GCN Circular 35865

G. P. Srinivasaragavan (UMD), E. Hammerstein (UMD), V. Karambelkar (CIT), I. Andreoni (UMD), M. Kasliwal (CIT), S. B. Cenko (UMD) report:

We observed the location of the X-ray flare EPW20240219aa (Zhang et al., ATel #16463; Zhang et al., GCN 35773
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) in the optical r' and i' bands with the Large Monolithic Imager (LMI) mounted at the 4-m Lowell Discovery Telescope (LDT) located in Flagstaff, AZ, under program M01 (PI: Andreoni).

We obtained ten 150-second exposures that covered the locations of all the VLA radio sources reported in Ho et al., GCN 35788
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. The observations began on UTC 2024-03-01T03:35:00.270. Karambelkar et al., GCN 35808
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 reported that VLA-3 was the most promising source to be associated with the X-ray transient, and we report a tentative detection in the i' band of i' = 24.78+/-0.24 mag, and an upper limit in the r' band of r' > 24.9 mag. Below we report the observations at the locations of all the radio sources.

VLA-1 : We detect a source with r' = 24.5+/-0.2 mag and i' = 23.0+/-0.2 mag. As reported in Karambelkar et al., GCN 35808
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, the source is ~ 0.8 arcseconds offset from the VLA position reported, and is likely unrelated to the X-ray transient since the source was visible in archival images from the UKIRT Galactic Plane Survey (Lucas et al. 2008).

VLA-2: We detect an extended source with r' = 24.1+/-0.1 mag and i' = 23.0+/-0.1 mag. It is unlikely that this is the optical transient counterpart to the X-ray transient.

VLA-3: We tentatively detect a source with i' = 24.8+/-0.2 mag, and do not detect a source in the r' band to an upper limit r' > 24.9 mag. Upon inspection of the image, a very faint source is visible at the location of VLA-3 in the i’ band. This source may be associated with the X-ray transient, and we encourage further follow-up observations.

VLA-4: We do not detect a source, to limits of r' > 25.1 mag and i' > 24.5 mag. This is consistent with the nondetection reported in Karambelkar et al., GCN 35808
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.

VLA-5: We tentatively detect a source with i' = 24.7+/-0.2 mag, and do not detect a source in the r' band to an upper limit r' > 25.1 mag. A non-detection was reported in Karambelkar et al., GCN 35808
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. This source may be associated with the X-ray transient, and we encourage further follow-up observations.

VLA-6: We detect an extended source with r' = 21.5+/-0.1 mag and i' = 20.6+/-0.1 mag. It is unlikely that this is the optical transient counterpart to the X-ray transient.

VLA-7: We detect an extended source with r' = 21.7+/-0.1 mag and i' = 20.6+/-0.1 mag. It is unlikely that this is the optical transient counterpart to the X-ray transient.

All magnitudes are in AB mag, calibrated against the PS1 catalog, and are not corrected for Galactic extinction.

We thank the staff of the Lowell Discovery Telescope for assistance with these observations.