Hintze Centre for Astrophysical Surveys

Abstract:

This work presents comprehensive multifrequency radio monitoring of the black hole low-mass X-ray binary (LMXB) Swift J1727.8−1613, which underwent its first recorded outburst after its discovery in 2023 August. Through a considerable community effort, we have coalesced the data from multiple, distinct observing programs; the light curves include ∼10 months and 197 epochs of monitoring from seven radio facilities with observing frequencies ranging from (approximately) 0.3–230 GHz. The primary purpose of this work is to provide the broader astronomical community with these light curves to assist with the interpretation of other observing campaigns, particularly nonradio observing frequencies. We discuss the phenomenological evolution of the source, which included (i) multiple radio flares consistent with the launching of discrete jet ejections, the brightest of which reached ∼1 Jy; (ii) temporally evolving radio spectral indices (α), reaching values steeper than expected for optically thin synchrotron emission (α < −1) and emission with significant radiative cooling (α < −1.5). We have published a digital copy of the data and intend for this work to set a precedent for the community to continue releasing comprehensive radio light curves of future LMXB outbursts.

Abstract:

We present results from the Pan-STARRS optical search for kilonovae without the aid of gravitational wave and gamma-ray burst triggers. The search was conducted from 2019 October 26 to 2022 December 15. During this time, we reported 29 740 transients observed by Pan-STARRS to the IAU Transient Name Server. Of these, 175 were Pan-STARRS credited discoveries that had a host galaxy within 200 Mpc and had discovery absolute magnitudes . A subset of 11 transients was plausibly identified as kilonova candidates by our kilonova prediction algorithm. Through a combination of historical forced photometry, extensive follow-up, and aggregating observations from multiple sky surveys, we eliminated all as kilonova candidates. Rapidly evolving outbursts from massive stars (likely to be Luminous Blue Variable eruptions) accounted for 55 per cent of the subset’s contaminating sources. We estimate the rate of such eruptions using the Asteroid Terrestrial-impact Last Alert System 100 Mpc volume-limited survey data. As these outbursts appear to be significant contaminants in kilonova searches, we estimate contaminating numbers when searching gravitational wave skymaps produced by the LIGO-Virgo-Kagra science collaboration during the Rubin era. The Legacy Survey of Space and time, reaching limiting magnitudes of , could detect 2–6 massive stellar outbursts per 500 deg within a 4-d observing window, within the skymaps and volumes typical for binary neutron star mergers projected for Ligo-Virgo-Kagra Observing run 5. We conclude that while they may be a contaminant, they can be photometrically identified.

Abstract:

With a small sample of fast X-ray transients (FXTs) with multiwavelength counterparts discovered to date, their progenitors and connections to γ-ray bursts (GRBs) and supernovae (SNe) remain ambiguous. Here, we present photometric and spectroscopic observations of SN 2025kg, the SN counterpart to the FXT EP 250108a. At z = 0.17641, this is the closest known SN discovered following an Einstein Probe (EP) FXT. We show that SN 2025kg’s optical spectra reveal the hallmark features of a broad-lined Type Ic SN. Its light-curve evolution and expansion velocities are comparable to those of GRB-SNe, including SN 1998bw, and two past FXT-SNe. We present JWST/NIRSpec spectroscopy taken around SN 2025kg’s maximum light, and find weak absorption due to He I 1.0830 μm and 2.0581 μm and a broad, unidentified emission feature at ∼4–4.5 μm. Further, we observe broadened Hα in optical data at 42.5 days that is not detected at other epochs, indicating interaction with H-rich material. From its light curve, we derive a 56Ni mass of 0.2–0.6 M⊙. Together with our companion Letter, our broadband data are consistent with a trapped or low-energy (≲1051 erg) jet-driven explosion from a collapsar with a zero-age main-sequence mass of 15–30 M⊙. Finally, we show that the sample of EP FXT-SNe supports past estimates that low-luminosity jets seen through FXTs are more common than successful (GRB) jets, and that similar FXT-like signatures are likely present in at least a few percent of the brightest Type Ic-BL SNe.

Abstract:

We present a simulation of the time-domain catalog for the Nancy Grace Roman Space Telescope’s High-Latitude Time-Domain Core Community Survey. This simulation, called the Hourglass simulation, uses the most up-to-date spectral energy distribution models and rate measurements for 10 extragalactic time-domain sources. We simulate these models through the design reference Roman Space Telescope survey: four filters per tier, a five-day cadence, over 2 yr, a wide tier of 19 deg2, and a deep tier of 4.2 deg2, with ∼20% of those areas also covered with prism observations. We find that a science-independent Roman time-domain catalog, assuming a signal-to-noise ratio at a max of >5, would have approximately 21,000 Type Ia supernovae, 40,000 core-collapse supernovae, around 70 superluminous supernovae, ∼35 tidal disruption events, three kilonovae, and possibly pair-instability supernovae. In total, Hourglass has over 64,000 transient objects, 11,000,000 photometric observations, and 500,000 spectra. Additionally, Hourglass is a useful data set to train machine learning classification algorithms. We show that SCONE is able to photometrically classify Type Ia supernovae with high precision (∼95%) to a z > 2. Finally, we present the first realistic simulations of non-Type Ia supernovae spectral time series data from Roman’s prism.