The Atacama Cosmology Telescope: Detection of Millimeter-wave Transient Sources

Choi S.K., Hasselfield M., Ho S.P., Koopman B., Lungu M., Abitbol M.H., Addison G.E., Ade P.A., Aiola S., Alonso D., Amiri M., Amodeo S., Angile E., Austermann J.E., Baildon T., et. al.

We present the temperature and polarization angular power spectra of the CMB measured by the Atacama Cosmology Telescope (ACT) from 5400 deg2 of the 2013–2016 survey, which covers >15000 deg2 at 98 and 150 GHz. For this analysis we adopt a blinding strategy to help avoid confirmation bias and, related to this, show numerous checks for systematic error done before unblinding. Using the likelihood for the cosmological analysis we constrain secondary sources of anisotropy and foreground emission, and derive a "CMB-only" spectrum that extends to ℓ=4000. At large angular scales, foreground emission at 150 GHz is ∼1% of TT and EE within our selected regions and consistent with that found by Planck. Using the same likelihood, we obtain the cosmological parameters for ΛCDM for the ACT data alone with a prior on the optical depth of τ=0.065±0.015. ΛCDM is a good fit. The best-fit model has a reduced χ2 of 1.07 (PTE=0.07) with H0=67.9±1.5 km/s/Mpc. We show that the lensing BB signal is consistent with ΛCDM and limit the celestial EB polarization angle to ψP =−0.07o±0.09o. We directly cross correlate ACT with Planck and observe generally good agreement but with some discrepancies in TE. All data on which this analysis is based will be publicly released.

Aiola S., Calabrese E., Maurin L., Naess S., Schmitt B.L., Abitbol M.H., Addison G.E., Ade P.A., Alonso D., Amiri M., Amodeo S., Angile E., Austermann J.E., Baildon T., Battaglia N., et. al.

We present new arcminute-resolution maps of the Cosmic Microwave Background temperature and polarization anisotropy from the Atacama Cosmology Telescope, using data taken from 2013–2016 at 98 and 150 GHz. The maps cover more than 17,000 deg2, the deepest 600 deg2 with noise levels below 10μK-arcmin. We use the power spectrum derived from almost 6,000 deg2 of these maps to constrain cosmology. The ACT data enable a measurement of the angular scale of features in both the divergence-like polarization and the temperature anisotropy, tracing both the velocity and density at last-scattering. From these one can derive the distance to the last-scattering surface and thus infer the local expansion rate, H0. By combining ACT data with large-scale information from WMAP we measure H0=67.6± 1.1 km/s/Mpc, at 68% confidence, in excellent agreement with the independently-measured Planck satellite estimate (from ACT alone we find H0=67.9± 1.5 km/s/Mpc). The ΛCDM model provides a good fit to the ACT data, and we find no evidence for deviations: both the spatial curvature, and the departure from the standard lensing signal in the spectrum, are zero to within 1σ; the number of relativistic species, the primordial Helium fraction, and the running of the spectral index are consistent with ΛCDM predictions to within 1.5–2.2σ. We compare ACT, WMAP, and Planck at the parameter level and find good consistency; we investigate how the constraints on the correlated spectral index and baryon density parameters readjust when adding CMB large-scale information that ACT does not measure. The DR4 products presented here will be publicly released on the NASA Legacy Archive for Microwave Background Data Analysis.

Naess S., Aiola S., Austermann J.E., Battaglia N., Beall J.A., Becker D.T., Bond R.J., Calabrese E., Choi S.K., Cothard N.F., Crowley K.T., Darwish O., Datta R., Denison E.V., Devlin M., et. al.

Brown A.G., Vallenari A., Prusti T., de Bruijne J.H., Babusiaux C., Biermann M., Creevey O.L., Evans D.W., Eyer L., Hutton A., Jansen F., Jordi C., Klioner S.A., Lammers U., Lindegren L., et. al.

