[en] We present milliarcsecond-resolution radio very long baseline interferometry (VLBI) observations of the ultracool dwarfs TVLM 513-46546 (M8.5) and 2MASS J00361617+1821104 (L3.5) in an attempt to detect sub-stellar companions via direct imaging or reflex motion. Both objects are known radio emitters with strong evidence for periodic emission on timescales of about 2 hr and 3 hr, respectively. Using the inner seven VLBA antennas, we detect unresolved emission from TVLM 513-46546 on a scale of 2.5 mas (∼50 stellar radii), leading to a direct limit on the radio emission brightness temperature of T_B ∼> 4 x 10⁵ K. However, with the higher spatial resolution afforded by the full VLBA we find that the source appears to be marginally resolved at a low signal-to-noise ratio, possibly indicating that TVLM 513-46546 is a binary with a projected separation of ∼1 mas (∼20 stellar radii). Using the 7 hr baseline of our observation, we find no astrometric shift in the position of TVLM 513-46546, with a 3σ limit of about 0.6 mas. This is about three times larger than expected for an equal-mass companion with a few-hour orbital period. Future monitoring of its position on a range of timescales will provide the required astrometric sensitivity to detect a planetary companion with a mass of ∼10 M _J in a ∼>15 day (∼>0.06 AU) orbit, or with a mass of ∼2 M _J in an orbit of ∼>0.5 yr (∼>0.3 AU).

[en] We conducted multi-epoch very long baseline interferometry observations to search for astrometric reflex motion that would be caused by a sub-stellar companion of the M8.5 dwarf TVLM 513–46546. The observations yield an absolute parallax corresponding to a distance of 10.762 ± 0.027 pc and a proper motion of 78.09 ± 0.17 mas yr^–1. The averaged flux density per epoch varies by a factor of at least three. From the absence of significant residual motion, we place an upper limit on any reflex motion caused by a companion, extending the parameter space covered by previous near-infrared direct-imaging searches. The data exclude a phase space of companion masses and orbital periods ranging from 3.8 M_Jup with an orbital radius of ∼0.05 AU (and an orbital period of 16 days) to 0.3 M_Jup with an orbital radius of ∼0.7 AU (and an orbital period of 710 days)

[en] Radio detections of ultracool dwarfs provide insight into their magnetic fields and the dynamos that maintain them, especially at the very bottom of the main sequence, where other activity indicators dramatically weaken. Until recently, radio emission was only detected in the M and L dwarf regimes, but this has changed with the Arecibo detection of rapid polarized flares from the T6.5 dwarf 2MASS J10475385+2124234. Here, we report the detection of quasi-quiescent radio emission from this source at 5.8 GHz using the Karl G. Jansky Very Large Array. The spectral luminosity is L_ν = (2.2 ± 0.7) × 10¹² erg s^–1 Hz^–1, a factor of ∼100 times fainter than the Arecibo flares. Our detection is the lowest luminosity yet achieved for an ultracool dwarf. Although the emission is fully consistent with being steady, unpolarized, and broad band, we find tantalizing hints for variability. We exclude the presence of short-duration flares as seen by Arecibo, although this is not unexpected given estimates of the duty cycle. Follow-up observations of this object will offer the potential to constrain its rotation period, electron density, and the strength and configuration of the magnetic field. Equally important, follow-up observations will address the question of whether the electron cyclotron maser instability, which is thought to produce the flares seen by Arecibo, also operates in the very different parameter regime of the emission we detect, or whether instead this ultracool dwarf exhibits both maser and gyrosynchrotron radiation, potentially originating from substantially different locations.

[en] Subarcsecond localization of the repeating fast radio burst FRB 121102 revealed its coincidence with a dwarf host galaxy and a steady (“quiescent”) nonthermal radio source. We show that the properties of the host galaxy are consistent with those of long-duration gamma-ray bursts (LGRB) and hydrogen-poor superluminous supernovae (SLSNe-I). Both LGRBs and SLSNe-I were previously hypothesized to be powered by the electromagnetic spin-down of newly formed, strongly magnetized neutron stars with millisecond birth rotation periods (“millisecond magnetars”). This motivates considering a scenario whereby the repeated bursts from FRB 121102 originate from a young magnetar remnant embedded within a young hydrogen-poor supernova (SN) remnant. Requirements on the gigahertz free–free optical depth through the expanding SN ejecta (accounting for photoionization by the rotationally powered magnetar nebula), energetic constraints on the bursts, and constraints on the size of the quiescent source all point to an age of less than a few decades. The quiescent radio source can be attributed to synchrotron emission from the shock interaction between the fast outer layer of the supernova ejecta with the surrounding wind of the progenitor star, or the radio source can from deeper within the magnetar wind nebula as outlined in Metzger et al. Alternatively, the radio emission could be an orphan afterglow from an initially off-axis LGRB jet, though this might require the source to be too young. The young age of the source can be tested by searching for a time derivative of the dispersion measure and the predicted fading of the quiescent radio source. We propose future tests of the SLSNe-I/LGRB/FRB connection, such as searches for FRBs from nearby SLSNe-I/LGRBs on timescales of decades after their explosions.

