TITLE:   GCN CIRCULAR
NUMBER:  36777
SUBJECT: GRB 240615A: SAI CMO J- band observations
DATE:    24/06/28 12:50:00 GMT
FROM:    Alexei Pozanenko at IKI, Moscow <grb.alex@gmail.com>

A. Volnova (IKI), A.A. Tatarnikov  (SAI MSU), A.M. Tatarnikov  (SAI MSU),
 N. Pankov (HSE, IKI), A. Pozanenko (IKI), S. Belkin (IKI, HSE) report on
behalf of GRB-IKI-FuN:

We observed the field of GRB 240615A (Fermi GBM team, GCN 36671; DeLaunay
et al, GCN 36672; Evans, GCN 36673; Frederiks et al, GCN 36677; Tan et al,
GCN 36682; Page et al, GCN 36683; Roberts and Meegan, GCN 36685; Kozyrev et
al, GCN 36726) with 2.5-meter SAI-25 telescope of the Caucasian Mountain
Observatory of Sternberg Astronomical Institute (SAI CMO), equipped with
the IR camera ASTRONIRCAM. The observations in the J-filter were carried
out on 2024-06-20, and 2024-06-23. The infrared point source (Busmann et
al, GCN 36706) within XRT S1 localization is well detected in stacked
images of both epochs at coordinates (J2000)  21:44:23.66, +38:34:08.81
with an uncertainty of 0.1 arcsec in both coordinates.

Preliminary photometry of the source is as follows:

Date       UT start  t-T0     Exp.    Filter   OT     Err.  UL(3 sigma)
                   (mid, days)  (s)
2024-06-20 19:39:18  5.10936  46x96.5 J        19.23  0.07  21.1
2024-06-23 21:58:41  8.19335  35x96.5 J        19.76  0.07  21.8

The magnitudes are calibrated against nearby stars of 2MASS and are not
corrected for the Galactic extinction.

The light curve fit yields a power law exponent of about -1.0, which is
typical for a gamma-ray burst afterglow. We suggest the source is an
afterglow of GRB 240615A.
Our photometry is consistent with photometry reported early (Kumar et al,
GCN 36676, Kumar et al, GCN 36680; Karambelkar et al, GCN 36686; Gupta and
Misra, GCN 36696; Busmann et al, GCN 36706; Jiang et al, GCN 36713;
Rastinejad et al, GCN 36746, GCN 36751).

Due to the extremely red color of the source, i.e. marginal detection in
the i-filter by NOT (Jiang et al, GCN 36713) we tentatively set a lower
limit on the redshift of the GRB source  of ~4.5. Further observations of
GRB 240615A are encouraged.