Context.We present the early installment of the thirdGaiadata release,GaiaEDR3, consisting of astrometry and photometry for 1.8 billion sources brighter than magnitude 21, complemented with the list of radial velocities fromGaiaDR2.Aims.A summary of the contents ofGaiaEDR3 is presented, accompanied by a discussion on the differences with respect toGaiaDR2 and an overview of the main limitations which are present in the survey. Recommendations are made on the responsible use ofGaiaEDR3 results.Methods.The raw data collected with theGaiainstruments during the first 34 months of the mission have been processed by theGaiaData Processing and Analysis Consortium and turned into this early third data release, which represents a major advance with respect toGaiaDR2 in terms of astrometric and photometric precision, accuracy, and homogeneity.Results. GaiaEDR3 contains celestial positions and the apparent brightness inGfor approximately 1.8 billion sources. For 1.5 billion of those sources, parallaxes, proper motions, and the (GBP−GRP) colour are also available. The passbands forG,GBP, andGRPare provided as part of the release. For ease of use, the 7 million radial velocities fromGaiaDR2 are included in this release, after the removal of a small number of spurious values. New radial velocities will appear as part ofGaiaDR3. Finally,GaiaEDR3 represents an updated materialisation of the celestial reference frame (CRF) in the optical, theGaia-CRF3, which is based solely on extragalactic sources. The creation of the source list forGaiaEDR3 includes enhancements that make it more robust with respect to high proper motion stars, and the disturbing effects of spurious and partially resolved sources. The source list is largely the same as that forGaiaDR2, but it does feature new sources and there are some notable changes. The source list will not change forGaiaDR3.Conclusions. GaiaEDR3 represents a significant advance overGaiaDR2, with parallax precisions increased by 30 per cent, proper motion precisions increased by a factor of 2, and the systematic errors in the astrometry suppressed by 30–40% for the parallaxes and by a factor ~2.5 for the proper motions. The photometry also features increased precision, but above all much better homogeneity across colour, magnitude, and celestial position. A single passband forG,GBP, andGRPis valid over the entire magnitude and colour range, with no systematics above the 1% level

Miret-Roig N., Galli P.A., Brandner W., Bouy H., Barrado D., Olivares J., Antoja T., Romero-Gómez M., Figueras F., Lillo-Box J.

Context.TheβPictoris moving group is one of the most well-known young associations in the solar neighbourhood and several members are known to host circumstellar discs, planets, and comets. Measuring its age precisely is essential to the study of several astrophysical processes, such as planet formation and disc evolution, which are strongly age-dependent.Aims.We aim to determine a precise and accurate dynamical traceback age for theβPictoris moving group.Methods.Our sample combines the extremely preciseGaiaDR2 astrometry with ground-based radial velocities measured in an homogeneous manner. We use an updated version of our algorithm to determine dynamical ages. The new approach takes into account a robust estimate of the spatial and kinematic covariance matrices of the association to improve the sample selection process and to perform the traceback analysis.Results.We estimate a dynamical age of 18.5−2.4+2.0Myr for theβPictoris moving group. We investigated the spatial substructure of the association at the time of birth and we propose the existence of a core of stars that is more concentrated. We also provide precise radial velocity measurements for 81 members ofβPic, including ten stars with the first determinations of their radial velocities.Conclusions.Our dynamical traceback age is three times more precise than previous traceback age estimates and, more importantly, for the first time it reconciles the traceback age with the most recent estimates of other dynamical, lithium depletion boundaries and isochronal ages. This has been possible thanks to the excellent astrometric and spectroscopic precisions, the homogeneity of our sample, and the detailed analysis of binaries and membership.

MacGregor A.M., Osten R.A., Hughes A.M.

We report on two millimeter flares detected by ALMA at 220 GHz from AU Mic, a nearby M dwarf. The larger flare had a duration of only $\sim35$ sec, with peak $L_{R}=2\times10^{15}$ erg s$^{-1}$ Hz$^{-1}$, and lower limit on linear polarization of $|Q/I|>0.12\pm0.04$. We examine the characteristics common to these new AU Mic events and those from Proxima Cen previously reported in MacGregor et al. (2018) - namely short durations, negative spectral indices, and significant linear polarization - to provide new diagnostics of conditions in outer stellar atmospheres and details of stellar flare particle acceleration. The event rates ($\sim20$ and $4$ events day$^{-1}$ for AU Mic and Proxima Cen, respectively) suggest that millimeter flares occur commonly but have been undetected until now. Analysis of the flare observing frequency and consideration of possible incoherent emission mechanisms confirms the presence of MeV electrons in the stellar atmosphere occurring as part of the flare process. The spectral indices point to a hard distribution of electrons. The short durations and lack of pronounced exponential decay in the light curve are consistent with formation in a simple magnetic loop, with radio emission predominating from directly precipitating electrons. We consider the possibility of both synchrotron and gyrosynchrotron emission mechanisms, although synchrotron is favored given the linear polarization signal. This would imply that the emission must be occurring in a low density environment of only modest magnetic field strength. A deeper understanding of this newly discovered and apparently common stellar flare mechanism awaits more observations with better-studied flare components at other wavelengths.

Laskar T., Alexander K.D., Gill R., Granot J., Berger E., Mundell C.G., Duran R.B., Bolmer J., Duffell P., Eerten H.V., Fong W., Kobayashi S., Margutti R., Schady P.

