Jan van Roestel

Bio: Jan van Roestel is an academic researcher from California Institute of Technology. The author has contributed to research in topics: White dwarf & Subdwarf. The author has an hindex of 7, co-authored 13 publications receiving 114 citations.

A New Class of Large-amplitude Radial-mode Hot Subdwarf Pulsators

Abstract: Using high-cadence observations from the Zwicky Transient Facility at low Galactic latitudes, we have discovered a new class of pulsating, hot compact stars. We have found four candidates, exhibiting blue colors (g − r ≤ −0.1 mag), pulsation amplitudes of >5%, and pulsation periods of 200–475 s. Fourier transforms of the light curves show only one dominant frequency. Phase-resolved spectroscopy for three objects reveals significant radial velocity, T_(eff), and log(g) variations over the pulsation cycle, which are consistent with large-amplitude radial oscillations. The mean T_(eff) and log(g) for these stars are consistent with hot subdwarf B (sdB) effective temperatures and surface gravities. We calculate evolutionary tracks using MESA and adiabatic pulsations using GYRE for low-mass, helium-core pre-white dwarfs (pre-WDs) and low-mass helium-burning stars. Comparison of low-order radial oscillation mode periods with the observed pulsation periods show better agreement with the pre-WD models. Therefore, we suggest that these new pulsators and blue large-amplitude pulsators (BLAPs) could be members of the same class of pulsators, composed of young ≈0.25–0.35 M_⊙ helium-core pre-WDs.

The Kitt Peak Electron Multiplying CCD demonstrator

Abstract: The Kitt Peak Electron Multiplying CCD (EMCCD) demonstrator is a new instrument that has been developed for use at the Kitt Peak National Observatory’s 84-inch telescope. The EMCCD enables single-band optical imaging in the Sloan g and r bands and Johnson UVRI filters. The EMCCD is chosen for its sub-electron effective read noise using large multiplicative gains. With these systems, frame rates of greater than 1 Hz are possible. The field of view is 4.4 × 4.4 arcmin^2 and the pixel size is 0.259 arcsec. This camera, coupled with a fully roboticized telescope, is ideal for follow-up of short period, white dwarf binary candidates, as well as short duration transient and periodic sources identified by large field-of-view all-sky surveys such as the Zwicky Transient Facility.

ZTF20aajnksq (AT 2020blt): A Fast Optical Transient at z 2.9 with No Detected Gamma-Ray Burst Counterpart

Abstract: We present ZTF20aajnksq (AT 2020blt), a fast-fading (Δr = 2.3 mag in Δt = 1.3 days) red (g − r ≈ 0.6 mag) and luminous (M1626 A = −25.9 mag) optical transient at z = 2.9 discovered by the Zwicky Transient Facility (ZTF). AT 2020blt shares several features in common with afterglows to long-duration gamma-ray bursts (GRBs): (1) an optical light curve well-described by a broken power law with a break at tj = 1 d (observer frame); (2) a luminous (L0.3–10 KeV = 1046 erg s−1) X-ray counterpart; and (3) luminous (L10 GHz = 4 × 1031 erg s−1 Hz−1) radio emission. However, no GRB was detected in the 0.74 days between the last ZTF nondetection (r > 21.36 mag) and the first ZTF detection (r = 19.60 mag), with an upper limit on the isotropic-equivalent gamma-ray energy release of Eγ,iso < 7 × 1052 erg. AT 2020blt is thus the third afterglow-like transient discovered without a detected GRB counterpart (after PTF11agg and ZTF19abvizsw) and the second (after ZTF19abvizsw) with a redshift measurement. We conclude that the properties of AT 2020blt are consistent with a classical (initial Lorentz factor Γ0 gsim 100) on-axis GRB that was missed by high-energy satellites. Furthermore, by estimating the rate of transients with light curves similar to that of AT 2020blt in ZTF high-cadence data, we agree with previous results that there is no evidence for an afterglow-like phenomenon that is significantly more common than classical GRBs, such as dirty fireballs. We conclude by discussing the status and future of fast-transient searches in wide-field high-cadence optical surveys.

Year 1 of the ZTF high-cadence Galactic plane survey: strategy, goals, and early results on new single-mode hot subdwarf B-star pulsatos