[en] Long-duration gamma-ray bursts (GRBs) provide a premier tool for studying high-redshift star-forming galaxies thanks to their extreme brightness and association with massive stars. Here we use GRBs to study the galaxy stellar mass-metallicity (M_*-Z) relation at z ∼ 3-5, where conventional direct metallicity measurements are extremely challenging. We use the interstellar medium metallicities of long GRB hosts derived from afterglow absorption spectroscopy, in conjunction with host galaxy stellar masses determined from deep Spitzer 3.6 μm observations of 20 GRB hosts. We detect about 1/4 of the hosts with M_AB(I) ∼ -21.5 to -22.5 mag and place a limit of M_AB(I) ∼> -19 mag on the remaining hosts from a stacking analysis. Using these observations, we present the first rest-frame optical luminosity distribution of long GRB hosts at z ∼> 3 and find that it is similar to the distribution of long GRB hosts at z ∼ 1. In comparison to Lyman-break galaxies at the same redshift, GRB hosts are generally fainter, but the sample is too small to rule out an overall similar luminosity function. On the other hand, the GRB hosts appear to be more luminous than the population of Lyα emitters at z ∼ 3-4. Using a conservative range of mass-to-light ratios for simple stellar populations (with ages of 70 Myr to ∼2 Gyr), we infer the host stellar masses and present mass-metallicity measurements at z ∼ 3-5 ((z) ∼ 3.5). We find that the detected GRB hosts, with M_* ∼ 2 x 10¹⁰ M_sun, display a wide range of metallicities, but that the mean metallicity at this mass scale, Z ∼ 0.3 Z_sun, is lower than measurements at z ∼< 3. Combined with stacking of the non-detected hosts with M_* ∼< 3 x 10⁹ M_sun and Z ∼< 0.1 Z_sun, we find tentative evidence for the existence of an M_*-Z relation at z ∼ 3.5 and continued evolution of this relation to systematically lower metallicities from z ∼ 2.

[en] The duration–luminosity phase space (DLPS) of optical transients is used, mostly heuristically, to compare various classes of transient events, to explore the origin of new transients, and to influence optical survey observing strategies. For example, several observational searches have been guided by intriguing voids and gaps in this phase space. However, we should ask, do we expect to find transients in these voids given our understanding of the various heating sources operating in astrophysical transients? In this work, we explore a broad range of theoretical models and empirical relations to generate optical light curves and to populate the DLPS. We explore transients powered by adiabatic expansion, radioactive decay, magnetar spin-down, and circumstellar interaction. For each heating source, we provide a concise summary of the basic physical processes, a physically motivated choice of model parameter ranges, an overall summary of the resulting light curves and their occupied range in the DLPS, and how the various model input parameters affect the light curves. We specifically explore the key voids discussed in the literature: the intermediate-luminosity gap between classical novae and supernovae, and short-duration transients ($\lesssim <!--\; ???\; -->10$ days). We find that few physical models lead to transients that occupy these voids. Moreover, we find that only relativistic expansion can produce fast and luminous transients, while for all other heating sources events with durations $\lesssim <!--\; ???\; -->10$ days are dim ($M_{\mathrm{R}}\gtrsim <!--\; ???\; -->-<!--\; ???\; -->15$ mag). Finally, we explore the detection potential of optical surveys (e.g., Large Synoptic Survey Telescope) in the DLPS and quantify the notion that short-duration and dim transients are exponentially more difficult to discover in untargeted surveys.

[en] The LIGO/Virgo Scientific Collaboration (LVC) recently announced the detection of a compact object binary merger, GW190425, with a total mass of ${3.4}_{-<!--\; ???\; -->0.1}^{+0.3}$ M _⊙ and individual component masses in the range of about 1.1–2.5 M _⊙. If the constituent compact objects are neutron stars, then the total mass is five standard deviations higher than the mean of 2.66 ± 0.12 M _⊙ for Galactic binary neutron stars. LVC suggests that the nondetection of such massive binary neutron star (BNS) systems in the Galaxy is due to a selection effect. However, we are unable to reconcile the inferred formation efficiency from the reported merger rate, ${\mathcal{R}}_{\mathrm{G}\mathrm{W}190425}={460}_{-<!--\; ???\; -->390}^{+1050}$ yr⁻¹ Gpc⁻³, with predictions from our own study for fast-merging BNS systems. Moreover, the comparable merger rates of GW190425 and GW170817 are possibly in tension with our results for two reasons: (i) more massive systems are expected to have lower formation rates, and (ii) fast-merging channels should constitute ≲10% of the total BNS systems if case BB unstable mass transfer is permitted to take place as a formation pathway. We argue that, to account for the high merger rate of GW190425 as a BNS system, (i) our understanding of NS formation in supernova explosions must be revisited, or (ii) more massive NSs must be preferentially born with either very weak or very high magnetic fields so that they would be undetectable in the radio surveys. Perhaps the detected massive NSs in NS–white dwarf binaries are our clues to the formation path of GW190425 systems.