We present ALMA 97.5 GHz total intensity and linear polarization observations of the mm-band afterglow of GRB 190114C spanning 2.2 to 5.2 hours after the burst. We detect linear polarization at the $\approx 5\,\sigma$ level, decreasing from $\Pi=(0.87\pm0.13)\%$ to $(0.60\pm0.19)\%$, and evolving in polarization position angle from $(10\pm5)^\circ$ to $(-44\pm12)^\circ$ during the course of the observations. This represents the first detection of polarized millimeter emission in a $\gamma$-ray burst. We show that the optical and X-ray observations between $0.03$ days and $\sim0.3$ days are consistent with a fast cooling forward shock expanding into a wind environment. However, the optical observations at $\lesssim0.03$ days, as well as the radio and millimeter observations arise from a separate component, which we interpret as emission from the reverse-shocked ejecta. Using the measured linear polarization, we constrain the coherence scale of tangled magnetic fields in the ejecta to an angular size of $\theta_{\rm B} \approx10^{-3}$ radian, while the rotation of the polarization angle rules out the presence of large scale, ordered axisymmetric magnetic fields, and in particular a large scale toroidal field, in the jet.

Batygin K., Adams F.C., Brown M.E., Becker J.C.

Over the course of the past two decades, observational surveys have unveiled the intricate orbital structure of the Kuiper Belt, a field of icy bodies orbiting the Sun beyond Neptune. In addition to a host of readily-predictable orbital behavior, the emerging census of trans-Neptunian objects displays dynamical phenomena that cannot be accounted for by interactions with the known eight-planet solar system alone. Specifically, explanations for the observed physical clustering of orbits with semi-major axes in excess of $\sim250\,$AU, the detachment of perihelia of select Kuiper belt objects from Neptune, as well as the dynamical origin of highly inclined/retrograde long-period orbits remain elusive within the context of the classical view of the solar system. This newly outlined dynamical architecture of the distant solar system points to the existence of a new planet with mass of $m_9\sim 5-10\,M_{\oplus}$, residing on a moderately inclined orbit ($i_9\sim15-25\deg$) with semi-major axis $a_9\sim 400 - 800\,$AU and eccentricity between $e_9 \sim 0.2 - 0.5$. This paper reviews the observational motivation, dynamical constraints, and prospects for detection of this proposed object known as Planet Nine.

Zonca A., Singer L., Lenz D., Reinecke M., Rosset C., Hivon E., Gorski K.

Ivezić Ž., Kahn S.M., Tyson J.A., Abel B., Acosta E., Allsman R., Alonso D., AlSayyad Y., Anderson S.F., Andrew J., P. Angel J.R., Angeli G.Z., Ansari R., Antilogus P., Araujo C., et. al.

(Abridged) We describe here the most ambitious survey currently planned in the optical, the Large Synoptic Survey Telescope (LSST). A vast array of science will be enabled by a single wide-deep-fast sky survey, and LSST will have unique survey capability in the faint time domain. The LSST design is driven by four main science themes: probing dark energy and dark matter, taking an inventory of the Solar System, exploring the transient optical sky, and mapping the Milky Way. LSST will be a wide-field ground-based system sited at Cerro Pach\'{o}n in northern Chile. The telescope will have an 8.4 m (6.5 m effective) primary mirror, a 9.6 deg$^2$ field of view, and a 3.2 Gigapixel camera. The standard observing sequence will consist of pairs of 15-second exposures in a given field, with two such visits in each pointing in a given night. With these repeats, the LSST system is capable of imaging about 10,000 square degrees of sky in a single filter in three nights. The typical 5$\sigma$ point-source depth in a single visit in $r$ will be $\sim 24.5$ (AB). The project is in the construction phase and will begin regular survey operations by 2022. The survey area will be contained within 30,000 deg$^2$ with $\delta

Ade P., Aguirre J., Ahmed Z., Aiola S., Ali A., Alonso D., Alvarez M.A., Arnold K., Ashton P., Austermann J., Awan H., Baccigalupi C., Baildon T., Barron D., Battaglia N., et. al.

The Simons Observatory (SO) is a new cosmic microwave background experiment being built on Cerro Toco in Chile, due to begin observations in the early 2020s. We describe the scientific goals of the experiment, motivate the design, and forecast its performance. SO will measure the temperature and polarization anisotropy of the cosmic microwave background in six frequency bands centered at: 27, 39, 93, 145, 225 and 280 GHz. The initial configuration of SO will have three small-aperture 0.5-m telescopes and one large-aperture 6-m telescope, with a total of 60,000 cryogenic bolometers. Our key science goals are to characterize the primordial perturbations, measure the number of relativistic species and the mass of neutrinos, test for deviations from a cosmological constant, improve our understanding of galaxy evolution, and constrain the duration of reionization. The small aperture telescopes will target the largest angular scales observable from Chile, mapping ≈ 10% of the sky to a white noise level of 2 μK-arcmin in combined 93 and 145 GHz bands, to measure the primordial tensor-to-scalar ratio, r, at a target level of σ(r)=0.003. The large aperture telescope will map ≈ 40% of the sky at arcminute angular resolution to an expected white noise level of 6 μK-arcmin in combined 93 and 145 GHz bands, overlapping with the majority of the Large Synoptic Survey Telescope sky region and partially with the Dark Energy Spectroscopic Instrument. With up to an order of magnitude lower polarization noise than maps from the Planck satellite, the high-resolution sky maps will constrain cosmological parameters derived from the damping tail, gravitational lensing of the microwave background, the primordial bispectrum, and the thermal and kinematic Sunyaev-Zel'dovich effects, and will aid in delensing the large-angle polarization signal to measure the tensor-to-scalar ratio. The survey will also provide a legacy catalog of 16,000 galaxy clusters and more than 20,000 extragalactic sources.

