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GCN Circular 26863

Subject
LIGO/Virgo S200115j: Zwicky Transient Facility search for optical counterparts to Swift X-ray sources
Date
2020-01-23T05:39:23Z (5 years ago)
From
Igor Andreoni at Caltech <igor.andreoni@gmail.com>
Igor Andreoni (Caltech), Mansi M. Kasliwal (Caltech), S. Brad Cenko (NASA GSFC), Yuhan Yao (Caltech)
On behalf of the Zwicky Transient Facility (ZTF) and Global Relay of Observatories Watching Transients Happen (GROWTH) collaborations

The Zwicky Transient Facility (ZTF; Graham et al. 2019, Bellm et al. 2019) has repeatedly observed the location of nine X-ray sources, found with the Neil Gehrels Swift Observatory (Swift), that Evans et al. (GCN #26855Not found
 
 
) reported as candidate counterparts to S200115j (GCN #26759Not found
 
 
, GCN #26807Not found
 
 
) with rank = 2.

We searched the ZTF alert database using Kowalski (Duev et al., 2019). We found that only the Swift source S200115j_X136 has a ZTF alert within the spatial uncertainty indicated in GCN #26855Not found
 
 
. The ZTF source, labelled ZTF20aafapey, was found on 2020-01-13 (before the detection of S200115j) and it is located on top of the nucleus of the galaxy 2MASS 02401222-0233442 at redshift z=0.04333 (6dF Galaxy Survey; Jones et al., 2009). We performed forced photometry of ZTF20aafapey using ForcePhotZTF (Yao et al., 2019). The light curve (Table 1) shows rapid variability days before S200115j, with a significant brightening of >0.5 mag on 2020-01-13. Our results are in agreement with Oates et al. (GCN #26808Not found
 
 
) and further suggest that S200115j_X136/ZTF20aafapey is caused by AGN activity. We conclude that S200115j_X136 is unlikely associated with S200115j.

Table 1: Light curve of ZTF20aafapey
+---------------------+------+------+-----+
|       date_obs      |filter| mag  | err |
+---------------------+------+------+-----+
| 2020-01-12 04:52:53 |  r   | 19.4 | 0.1 |
| 2020-01-13 04:15:19 |  r   | 19.5 | 0.1 |
| 2020-01-13 04:15:59 |  r   | 19.3 | 0.1 |
| 2020-01-13 04:43:20 |  r   | 19.5 | 0.2 |
| 2020-01-13 04:43:59 |  r   | 18.9 | 0.1 |
| 2020-01-13 05:28:55 |  r   | 19.5 | 0.2 |
| 2020-01-13 05:29:33 |  r   | 19.0 | 0.1 |
+---------------------+------+------+-----+


For other Swift X-ray sources no ZTF alerts were issued, although the transient candidate locations were recently observed in most cases. In Table 2 we report ZTF detection upper limits at the approximate location of the Swift sources - other than S200115j_X136 - from 2020-01-13 to 2020-01-22. The exposure times are provided in seconds.

Table 2: ZTF upper limits
+---------------+---------------------+---------+--------+----------+
|  source_name  |       date_obs      | exptime | filter | maglimit |
+---------------+---------------------+---------+--------+----------+
| S200115j_X130 | 2020-01-13 02:28:48 |    30   |   g    |  >20.6   |
| S200115j_X130 | 2020-01-13 02:29:28 |    30   |   g    |  >20.5   |
| S200115j_X130 | 2020-01-15 03:12:49 |    30   |   g    |  >20.9   |
| S200115j_X130 | 2020-01-15 06:34:05 |   300   |   g    |  >20.7   |
| S200115j_X130 | 2020-01-18 05:45:35 |   300   |   g    |  >21.3   |
| S200115j_X136 | 2020-01-13 04:15:20 |    30   |   r    |  >20.0   |
| S200115j_X136 | 2020-01-13 04:15:59 |    30   |   r    |  >20.1   |
| S200115j_X136 | 2020-01-13 04:43:21 |    30   |   r    |  >19.5   |
| S200115j_X136 | 2020-01-13 04:44:00 |    30   |   r    |  >19.5   |
| S200115j_X136 | 2020-01-13 05:28:55 |    30   |   r    |  >19.4   |
| S200115j_X136 | 2020-01-13 05:29:34 |    30   |   r    |  >19.3   |
| S200115j_X487 | 2020-01-13 04:43:21 |    30   |   r    |  >19.5   |
| S200115j_X487 | 2020-01-13 04:44:00 |    30   |   r    |  >19.5   |
| S200115j_X488 | 2020-01-13 04:43:21 |    30   |   r    |  >19.6   |
| S200115j_X488 | 2020-01-13 04:44:00 |    30   |   r    |  >19.6   |
| S200115j_X707 | 2020-01-15 06:28:56 |   300   |   g    |  >21.0   |
| S200115j_X707 | 2020-01-15 07:20:39 |   300   |   g    |  >20.6   |
| S200115j_X707 | 2020-01-18 05:40:26 |   300   |   g    |  >21.4   |
| S200115j_X717 | 2020-01-13 05:28:55 |    30   |   r    |  >19.4   |
| S200115j_X717 | 2020-01-13 05:29:34 |    30   |   r    |  >19.4   |
| S200115j_X745 | 2020-01-13 05:28:55 |    30   |   r    |  >19.4   |
| S200115j_X745 | 2020-01-13 05:29:34 |    30   |   r    |  >19.4   |
| S200115j_X746 | 2020-01-13 04:13:23 |    30   |   r    |  >20.0   |
| S200115j_X746 | 2020-01-13 04:14:02 |    30   |   r    |  >19.9   |
| S200115j_X746 | 2020-01-15 06:34:05 |   300   |   g    |  >20.6   |
| S200115j_X746 | 2020-01-18 05:45:35 |   300   |   g    |  >21.2   |
| S200115j_X748 |          -          |    -    |   -    |    -     | 
+---------------+---------------------+---------+--------+----------+

Evans et al. (GCN #26855Not found
 
 
) recommended the investigation of three sources in particular: S200115j_X136, S200115j_X707, and S200115j_X746. A discussion on S200115j_X136 is presented above. The coordinates of both S200115j_X707 and S200115j_X746 were observed with ZTF 2 days before S200115j, 2-3 hours after S200115j, and 3 days after S200115j. The non-detection of counterparts in ZTF data well constrains the presence of a bright optical transient evolving on hours to days timescales associated with S200115j_X707 or S200115j_X746. Further follow-up is encouraged.



Forced photometry was performed on images processed through the ZTF reduction and image subtraction pipelines at IPAC (Masci et al. 2019). 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 generous support of the NSF under PIRE Grant No 1545949.