Abstract: We present the goals, strategy and first results of the high-cadence Galactic plane survey using the Zwicky Transient Facility (ZTF). The goal of the survey is to unveil the Galactic population of short-period variable stars, including short period binaries and stellar pulsators with periods less than a few hours. Between June 2018 and January 2019, we observed 64 ZTF fields resulting in 2990 deg$^2$ of high stellar density in ZTF-$r$ band along the Galactic Plane. Each field was observed continuously for 1.5 to 6 hrs with a cadence of 40 sec. Most fields have between 200 and 400 observations obtained over 2-3 continuous nights. As part of this survey we extract a total of $\approx$230 million individual objects with at least 80 epochs obtained during the high-cadence Galactic Plane survey reaching an average depth of ZTF-$r$ $\approx$20.5 mag. For four selected fields with 2 million to 10 million individual objects per field we calculate different variability statistics and find that $\approx$1-2% of the objects are astrophysically variable over the observed period. We present a progress report on recent discoveries, including a new class of compact pulsators, the first members of a new class of Roche Lobe filling hot subdwarf binaries as well as new ultracompact double white dwarfs and flaring stars. Finally we present a sample of 12 new single-mode hot subdwarf B-star pulsators with pulsation amplitudes between ZTF-$r$ = 20-76 mmag and pulsation periods between $P$ = 5.8-16 min with a strong cluster of systems with periods $\approx$ 6 min. All of the data have now been released in either ZTF Data Release 3 or data release 4.

ZTF J1901+5309: A 40.6-min orbital period eclipsing double white dwarf system

Abstract: The Zwicky Transient Facility has begun to discover binary systems with orbital periods that are less than 1 h. Combined with dedicated follow-up systems, which allow for high-cadence photometry of these sources, systematic confirmation and characterization of these sources are now possible. Here, we report the discovery of ZTF J190125.42+530929.5, a 40.6-min orbital period, eclipsing double white dwarf binary. Both photometric modelling and spectroscopic modelling confirm its nature, yielding an estimated inclination of i=86.2^(+0.6)_(−0.2) deg and primary and secondary effective temperatures of {T}_(eff)=28000⁺⁵⁰⁰₋₅₀₀ and 17600⁺⁴⁰⁰₋₄₀₀, respectively. This system adds to a growing list of sources for future gravitational-wave detectors and contributes to the demographic analysis of double degenerates.

Overview of the DESI Legacy Imaging Surveys

Abstract: Author(s): Huang, L; Dey, A; Schlegel, DJ; Lang, D; Blum, R; Burleigh, K; Fan, X; Findlay, JR; Finkbeiner, D; Herrera, D; Juneau, S; Landriau, M; Levi, M; McGreer, I; Meisner, A; Myers, AD; Moustakas, J; Nugent, P; Patej, A; Schlafly, EF; Walker, AR; Valdes, F; Weaver, BA; Yeche, C; Zou, H; Zhou, X; Abareshi, B; Abbott, TMC; Abolfathi, B; Aguilera, C; Alam, S; Allen, L; Alvarez, A; Annis, J; Ansarinejad, B; Aubert, M; Beechert, J; Bell, EF; Benzvi, SY; Beutler, F; Bielby, RM; Bolton, AS; Briceno, C; Buckley-Geer, EJ; Butler, K; Calamida, A; Carlberg, RG; Carter, P; Casas, R; Castander, FJ; Choi, Y; Comparat, J; Cukanovaite, E; Delubac, T; Devries, K; Dey, S; Dhungana, G; Dickinson, M; Ding, Z; Donaldson, JB; Duan, Y; Duckworth, CJ; Eftekharzadeh, S; Eisenstein, DJ; Etourneau, T; Fagrelius, PA; Farihi, J; Fitzpatrick, M; Font-Ribera, A; Fulmer, L; Gansicke, BT; Gaztanaga, E; George, K; Gerdes, DW; A Gontcho, SG; Gorgoni, C; Green, G; Guy, J; Harmer, D; Hernandez, M; Honscheid, K; Huang, LW; James, DJ; Jannuzi, BT; Jiang, L | Abstract: © 2019. The American Astronomical Society. All rights reserved.. The DESI Legacy Imaging Surveys (http://legacysurvey.org/) are a combination of three public projects (the Dark Energy Camera Legacy Survey, the Beijing-Arizona Sky Survey, and the Mayall z-band Legacy Survey) that will jointly image ≈14,000 deg2 of the extragalactic sky visible from the northern hemisphere in three optical bands (g, r, and z) using telescopes at the Kitt Peak National Observatory and the Cerro Tololo Inter-American Observatory. The combined survey footprint is split into two contiguous areas by the Galactic plane. The optical imaging is conducted using a unique strategy of dynamically adjusting the exposure times and pointing selection during observing that results in a survey of nearly uniform depth. In addition to calibrated images, the project is delivering a catalog, constructed by using a probabilistic inference-based approach to estimate source shapes and brightnesses. The catalog includes photometry from the grz optical bands and from four mid-infrared bands (at 3.4, 4.6, 12, and 22 μm) observed by the Wide-field Infrared Survey Explorer satellite during its full operational lifetime. The project plans two public data releases each year. All the software used to generate the catalogs is also released with the data. This paper provides an overview of the Legacy Surveys project.