Ho A.Y., Phinney E.S., Ravi V., Kulkarni S.R., Petitpas G., Emonts B., Bhalerao V., Blundell R., Cenko S.B., Dobie D., Howie R., Kamraj N., Kasliwal M.M., Murphy T., Perley D.A., et. al.

We present detailed submillimeter- through centimeter-wave observations of the extraordinary extragalactic transient AT2018cow. The apparent characteristics -- the high radio luminosity, the long-lived emission plateau at millimeter bands, and the sub-relativistic velocity -- have no precedent. A basic interpretation of the data suggests $E_k \gtrsim 10^{48}$ erg coupled to a fast but sub-relativistic ($v \approx 0.13c$) shock in a dense ($n_e \approx 3 \times 10^{5}\,$cm$^{-3}$) medium. We find that the X-ray emission is not naturally explained by an extension of the radio-submm synchrotron spectrum, nor by inverse Compton scattering of the dominant blackbody UVOIR photons by energetic electrons within the forward shock. By $\Delta t \approx20\,$days, the X-ray emission shows spectral softening and erratic inter-day variability. Taken together, we are led to invoke an additional source of X-ray emission: the central engine of the event. Regardless of the nature of this central engine, this source heralds a new class of energetic transients shocking a dense medium, which at early times are most readily observed at millimeter wavelengths.

Mairs S., Lalchand B., Bower G.C., Forbrich J., Bell G.S., Herczeg G.J., Johnstone D., Chen W., Lee J., Hacar A.

The binary T Tauri system JW 566 in the Orion Molecular Cloud underwent an energetic, short-lived flare observed at submillimetre wavelengths by the SCUBA-2 instrument on 26 November 2016 (UT). The emission faded by nearly 50% during the 31 minute integration. The simultaneous source fluxes averaged over the observation are 500 +/- 107 mJy/beam at 450 microns and 466 +/- 47 mJy/beam at 850 microns. The 850 micron flux corresponds to a radio luminosity of $L_{\nu}=8\times10^{19}$ erg/s/Hz, approximately one order of magnitude brighter (in terms of $\nu L_{\nu}$) than that of a flare of the young star GMR-A, detected in Orion in 2003 at 3mm. The event may be the most luminous known flare associated with a young stellar object and is also the first coronal flare discovered at sub-mm wavelengths. The spectral index between 450 microns and 850 microns of $\alpha = 0.11$ is broadly consistent with non-thermal emission. The brightness temperature was in excess of $6\times10^{4}$ K. We interpret this event to be a magnetic reconnection that energised charged particles to emit gyrosynchrotron/synchrotron radiation.

Zuckerman B.

The reality of a field Argus Association has been doubted in some papers in the literature. We apply Gaia DR2 data to stars previously suggested to be Argus members and conclude that a true association exists with age 40-50 Myr and containing many stars within 100 pc of Earth; Beta Leo and 49 Cet are two especially interesting members. Based on youth and proximity to Earth, Argus is one of the better nearby moving groups to target in direct imaging programs for dusty debris disks and young planets.

Crowley K.T., Austermann J.E., Choi S.K., Duff S.M., Gallardo P.A., Ho S.P., Hubmayr J., Koopman B.J., Nati F., Niemack M.D., Salatino M., Simon S.M., Staggs S.T., Stevens J.R., Ullom J.N., et. al.

The Advanced ACTPol (AdvACT) upgrade to the Atacama Cosmology Telescope (ACT) features arrays of aluminum manganese transition-edge sensors (TESes) optimized for ground-based observations of the cosmic microwave background (CMB). Array testing shows highly responsive detectors with anticipated in-band noise performance under optical loading. We report on TES parameters measured with impedance data taken on a subset of TESes. We then compare modeled noise spectral densities to measurements. We find excess noise at frequencies around 100 Hz, nearly outside of the signal band of CMB measurements. In addition, we describe full-array noise measurements in the laboratory and in the field for two new AdvACT mid-frequency arrays, sensitive at bands centered on 90 and 150 GHz, and data for the high-frequency array (150/230 GHz) as deployed.

Yurk N.Y., Ravi V., Hodges M.W., Lamb J.W., Hobbs R., Woody D.P., Readhead A.C.

