We present a study of short time-scale variability of OH masers within a contiguous 15-hour Very Long Baseline Array observation of the high-mass star-forming region, W3 (OH). With an angular ...resolution of ~7 mas and a velocity resolution of 53 m s−1, we isolate emission from masers in the field into individual Gaussian-shaped components, each a few milliarcseconds in size. We compute dynamic spectra for individual maser features with a time resolution of 1 minute by fitting for the flux density of all sources in the field simultaneously in the uv-domain. We isolate intrinsic maser variability from interstellar scintillation and instrumental effects. We find fluctuations in the maser line shape on time scales of 5 to 20 minutes, corresponding to maser column lengths of 0.5 to 2.0 Astronomical Units.
We present radio observations of 23 optically-discovered tidal disruption
events (TDEs) on timescales of about 500-3200 days post-discovery. We detect 9
new TDEs that did not have detectable radio ...emission at earlier times,
indicating a late-time brightening after several hundred (and up to 2300 days);
an additional 6 TDEs exhibit radio emission whose origin is ambiguous or may be
attributed to the host galaxy or an AGN. We also report new rising components
in two TDEs previously detected in the radio (iPTF16fnl and AT2019dsg) at ~1000
days. While the radio emission in some of the detected TDEs peaked on a
timescale of ~2-4 years, more than half of the sample still shows rising
emission. The range of luminosities for the sample is 10^37-10^39 erg/s, about
two orders of magnitude below the radio luminosity of the relativistic TDE
Sw1644+57. Our data set indicates that about 40% of all optical TDEs are
detected in the radio hundreds to thousands of days after discovery, and that
this is probably more common than early radio emission peaking at ~100 days.
Using an equipartition analysis, we find evidence for a delayed launch of the
radio-emitting outflows, with delay timescales of ~500-2000 days, inferred
velocities of ~0.02-0.15c, and kinetic energies of ~10^47-10^49 erg. We rule
out off-axis relativistic jets as a viable explanation for this population, and
conclude delayed outflows are a more likely explanation, such as from delayed
disk formation. Finally, we find comparable densities in the circumnuclear
environments of these TDEs as for those with early radio emission, and find the
TDEs still rising in luminosity are consistent with free expansion. We conclude
that late radio emission marks a fairly ubiquitous but heretofore overlooked
phase of TDE evolution.
We present 1.3 mm (230 GHz) observations of the recent and nearby Type II supernova, SN2023ixf, obtained with the Submillimeter Array (SMA) at 2.6-18.6 days after explosion. The observations were ...obtained as part the SMA Large Program POETS (Pursuit of Extragalactic Transients with the SMA). We do not detect any emission at the location of SN2023ixf, with the deepest limits of \(L_\nu(230\,{\rm GHz})\lesssim 8.6\times 10^{25}\) erg s\(^{-1}\) Hz\(^{-1}\) at 2.7 and 7.7 days, and \(L_\nu(230\,{\rm GHz})\lesssim 3.4\times 10^{25}\) erg s\(^{-1}\) Hz\(^{-1}\) at 18.6 days. These limits are about a factor of 2 times dimmer than the mm emission from SN2011dh (IIb), about an order of magnitude dimmer compared to SN1993J (IIb) and SN2018ivc (IIL), and about 30 times dimmer than the most luminous non-relativistic SNe in the mm-band (Type IIb/Ib/Ic). Using these limits in the context of analytical models that include synchrotron self-absorption and free-free absorption we place constraints on the proximate circumstellar medium around the progenitor star, to a scale of \(\sim 2\times 10^{15}\) cm, excluding the range \(\dot{M}\sim {\rm few}\times 10^{-6}-10^{-2}\) M\(_\odot\) yr\(^{-1}\) (for a wind velocity, \(v_w=115\) km s\(^{-1}\), and ejecta velocity, \(v_{\rm eje}\sim (1-2)\times 10^4\) km s\(^{-1}\)). These results are consistent with an inference of the mass loss rate based on optical spectroscopy (\(\sim 2\times 10^{-2}\) M\(_\odot\) yr\(^{-1}\) for \(v_w=115\) km s\(^{-1}\)), but are in tension with the inference from hard X-rays (\(\sim 7\times 10^{-4}\) M\(_\odot\) yr\(^{-1}\) for \(v_w=115\) km s\(^{-1}\)). This tension may be alleviated by a non-homogeneous and confined CSM, consistent with results from high-resolution optical spectroscopy.
