TITLE:   GCN CIRCULAR
NUMBER:  36706
SUBJECT: GRB 240615A: possible transient detection from Wendelstein observations
DATE:    24/06/19 22:11:12 GMT
FROM:    Malte Busmann at Ludwig-Maximilians-Universität München <m.busmann@physik.lmu.de>

Malte Busmann (LMU), Daniel Gruen (LMU), Brendan O’Connor (Carnegie Mellon U.), Antonella Palmese (Carnegie Mellon U.), Christoph Ries (LMU), Arno Riffeser (LMU/MPE) report:

With the 3KK optical-NIR imager at the 2m Fraunhofer telescope at Wendelstein Observatory, Germany, we have followed up the GRB 240615A reported by GCN 36671, 36672. Our observations began at ~3.2 days after the GRB. 

We detect one source at 10sigma significance that is consistent with the location (90% error of ~9") of the Swift-XRT uncatalogued X-ray source reported in GCN 36683. We report the following photometric measurements and 3-sigma upper limits for this source (hereafter referred to as Source 1):

Name      RA(J2000)   dec(J2000)    UTC                  mag_r   mag_i         mag_J
Source 1  21:44:23.63 +38:34:08.89  2024-06-18T22:44:19  >24.0   24.0 +/- 0.5  20.36 +/- 0.09

We also obtained a tentative detection of Source 1 in images taken about 22 h before this detection (~2.3 days after the GRB). There is evidence at the 3sigma level that Source 1 was brightening between the two epochs. We note that Source 1 is extremely red in color with i-J>3 mag. 

A second source (Source 2) is marginally detected at RA, dec (J2000) = 21:44:23.8, +38:34:03.4 in both r and i-bands. We note the marginal detection of an underlying source in PS1 imaging at both of these locations (Source 1 and Source 2).

Further observations are scheduled and additional follow-up is encouraged. 

Magnitudes are in the AB system and not corrected for the significant Milky Way extinction along this line of sight.