Abstract Though the time-domain millimeter sky is yet to be well characterized, the scarcity of millimeter observing resources in the world at present hampers progress toward it. In efforts to bolster the exploration of millimeter transients, we present the Stokes Polarization Radio Interferometer for Time-Domain Experiments (SPRITEly). Located at the Owens Valley Radio Observatory, SPRITEly is currently deployed as a two-element short-baseline 90 GHz interferometer uniquely focused on monitoring bright variable millimeter-continuum sources. We leverage two existing 10.4 m antennas and their existing receiver systems to begin, but we make significant upgrades to the back-end system during the commissioning process. With the ability to achieve rms noise of a few mJy, we plan to monitor known variable sources along with new nearby transients detected from optical surveys at high cadence, with the goal of producing well-sampled light curves. Interpreting these data in conjunction with multiwavelength observations stands to provide insight into the physical properties of the sources that produce transient millimeter emission. We present commissioning and early-science observations that demonstrate the performance of the instrument, including observations of the flaring BL Lac object S2 0109+22 and a periastron passage of the binary T Tauri system DQ Tau.

Tandoi C., Guns S., Foster A., Ade P.A., Anderson A.J., Ansarinejad B., Archipley M., Balkenhol L., Benabed K., Bender A.N., Benson B.A., Bianchini F., Bleem L.E., Bouchet F.R., Bryant L., et. al.

Abstract We present a flare star catalog from 4 yr of nontargeted millimeter-wave survey data from the South Pole Telescope (SPT). The data were taken with the SPT-3G camera and cover a 1500 deg2 region of the sky from 20h40m0s to 3h20m0s in right ascension and from −42° to −70° in declination. This region was observed on a nearly daily cadence from 2019 to 2022 and chosen to avoid the plane of the galaxy. A short-duration transient search of this survey yields 111 flaring events from 66 stars, increasing the number of both flaring events and detected flare stars by an order of magnitude from the previous SPT-3G data release. We provide cross-matching to Gaia DR3, as well as matches to X-ray point sources found in the second ROSAT all-sky survey. We have detected flaring stars across the main sequence, from early-type A stars to M dwarfs, as well as a large population of evolved stars. These stars are mostly nearby, spanning 10–1000 pc in distance. Most of the flare spectral indices are constant or gently rising as a function of frequency at 95/150/220 GHz. The timescale of these events can range from minutes to hours, and the peak ν L ν luminosities range from 1027 to 1031 erg s−1 in the SPT-3G frequency bands.

Plant K., Hallinan G., Bastian T.

Abstract BL and UV Ceti are a nearby (2.7 pc) binary system with similar masses, spectral types, and rapid rotation rates, but very different magnetic activity. UV Ceti’s much stronger large-scale magnetic field may cause this difference, highlighting key unanswered questions about dynamo processes in fully convective objects. Here, we present multiepoch characterization of the radio spectrum of UV Ceti spanning 1–105 GHz, exhibiting flared emission similar to coronal activity, auroral-like emission analogous to planetary magnetospheres, and slowly varying persistent emission. Radio observations are a powerful means to probe the role that the large-scale magnetic field of UV Ceti has in nonthermal particle acceleration because radio-frequency phenomena result from both the activity of small-scale field features as well as large-scale auroral current systems. We find temporal variability at all bands observed, and a hint of rotational modulation in the degree of circular polarization up to 40 GHz. The persistent component of the emission is fairly constant from 1 to 105 GHz, making optically thick emission or optically thin gyrosynchrotron from electrons with an isotropic pitch angle distribution unlikely. We discuss the possibility of emission mechanisms analogous to Jupiter’s radiation belts.

Mairs S., Lee S., Johnstone D., Broughton C., Lee J., Herczeg G.J., Bell G.S., Chen Z., Contreras-Peña C., Francis L., Hatchell J., Kim M., Liu S., Park G., Qiu K., et. al.

Abstract The James Clerk Maxwell Telescope (JCMT) Transient Survey has been monitoring eight Gould Belt low-mass star-forming regions since 2015 December and six somewhat more distant intermediate-mass star-forming regions since 2020 February with the Submillimeter Common User Bolometer Array 2 on board JCMT at 450 and 850 μm and with an approximately monthly cadence. We introduce our pipeline v2 relative calibration procedures for image alignment and flux calibration across epochs, improving on our previous pipeline v1 by decreasing measurement uncertainties and providing additional robustness. These new techniques work at both 850 and 450 μm, where version 1 only allowed investigation of the 850 μm data. Pipeline v2 achieves better than 0.″5 relative image alignment, less than a tenth of the submillimeter beam widths. The version 2 relative flux calibration is found to be 1% at 850 μm and <5% at 450 μm. The improvement in the calibration is demonstrated by comparing the two pipelines over the first 4 yr of the survey and recovering additional robust variables with version 2. Using the full 6 yr of the Gould Belt survey, the number of robust variables increases by 50%, and at 450 μm we identify four robust variables, all of which are also robust at 850 μm. The multiwavelength light curves for these sources are investigated and found to be consistent with the variability being due to dust heating within the envelope in response to accretion luminosity changes from the central source.