We present late-time radio/millimeter (as well as optical/UV and X-ray) detections of the tidal disruption event (TDE) AT2018hyz, spanning \(970 - 1300\) d after optical discovery. In conjunction ...with earlier deeper limits, including at \(\approx 700\) d, our observations reveal rapidly rising emission at \(0.8-240\) GHz, steeper than \(F_\nu\propto t^5\) relative to the time of optical discovery. Such a steep rise cannot be explained in any reasonable scenario of an outflow launched at the time of disruption (e.g., off-axis jet, sudden increase in the ambient density), and instead points to a delayed launch. Our multi-frequency data allow us to directly determine the radius and energy of the radio-emitting outflow, showing that it was launched \(\approx 750\) d after optical discovery. The outflow velocity is mildly relativistic, with \(\beta\approx 0.25\) and \(\approx 0.6\) for a spherical and a \(10^\circ\) jet geometry, respectively, and the minimum kinetic energy is \(E_K\approx 5.8\times 10^{49}\) and \(\approx 6.3\times 10^{49}\) erg, respectively. This is the first definitive evidence for the production of a delayed mildly-relativistic outflow in a TDE; a comparison to the recently-published radio light curve of ASASSN-15oi suggests that the final re-brightening observed in that event (at a single frequency and time) may be due to a similar outflow with a comparable velocity and energy. Finally, we note that the energy and velocity of the delayed outflow in AT2018hyz are intermediate between those of past non-relativistic TDEs (e.g., ASASSN-14li, AT2019dsg) and the relativistic TDE Sw\,J1644+57. We suggest that such delayed outflows may be common in TDEs.
We present detailed radio observations of the tidal disruption event (TDE) ASASSN-19bt/AT2019ahk, obtained with the Australia Telescope Compact Array (ATCA), the Atacama Large ...Millimeter/submillimeter Array (ALMA), and the MeerKAT radio telescopes, spanning 40 to 1464 days after the onset of the optical flare. We find that ASASSN-19bt displays unusual radio evolution compared to other TDEs, as the peak brightness of its radio emission increases rapidly until 457 days post-optical discovery and then plateaus. Using a generalized approach to standard equipartition techniques, we estimate the energy and corresponding physical parameters for two possible emission geometries: a non-relativistic spherical outflow and a relativistic outflow observed from an arbitrary viewing angle. We find that the non-relativistic solution implies a continuous energy rise in the outflow from \(E\sim10^{46}\) erg to \(E\sim10^{49}\) erg with \(\beta \approx 0.05\), while the off-axis relativistic jet solution instead suggests \(E\approx10^{52}\) erg with \(\Gamma\sim10\) erg at late times in the maximally off-axis case. We find that neither model provides a holistic explanation for the origin and evolution of the radio emission, emphasizing the need for more complex models. ASASSN-19bt joins the population of TDEs that display unusual radio emission at late times. Conducting long-term radio observations of these TDEs, especially during the later phases, will be crucial for understanding how these types of radio emission in TDEs are produced.
Polarization measurements of gamma-ray burst (GRB) afterglows are a promising means of probing the structure, geometry, and magnetic composition of relativistic GRB jets. However, a precise treatment ...of instrumental calibration is vital for a robust physical interpretation of polarization data, requiring tests of and validations against potential instrumental systematics. We illustrate this with ALMA Band 3 (97.5 GHz) observations of GRB 171205A taken \(\approx5.19\) days after the burst, where a detection of linear polarization was recently claimed. We describe a series of tests for evaluating the stability of polarization measurements with ALMA. Using these tests to re-analyze and evaluate the archival ALMA data, we uncover systematics in the polarization calibration at the \(\approx0.09\%\) level. We derive a 3\(\sigma\) upper limit on the linearly polarized intensity of \(P<97.2~\mu\)Jy, corresponding to an upper limit on the linear fractional polarization of \(\Pi_{\rm L}<0.30\%\), in contrast to the previously claimed detection. Our upper limit improves upon existing constraints on the intrinsic polarization of GRB radio afterglows by a factor of 3. We discuss this measurement in the context of constraints on the jet magnetic field geometry. We present a compilation of polarization observations of GRB radio afterglows, and demonstrate that a significant improvement in sensitivity is desirable for eventually detecting signals polarized at the \(\approx0.1\%\) level from typical radio afterglows.