[en] We present the results of an extensive Hubble Space Telescope imaging study of 105, mostly Swift, long-duration gamma-ray bursts (LGRBs) spanning $0.03\lesssim <!--\; ???\; -->z\lesssim <!--\; ???\; -->9.4$, which were localized using relative astrometry from ground- and space-based afterglow observations. We measure the distribution of LGRB offsets from their host centers and their relation to the underlying host light distribution. We find that the host-normalized offsets of LGRBs are more centrally concentrated than expected for an exponential disk profile, $⟨<!--\; ???\; -->R/R_h⟩<!--\; ???\; -->$ = 0.63, and in particular they are more concentrated than the underlying surface brightness profiles of their host galaxies and more concentrated than supernovae. The fractional flux distribution, with a median of 0.78, indicates that LGRBs prefer some of the brightest locations in their host galaxies but are not as strongly correlated as previous studies indicated. Importantly, we find a clear correlation between offset and fractional flux, where bursts at offsets $R/R_h\lesssim <!--\; ???\; -->0.5$ exclusively occur at fractional fluxes $\gtrsim <!--\; ???\; -->0.6$, while bursts at $R/R_h\gtrsim <!--\; ???\; -->0.5$ have a uniform fractional flux distribution. This indicates that the spatial correlation of LGRBs with bright star-forming regions seen in the full sample is dominated by the contribution from bursts at small offset and that LGRBs in the outer parts of galaxies show no preference for unusually bright regions. We conclude that LGRBs strongly prefer the bright, inner regions of their hosts, indicating that the star formation taking place there is more favorable for LGRB progenitor production. This indicates that environmental factors beyond metallicity, such as binary interactions or IMF differences, may operate in the central regions of LGRB hosts.

[en] We present a statistically robust mass-metallicity relation for long-duration gamma-ray burst (LGRB) host galaxies at z < 1. By comparing the LGRB host mass-metallicity relation to samples representative of the general star-forming galaxy population, we conclude that LGRBs occur in host galaxies with lower metallicities than the general population, and that this trend extends to z ∼ 1, with an average offset of -0.42 ± 0.18 from the M-Z relation for star-forming galaxies. Our sample in this work includes new spectroscopic data for six LGRB host galaxies obtained at the Keck and Magellan telescopes, as well as two new host galaxies from the literature. Combined with data from our previous work, this yields a total sample of six LGRB host galaxies at z < 0.3 and 10 host galaxies at 0.3 < z < 1. We have determined a number of interstellar medium properties for our host galaxies using optical emission-line diagnostics including metallicity, ionization parameter, young stellar population age, and star formation rate. Across our full sample of 16 LGRB hosts we find an average metallicity of log(O/H) + 12 = 8.4 ± 0.3. Notably, we also measure a comparatively high metallicity of log(O/H) + 12 = 8.83 ± 0.1 for the z = 0.296 host galaxy of GRB 050826. We also determine stellar masses (M_*) for our LGRB host galaxy sample, finding a mean stellar mass of log(M_*/M_sun) = 9.25^+0.19_-0.23.

[en] We present measurements of the pseudo-equivalent width of the Fe ii λ5018 absorption feature in the spectra of 12 Type IIP supernovae (SNe II) in low-luminosity (M > −17) dwarf host galaxies. The Fe ii λ5018 line has been called a useful diagnostic of the metallicity of the supernova (SN) progenitor stars. The events in our sample were discovered photometrically by the PanSTARRS Survey for Transients, and classified spectroscopically by us. Comparing our sample to 24 literature SNe II, we find that those in low-luminosity hosts have significantly weaker Fe ii features, with a probability of 10⁻⁴ that the two samples are drawn from the same distribution. Because low-mass galaxies are expected to contain a lower fraction of metals, our findings are consistent with a metallicity dependence for Fe ii λ5018, and therefore support the use of this line as a metallicity probe, in agreement with a number of recent works. In addition, we find that the SNe in faint (low-metallicity) hosts may be more luminous on average than those in the literature sample, suggesting possible physical differences between Type IIP SNe at low and high metallicity. However, accurate determinations of host galaxy extinction will be needed to quantify such an effect.