Hervías-Caimapo C., Naess S., Hincks A.D., Calabrese E., Devlin M.J., Dunkley J., Dünner R., Gallardo P.A., Hilton M., Ho A.Y., Huffenberger K.M., Ma X., Madhavacheril M.S., Niemack M.D., Orlowski-Scherer J., et. al.

Abstract We have performed targeted searches of known extragalactic transient events at millimetre wavelengths using nine seasons (2013–2021) of 98, 150, and 229 GHz Atacama Cosmology Telescope (ACT) observations that mapped ∼40 per cent of the sky for most of the data volume. Our data cover 88 gamma-ray bursts (GRBs), 12 tidal disruption events (TDEs) and 203 other transients, including supernovae (SNe). We stack our ACT observations to increase the signal-to-noise ratio of the maps. In all cases but one, we do not detect these transients in the ACT data. The single candidate detection (event AT2019ppm), seen at ∼5σ significance in our data, appears to be due to active galactic nuclei (AGN) activity in the host galaxy coincident with a transient alert. For each source in our search we provide flux upper limits. For example, the medians for the 95% confidence upper limits at 98 GHz are 15, 18, and 16 mJy for GRBs, SNe, and TDEs respectively, in the first month after discovery. The projected sensitivity of future wide-area cosmic microwave background (CMB) surveys should be sufficient to detect many of these events using the methods described in this paper.

Lovell J.B., Keating G.K., Wilner D.J., Andrews S.M., MacGregor M., Rahman R.A., Rao R., Williams J.P.

Abstract We present evidence of variable 1.3 mm emission from the 1 to 3 Myr, spectral-type G2–G5 class III young stellar object (YSO), HD 283572. HD 283572 was observed on eight dates with the Submillimeter Array between 2021 December and 2023 May, with a total on-source time of 10.2 hr, probing a range of timescales down to 5.2 s. Averaging all data obtained on 2022 January 17 shows a 4.4 mJy (8.8σ) point source detection with a negative spectral index (α = −2.7 ± 1.2), with peak emission rising to 13.8 mJy in one 3 minute span, and 25 mJy in one 29.7 s integration (L ν = 4.7 × 1017 erg s−1 Hz−1). Combining our data for the other seven dates shows no detection, with an rms noise of 0.24 mJy beam−1. The stochastic millimeter enhancements on time frames of seconds–minutes–hours with negative spectral indices are most plausibly explained by synchrotron or gyrosynchrotron radiation from stellar activity. HD 283572's 1.3 mm lightcurve has similarities with variable binaries, suggesting HD 283572's activity may have been triggered by interactions with an as-yet undetected companion. We additionally identify variability of HD 283572 at 10 cm, from VLASS data. This study highlights the challenges of interpreting faint millimeter emission from evolved YSOs that may host tenuous disks, and suggests that a more detailed temporal analysis of spatially unresolved data is generally warranted. The variability of class III stars may open up new ground for understanding the physics of flares in the context of terrestrial planet formation.

Li Y., Biermann E., Naess S., Aiola S., An R., Battaglia N., Bhandarkar T., Calabrese E., Choi S.K., Crowley K.T., Devlin M., Duell C.J., Duff S.M., Dunkley J., Dünner R., et. al.

Abstract We conduct a systematic search for transients in 3 yr of data (2017–2019) from the Atacama Cosmology Telescope (ACT). ACT covers 40% of the sky at three bands spanning from 77–277 GHz. Analysis of 3 day mean-subtracted sky maps, which were match filtered for point sources, yielded 29 transient detections. Eight of these transients are due to known asteroids, and three others were previously published. Four of these events occur in areas with poor noise models and thus we cannot be confident they are real transients. We are left with 14 new transient events occurring at 11 unique locations. All of these events are associated with either rotationally variable stars or cool stars. Ten events have flat or falling spectra indicating radiation from synchrotron emission. One event has a rising spectrum indicating a different engine for the flare.

Tagawa H., Kimura S.S., Haiman Z., Perna R., Bartos I.

Abstract Stellar-mass black holes (BHs) are predicted to be embedded in the disks of active galactic nuclei (AGNs) due to gravitational drag and in situ star formation. However, clear evidence for AGN disk-embedded BHs is currently lacking. Here, as possible electromagnetic signatures of these BHs, we investigate breakout emission from shocks emerging around Blandford–Znajek jets launched from accreting BHs in AGN disks. We assume that most of the highly super-Eddington flow reaches the BH and produces a strong jet, and the jet produces feedback that shuts off accretion and thus leads to episodic flaring. These assumptions, while poorly understood at present, yield observable consequences that can probe the presence of AGN-embedded BHs as well as the accretion process itself. They predict a breakout emission characterized by luminous thermal emission in the X-ray bands and bright broadband nonthermal emission from the infrared to the gamma-ray bands. The flare duration depends on the BH’s distance r from the central supermassive BH, varying between 103–106 s for r ∼ 0.01–1 pc. This emission can be discovered by current and future infrared, optical, and X-ray wide-field surveys and monitoring campaigns of nearby AGNs.

