2015-16 and IUCAA.

P. D'Avanzo (INAF-OAB), A. Reguitti (INAF-OAB / INAF-OAPd), L. Tomasella (INAF-OAPd), E. Cappellaro (INAF-OAPd), F. Onori (INAF-OAAb), 
F. De Luise (INAF-OAAb), L. Tartaglia (INAF-OAAb), R. Brivio (INAF-OAB), M. Ferro (INAF-OAB), V. D’Elia (ASI/SSDC), E. Brocato (INAF-OAAb) 
report on behalf ot the GRAWITA collaboration:


We carried out follow-up observations of the LIGO/Virgo/KAGRA GW trigger S230917af with the Schmidt telescopes sited in the INAF Asiago 
and Campo Imperatore observatories (Italy). 

Observations from Asiago have been carried out starting on 2023-09-19 at 00:44:52 UT (~ 1.61 days after the GW trigger) with the Clear, g and r filters. 
A second epoch has been carried out starting on 2023-09-19 at 21:14:12 UT (~ 2.47 days after the GW trigger) with the r filter. These observations 
covered ~ 86% of the localisation region of the low significance X-ray burst detected by MAXI (Negoro et al., GCN Circ. 34723
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 ) and the position of sources 
S230917af_X3, S230917af_X4 and S230917af_X6 detected by Swift/XRT (Evans et al., GCN Circ. 34736
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 ). 
Preliminary analysis, which includes image subtraction between the two epochs obtained in the r band, does not show evidence for promising candidate counterparts. 
The typical 3sigma limiting AB magnitudes are r ~ 21 mag and r ~ 22 mag for the first and second epoch, respectively. 


Observations from Campo Imperatore have have been carried out starting on 2023-09-19 at 20:14:33 UT (~ 2.42 days after the GW trigger) with the g and r filters.  
These observations covered ~ 86% of the localisation region of the low significance X-ray burst detected by MAXI and the position of sources S230917af_X3, 
S230917af_X4 and S230917af_X6 detected by Swift/XRT.
Preliminary analysis, which includes image subtraction with the images obtained in the first Asiago epoch, does not show evidence for promising candidate counterparts. 
The typical 3sigma limiting AB magnitudes are g ~ 21.4 mag and r ~ 20.8 mag.