PEARLS: 21 Transients Found in the Three-Epoch NIRCam Observations in the Continuous Viewing Zone of the James Webb Space Telescope

TL;DR

PEARLS reports 21 transients found in three-epoch, four-band JWST/NIRCam imaging of 14.16 arcmin^2 in the Spitzer IRAC Dark Field, supplemented by contemporaneous HST/ACS data and NIRSpec follow-up for three targets. The authors perform a difference-imaging search across epoch pairs, grouping transients by host association into hostless, outskirts, and deeply embedded categories, and derive photometric redshifts for hosts with spectroscopic confirmations for three transients: a SN Ia at $z=1.64$ and host redshifts of $z=2.64$ and $z=1.90$. They identify two broad populations: a mid-$z$ group with $z>1.6$ and $M_V<-16.0$ mag and a low-$z$ group with $z<0.4$ and $M_H>-14.0$ mag, noting a notable gap in the redshift distribution around $0.4<z<1.6$ and a tentative interpretation of low-$z$ events as gap transients. The study underscores NIRCam’s power for time-domain astrophysics, highlights the need for long-term, high-cadence monitoring across more passbands, and calls for prompt spectroscopy to confirm the nature of the low-$z$ candidates and to refine transient rates in blank-field surveys.

Abstract

We present 21 transients from our three-epoch, four-band NIRCam observations covering 14.16 arcmin^2 in the Spitzer IRAC Dark Field (IDF), taken by the JWST Prime Extragalactic Areas for Reionization and Lensing Science program with a time cadence of ~6 months. A separate Hubble Space Telescope program provided Advanced Camera for Surveys optical imaging contemporaneous with the second and third epochs of the NIRCam observations. The NIRSpec spectroscopy on three transients confirmed a Type Ia supernova at z=1.63 and the host galaxies of the other two at z=2.64 and 1.90, respectively. Combining these with the photometric redshifts (z_ph) of the host galaxies in the rest of the sample, we find that the transients are in either a "mid-z" group at z>1.6 with M_V < -16.0 mag or a "low-z" group at z < 0.4 with M_H > -14.0 mag. The mid-z transients are consistent with supernovae. In contrast, the low-z transients' luminosities fall in the range of the so-called "gap transients" between supernovae and novae. However, this latter conclusion is only tentative due to possible catastrophic failures in z_ph that could bias them to low-z. Conversely, if they are indeed at z < 0.4, it would be worth studying similar transients in the future. Our work further demonstrates the power of NIRCam in transient science and also shows that it would be more fruitful to carry out a long-term monitoring program with more passbands, a higher cadence and prompt follw-up spectroscopy. Being in the continuous viewing zone of the JWST, the IDF is an ideal field for this purpose.

Figures (13)

• Figure 1: Combined Ep1+Ep2+Ep3 F356W negative image of the JWIDF. Image scale and orientation are indicated. The locations of the D13, D31, and D21 transients are marked by the red, green, and blue circles, respectively.
• Figure 2: Distribution of the NIRCam magnitudes for all transients in their discovery epoch. Magnitude limits are not included. (By construction there are no limits in the $m_{356}$ panel.)
• Figure 3: EAZY SED-fitting results for the host galaxies of transients that have detectable hosts. The IDs of the corresponding transients are indicated. In each panel, the blue symbols (circles with error bars and upper limits) show the observations, the curve is the best-fit template, and the red symbols are the synthesized magnitudes based on the best-fit template. The derived photometric redshift ($z_{\rm ph}$) and the $\chi^2$ value of the fit are indicated in each panel, and the inset shows the probability distribution of $z_{\rm ph}$. The hosts of T-D31-G and T-D31-H are not included because they have spectroscopic redshifts (Section 5.2).
• Figure 4: Image stamps and SEDs for D13 transients. All images are 24 on a side and have north up, east to the left. For each object, the left three panels show NIRCam color image stamps in each epoch. These have F150W as blue, F200W as green, and F356W + F444W as red. The next two panels show negative D13 and D23 difference images. The right panel show the per-epoch SEDs with points for different epochs shifted by $0.06~\mu$m for clarity. Red circles show the Ep1 (discovery) SED, blue triangles show the Ep2 SED, and for the two hostless transients, green squares show the Ep3 SED. As mentioned in Section \ref{['s:tphot']}, the photometry for transients that have visible hosts assumes the transient had zero flux in Ep3.
• Figure 5: Same as Figure \ref{['fig:t-d31-all']}, but for D31 transients.