Kimura S.S., Takasao S., Tomida K.

Abstract We investigate gamma-ray emission in the impulsive phase of solar flares and the detectability of nonthermal signatures from protostellar flares. Energetic solar flares emit high-energy gamma rays of GeV energies, but their production mechanism and emission site are still unknown. Young stellar objects, including protostars, also exhibit luminous X-ray flares, but the triggering mechanism of the flaring activity is still unclear owing to the strong obscuration. Nonthermal signatures in millimeter/submillimeter and gamma-ray bands are useful to probe protostellar flares owing to their strong penetration power. We develop a nonthermal emission model of the impulsive phase of solar flares, where cosmic-ray protons accelerated at the termination shock produce high-energy gamma rays via hadronuclear interaction with the evaporation plasma. This model can reproduce gamma-ray data in the impulsive phase of a solar flare. We apply our model to protostellar flares and show that the Cherenkov Telescope Array will be able to detect gamma rays of TeV energies if particle acceleration in protostellar flares is efficient. Nonthermal electrons accelerated together with protons can emit strong millimeter and submillimeter signals via synchrotron radiation, whose power is consistent with the energetic millimeter/submillimeter transients observed from young stars. Future gamma-ray and millimeter/submillimeter observations from protostars, coordinated with a hard X-ray observation, will unravel the nonthermal particle production and triggering mechanism of protostellar flares.

CCAT-Prime Collaboration, Aravena M., Austermann J.E., Basu K., Battaglia N., Beringue B., Bertoldi F., Bigiel F., Bond J.R., Breysse P.C., Broughton C., Bustos R., Chapman S.C., Charmetant M., Choi S.K., et. al.

Abstract We present a detailed overview of the science goals and predictions for the Prime-Cam direct-detection camera–spectrometer being constructed by the CCAT-prime collaboration for dedicated use on the Fred Young Submillimeter Telescope (FYST). The FYST is a wide-field, 6 m aperture submillimeter telescope being built (first light in late 2023) by an international consortium of institutions led by Cornell University and sited at more than 5600 m on Cerro Chajnantor in northern Chile. Prime-Cam is one of two instruments planned for FYST and will provide unprecedented spectroscopic and broadband measurement capabilities to address important astrophysical questions ranging from Big Bang cosmology through reionization and the formation of the first galaxies to star formation within our own Milky Way. Prime-Cam on the FYST will have a mapping speed that is over 10 times greater than existing and near-term facilities for high-redshift science and broadband polarimetric imaging at frequencies above 300 GHz. We describe details of the science program enabled by this system and our preliminary survey strategies.

Burton K., MacGregor M.A., Osten R.A.

Abstract We report the detection of three large millimeter flaring events from the nearby Sun-like, ϵ Eridani, found in archival Atacama Large Millimeter/submillimeter Array (ALMA) 12 m and Atacama Compact Array observations at 1.33 mm taken from 2015 January 17 to 18 and 2016 October 24 to November 23, respectively. This is the first time that flares have been detected from a Sun-like star at millimeter wavelengths. The largest flare among our data was detected in the ALMA observations on 2015 January 17 from 20:09:10.4–21:02:49.3 UT with a peak flux density of 28 ± 7 mJy and a duration of 9 s. The peak brightness of the largest flare is 3.4 ± 0.9 × 1014 erg s−1 Hz−1, a factor of >50× times brighter than the star’s quiescent luminosity and >10× brighter than solar flares observed at comparable wavelengths. We find changes in the spectral index (F ν ∝ ν α ) at the flare peak, with α = 1.81 ± 1.94 and a lower limit on the fractional linear polarization ∣Q/I∣ = 0.08 ± 0.12. This positive spectral index is more similar to millimeter solar flares, differing from M-dwarf flares also detected at millimeter wavelengths that exhibit steeply negative spectral indices.

Howard W.S., MacGregor M.A., Osten R., Forbrich J., Cranmer S.R., Tristan I., Weinberger A.J., Youngblood A., Barclay T., Parke Loyd R.O., Shkolnik E.L., Zic A., Wilner D.J.

Abstract We present millimeter, optical, and soft X-ray observations of a stellar flare with an energy squarely in the regime of typical X1 solar flares. The flare was observed from Proxima Cen on 2019 May 6 as part of a larger multi-wavelength flare monitoring campaign and was captured by Chandra, the Las Cumbres Observatory Global Telescope, the Iréné du Pont Telescope at Las Campanas Observatory, and the Atacama Large Millimeter Array. Millimeter emission appears to be a common occurrence in small stellar flares that had gone undetected until recently, making it difficult to interpret these events within the current multi-wavelength picture of the flaring process. The May 6 event is the smallest stellar millimeter flare detected to date. We compare the relationship between the soft X-ray and millimeter emission to that observed in solar flares. The X-ray and optical flare energies of 1030.3 ± 0.2 and 1028.9 ± 0.1 erg, respectively, the coronal temperature of T = 11.0 ± 2.1 MK, and the emission measure of 9.5 ± 2.2 × 1049 cm−3 are consistent with M-X class solar flares. We find the soft X-ray and millimeter emission during quiescence are consistent with the Güdel–Benz relation, but not during the flare. The millimeter luminosity is >100× higher than that of an equivalent X1 solar flare and lasts only seconds instead of minutes as seen for solar flares.