GROWTH on S190425z: Searching Thousands of Square Degrees to Identify an Optical or Infrared Counterpart to a Binary Neutron Star Merger with the Zwicky Transient Facility and Palomar Gattini-IR

Abstract: The third observing run by LVC has brought the discovery of many compact binary coalescences. Following the detection of the first binary neutron star merger in this run (LIGO/Virgo S190425z), we performed a dedicated follow-up campaign with the Zwicky Transient Facility (ZTF) and Palomar Gattini-IR telescopes. The initial skymap of this single-detector gravitational wave (GW) trigger spanned most of the sky observable from Palomar Observatory. Covering 8000 deg2 of the initial skymap over the next two nights, corresponding to 46% integrated probability, ZTF system achieved a depth of ≈21 m AB in g- and r-bands. Palomar Gattini-IR covered 2200 square degrees in J-band to a depth of 15.5 mag, including 32% integrated probability based on the initial skymap. The revised skymap issued the following day reduced these numbers to 21% for the ZTF and 19% for Palomar Gattini-IR. We narrowed 338,646 ZTF transient "alerts" over the first two nights of observations to 15 candidate counterparts. Two candidates, ZTF19aarykkb and ZTF19aarzaod, were particularly compelling given that their location, distance, and age were consistent with the GW event, and their early optical light curves were photometrically consistent with that of kilonovae. These two candidates were spectroscopically classified as young core-collapse supernovae. The remaining candidates were ruled out as supernovae. Palomar Gattini-IR did not identify any viable candidates with multiple detections only after merger time. We demonstrate that even with single-detector GW events localized to thousands of square degrees, systematic kilonova discovery is feasible.

General relativistic orbital decay in a seven-minute-orbital-period eclipsing binary system

Abstract: General relativity1 predicts that short-orbital-period binaries emit considerable amounts of gravitational radiation. The upcoming Laser Interferometer Space Antenna2 (LISA) is expected to detect tens of thousands of such systems3 but few have been identified4, of which only one5 is eclipsing-the double-white-dwarf binary SDSS J065133.338+284423.37, which has an orbital period of 12.75 minutes. Here we report the discovery of an eclipsing double-white-dwarf binary system, ZTF J153932.16+502738.8, with an orbital period of 6.91 minutes. This system has an orbit so compact that the entire binary could fit within the diameter of the planet Saturn. The system exhibits a deep eclipse, and a double-lined spectroscopic nature. We see rapid orbital decay, consistent with that expected from general relativity. ZTF J153932.16+502738.8 is a strong source of gravitational radiation close to the peak of LISA's sensitivity, and we expect it to be detected within the first week of LISA observations, once LISA launches in approximately 2034.

A Reverse Shock in GRB 130427A

Abstract: We present extensive radio and millimeter observations of the unusually bright GRB 130427A at z = 0.340, spanning 0.67-12 days after the burst. We combine these data with detailed multi-band UV, optical, NIR, and Swift X-ray observations and find that the broadband afterglow emission is composed of distinct reverse shock and forward shock contributions. The reverse shock emission dominates in the radio/millimeter and at 0.1 days in the UV/optical/NIR, while the forward shock emission dominates in the X-rays and at 0.1 days in the UV/optical/NIR. We further find that the optical and X-ray data require a wind circumburst environment, pointing to a massive star progenitor. Using the combined forward and reverse shock emission, we find that the parameters of the burst include an isotropic kinetic energy of E K, iso 2 × 1053 erg, a mass loss rate of M ☉ yr–1 (for a wind velocity of 1000 km s–1), and a Lorentz factor at the deceleration time of Γ(200 s) 130. Due to the low density and large isotropic energy, the absence of a jet break to 15 days places only a weak constraint on the opening angle, θj 2.°5, and therefore a total energy of E γ + EK 1.2 × 1051 erg, similar to other gamma-ray bursts (GRBs). The reverse shock emission is detectable in this burst due to the low circumburst density, which leads to a slow cooling shock. We speculate that this property is required for the detectability of reverse shocks in radio and millimeter bands. Following on GRB 130427A as a benchmark event, observations of future GRBs with the exquisite sensitivity of the Very Large Array and ALMA, coupled with detailed modeling of the reverse and forward shock contributions, will test this hypothesis.

The Zwicky Transient Facility Bright Transient Survey. II. A Public Statistical Sample for Exploring Supernova Demographics

Abstract: We present a public catalog of transients from the Zwicky Transient Facility (ZTF) Bright Transient Survey (BTS), a magnitude-limited (m -16 mag), 10% in red-sequence galaxies, and 1% in massive ellipticals. We find no significant difference in the luminosity or color distributions between the host galaxies of Type II and Type Ib/c supernovae, suggesting that line-driven wind stripping does not play a major role in the loss of the hydrogen envelope from their progenitors. Future large-scale classification efforts with ZTF and other wide-area surveys will provide high-quality measurements of the rates, properties, and environments of all known types of optical transients and limits on the existence of theoretically predicted but as of yet unobserved explosions.