We report three faint, long rising hostless candidates from the LSST alert stream flagged by our ELEPHANT hostless detection pipeline (Pessi et al. 2024). The pipeline has been integrated into the Fink broker (Möller et al. 2021) to process the real time ZTF alert stream and has now been modified to also process the real time LSST alert stream. Given that the alerts of ZTF and LSST are different in the metadata and the size of the stamps, we plan to further fine-tune the pipeline parameters as more data is acquired. The pipeline has been operating for approximately 2 years now, and many of the transients we have reported on TNS have been classified by different spectroscopic follow-up initiatives. The performance of the pipeline is being evaluated and will be presented in Durgesh et al. in prep, this work will also describe the modifications implemented for the pipeline to process the LSST alert stream.
The transients reported here are rather faint, with the brightest detections being around 21.5 magnitudes. We expect such faint transients to be the norm rather than the exception in the LSST stream. We note that such faint transients are not reachable by most spectroscopic facilities, deeming spectroscopic follow up unlikely. Therefore we have decided to stop submitting astronotes to report hostless candidates in the future. We do, however, strongly encourage researchers to regularly inspect the Fink broker to investigate the flagged candidates for their science cases.
The characteristics of the events included in this, our last hostless candidate Astronote report, can be found below. For consistency check, we use the classification and host association of Sherlock (Smith et al. 2020) available on Lasair (Smith et al. 2019) and image cutouts from Euclid Q1 data MER catalog (Romelli et al. 2025).
| Catalog | Name | Reported RA | Reported DEC | Reported Obj-Type | Reported Redshift | Host Name | Host Redshift | Remarks | TNS RA | TNS DEC | TNS Obj-Type | TNS Redshift |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| TNS | 2026ejf [313831458352922664] | 04:04:37.779 | -48:30:07.25 | The peak apparent magnitude of the transient (https://lsst.fink-portal.org/313831458352922664) is around 21.21 in the r-band. The light curve has been rising for the last 2 months and is currently at its peak. Sherlock classified the target as an orphan and could not associate it with a host (https://lasair.lsst.ac.uk/objects/313831458352922664/), in agreement with our findings. However, visual inspection of the target in the Euclid cutout reveals an extended source close to the target. It is unclear if the target is physically associated to that extended source. | 04:04:37.779 | -48:30:07.25 | SLSN-I | 0.96 | ||||
| TNS | 2025altt | 04:16:51.717 | -46:34:29.09 | The transient (https://lsst.fink-portal.org/313831458514927674) has been rising for 2 months and is now starting to decline. The peak magnitude is around 21.61 mag in the z-band. Sherlock reports this object as a possible supernova and associates it with a 24.33 mag galaxy in the DESI catalogue (https://lasair.lsst.ac.uk/objects/313831458514927674/). At such magnitude it is consistent that our pipeline does not find an associated host. It could be possible that the transient is associated to an extended source that is visible in the Euclid cutout. Given the dispersion in the astrometry and the aspect of the light curve, we can’t discard this event as a nuclear transient. | 04:16:51.717 | -46:34:29.09 | ||||||
| TNS | 2025altn | 10:07:04.516 | +02:32:49.00 | The transient (https://lsst.fink-portal.org/313928194567700757) has been rising for almost two months now. The current peak magnitude is around 21.38 in the I-band. This event has been reported as a possible supernova by Murphey et al. 2026 (https://www.wis-tns.org/object/2025altn). Sherlock classifies it as an orphan and could not associate it with a host (https://lasair.lsst.ac.uk/objects/313928194567700757/), in agreement with the findings of our pipeline. By visually inspecting the target on the Aladin sky map (Matthieu et al. 2022), we see an extended source close to the target but outside the area of the alert stamp, we cannot discard that the transient is physically associated to the mentioned extended source. | 10:07:04.516 | +02:32:49.00 | SLSN-I | 1.056 |
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