Johnstone D., Lalchand B., Mairs S., Shang H., Chen W.P., Bower G.C., Herczeg G.J., Lee J., Forbrich J., Chen B., Contreras Peña C., Lee Y., Park W., Broughton C., Plovie S.

Abstract Short-duration flares at millimeter wavelengths provide unique insights into the strongest magnetic reconnection events in stellar coronae and combine with longer-term variability to introduce complications to next-generation cosmology surveys. We analyze 5.5 yr of JCMT Transient Survey 850 μm submillimeter monitoring observations toward eight Gould Belt star-forming regions to search for evidence of transient events or long-duration variability from faint sources. The eight regions (30′ diameter fields), including ∼1200 infrared-selected YSOs, have been observed on average 47 times with integrations of approximately half an hour, or one day total, spread over 5.5 yr. Within this large data set, only two robust faint source detections are recovered: JW 566 in OMC 2/3 and MGM12 2864 in NGC 2023. JW 566, a Class II T Tauri binary system previously identified as an extraordinary submillimeter flare, remains unique, the only clear single-epoch transient detection in this sample with a flare eight times bright than our ∼4.5σ detection threshold of 55 mJy beam−1. The lack of additional recovered flares intermediate between JW 566 and our detection limit is puzzling if smaller events are more common than larger events. In contrast, the other submillimeter variable identified in our analysis, Source 2864, is highly variable on all observed timescales. Although Source 2864 is occasionally classified as a YSO, the source is most likely a blazar. The degree of variability across the electromagnetic spectrum may be used to aid source classification.

Eftekhari T., Berger E., Metzger B.D., Laskar T., Villar V.A., Alexander K.D., Holder G.P., Vieira J.D., Whitehorn N., Williams P.K.

Abstract The next generation of wide-field cosmic microwave background (CMB) surveys are uniquely poised to open a new window into time-domain astronomy in the millimeter band. Here, we explore the discovery phase space for extragalactic transients with near-term and future CMB experiments to characterize the expected population. We use existing millimeter-band light curves of known transients (gamma-ray bursts, tidal disruption events, fast blue optical transients (FBOTs), neutron star mergers) and theoretical models, in conjunction with known and estimated volumetric rates. Using Monte Carlo simulations of various CMB survey designs (area, cadence, depth, duration) we estimate the detection rates and the resulting light-curve characteristics. We find that existing and near-term surveys will find tens to hundreds of long-duration gamma-ray bursts (LGRBs), driven primarily by detections of the reverse shock emission, and including off-axis LGRBs. Next-generation experiments (CMB-S4, CMB-HD) will find tens of FBOTs in the nearby universe and will detect a few tidal disruption events. CMB-HD will additionally detect a small number of short gamma-ray bursts, where these will be discovered within the detection volume of next-generation gravitational wave experiments like the Cosmic Explorer.

Ho A.Y., Margalit B., Bremer M., Perley D.A., Yao Y., Dobie D., Kaplan D.L., O’Brien A., Petitpas G., Zic A.

Abstract Observations of the extragalactic (z = 0.0141) transient AT 2018cow established a new class of energetic explosions shocking a dense medium, producing luminous emission at millimeter and submillimeter wavelengths. Here we present detailed millimeter- through centimeter-wave observations of a similar transient, ZTF 20acigmel (AT 2020xnd), at z = 0.2433. Using observations from the NOrthern Extended Millimeter Array and the Very Large Array, we model the unusual millimeter and radio emission from AT 2020xnd under several different assumptions and ultimately favor synchrotron radiation from a thermal electron population (relativistic Maxwellian). The thermal electron model implies a fast but subrelativistic (v ≈ 0.3c) shock and a high ambient density (n e ≈ 4 × 103 cm−3) at Δt ≈ 40 days. The X-ray luminosity of L X ≈ 1043 erg s−1 exceeds simple predictions from the radio and UVOIR luminosity and likely has a separate physical origin, such as a central engine. Using the fact that month-long luminous (L ν ≈ 2 × 1030 erg s−1 Hz−1 at 100 GHz) millimeter emission appears to be a generic feature of transients with fast (t 1/2 ≈ 3 days) and luminous (M peak ≈ −21 mag) optical light curves, we estimate the rate at which transients like AT 2018cow and AT 2020xnd will be detected by future wide-field millimeter transient surveys such as CMB-S4 and conclude that energetic explosions in dense environments may represent a significant population of extragalactic transients in the 100 GHz sky.