Identifying the astrophysical sites of the $r$-process, one of the primary
mechanisms by which heavy elements are formed, is a key goal of modern
astrophysics. The discovery of the brightest ...gamma-ray burst of all time, GRB
221009A, at a relatively nearby redshift, presented the first opportunity to
spectroscopically test the idea that $r$-process elements are produced
following the collapse of rapidly rotating massive stars. Here we present
spectroscopic and photometric $\textit{James Webb Space Telescope}$ (JWST)
observations of GRB 221009A obtained $+168$ and $+170$ rest-frame days after
the initial gamma-ray trigger, and demonstrate they are well-described by a
supernova (SN) and power-law afterglow, with no evidence for an additional
component from $r$-process emission, and that the SN component strongly
resembles the near-infrared spectra of previous SNe, including SN 1998bw. We
further find that the SN associated with GRB 221009A is slightly fainter than
the expected brightness of SN 1998bw at this phase, concluding that the SN is
therefore not an unusual GRB-SN. We infer a nickel mass of $\approx0.09$
M$_{\odot}$, consistent with the lack of an obvious SN detection in the
early-time data. We find that the host galaxy of GRB 221009A has a very low
metallicity of $\approx0.12$ Z$_{\odot}$ and our resolved host spectrum shows
that GRB 221009A occurred in a unique environment in its host characterized by
strong H$_2$ emission lines consistent with recent star formation, which may
hint at environmental factors being responsible for its extreme energetics.
We present the discovery of the radio afterglow of the short \(\gamma\)-ray burst (GRB) 210726A, localized to a galaxy at a photometric redshift of \(z\sim 2.4\). While radio observations commenced ...\(\lesssim 1~\)day after the burst, no radio emission was detected until \(\sim11~\)days. The radio afterglow subsequently brightened by a factor of \(\sim 3\) in the span of a week, followed by a rapid decay (a "radio flare"). We find that a forward shock afterglow model cannot self-consistently describe the multi-wavelength X-ray and radio data, and underpredicts the flux of the radio flare by a factor of \(\approx 5\). We find that the addition of substantial energy injection, which increases the isotropic kinetic energy of the burst by a factor of \(\approx 4\), or a reverse shock from a shell collision are viable solutions to match the broad-band behavior. At \(z\sim 2.4\), GRB 210726A is among the highest redshift short GRBs discovered to date as well as the most luminous in radio and X-rays. Combining and comparing all previous radio afterglow observations of short GRBs, we find that the majority of published radio searches conclude by \(\lesssim 10~\)days after the burst, potentially missing these late rising, luminous radio afterglows.
We present multiwavelength observations of the Swift short \(\gamma\)-ray burst GRB 231117A, localized to an underlying galaxy at redshift \(z = 0.257\) at a small projected offset (\(\sim 2~\)kpc). ...We uncover long-lived X-ray (Chandra) and radio/millimeter (VLA, MeerKAT, and ALMA) afterglow emission, detected to \(\sim 37~\)days and \(\sim 20~\)days (rest frame), respectively. We measure a wide jet (\(\sim 10.4^\circ\)) and relatively high circumburst density (\(\sim 0.07~{\rm cm}^{-3}\)) compared to the short GRB population. Our data cannot be easily fit with a standard forward shock model, but they are generally well fit with the incorporation of a refreshed forward shock and a reverse shock at \(< 1~\)day. We incorporate GRB 231117A into a larger sample of 132 X-ray detected events, 71 of which were radio-observed (17 cm-band detections), for a systematic study of the distributions of redshifts, jet and afterglow properties, galactocentric offsets, and local environments of events with and without detected radio afterglows. Compared to the entire short GRB population, the majority of radio-detected GRBs are at relatively low redshifts (\(z < 0.6\)) and have high circumburst densities (\(> 10^{-2}~{\rm cm}^{-3}\)), consistent with their smaller (\(< 8~\)kpc) projected galactocentric offsets. We additionally find that 70% of short GRBs with opening angle measurements were radio-detected, indicating the importance of radio afterglows in jet measurements, especially in the cases of wide (\(> 10^\circ\)) jets where observational evidence of collimation may only be detectable at radio wavelengths. Owing to improved observing strategies and the emergence of sensitive radio facilities, the number of radio-detected short GRBs has quadrupled in the past decade.
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