GCN Circular 35808

Viraj Karambelkar (Caltech), Tomas Ahumada (Caltech), Mansi Kasliwal
(Caltech)

We observed the location of the X-ray flare EPW20240219aa (Zhang et al.,
ATel #16463; Zhang et al., GCN #35773) in the near-infrared K prime-band
(Kp) with the NIRES acquisition camera on the Keck II telescope starting
UTC 2024-02-25T05:12:12.45.

We obtained four 300 second exposures that covered the locations of the VLA
radio sources # 1-6 reported in GCN #35788. The Kp-band observations
reached a depth of ~19.5 mag (Vega). We use an archival Ks-band image of
the field from the UKIRT Galactic Plane Survey (Lucas et al. 2008) to
calibrate the NIRES images. The depth of the UKIRT image is ~19 mag (Vega).

We detect sources at the locations of the VLA sources #1, 2, 3 and 6 in the
NIRES image.

VLA-1 : We detect a source with m_Kp ~ 19 mag (Vega) that is ~0.8 arcsec
offset from the VLA position reported in GCN #35788. The source is also
marginally visible in the archival UKIRT image (which is shallower than the
new NIRES image.)

VLA-2 has m_Kp ~ 17 mag (Vega) and appears as an extended source in the
NIRES as well as archival UKIRT image. There is no substantial variation
between the two epochs.

VLA-3 has m_Kp ~ 19 mag (Vega) and is not detected in the archival image
(limit ~19 mag).

VLA-4 and VLA-5 are not detected in our images.

VLA-6 is clearly detected as an extended source in both the NIRES and
archival images. As reported in GCN #35803, this source does not show any
variations, and is likely a galaxy unrelated to the transient.

VLA-7 was not covered by our observations.

In addition to these, we do not detect any new sources in the NIRES
observations.

VLA-3 is the most promising source in terms of association with the X-ray
transient. We encourage further follow-up.