Advanced LIGO and Virgo detectors reported the first binary neutron star merger candidate in the third observing run, S190425z , on 25th April 2019. A weak γ-ray excess was reported nearly coincidentally by the INTEGRAL satellite which accidentally covered the entire localization region of AdvLIGO/VIRGO. Electromagnetic follow-up in longer wavelengths has not lead to the detection of any associated counterparts. Here we combine the available information from gravitational wave measurements and upper limits of fluence from IN-TEGRAL to show that the observations are completely consistent with a relativistic Gaussian structured jet and a typical short duration Gamma Ray Burst (GRB) being produced in the merger. We obtain posterior bounds on the on-axis isotropic equivalent energy of the associated GRB under different prior distributions. This study demonstrates that even limited GW and EM information could be combined to produce valuable insights about outflows from mergers. Future follow ups may help constrain the jet structure further, especially if there is an orphan afterglow detection associated with the candidate.

The HH80-81 system is one of the most powerful jets driven by a massive protostar. We present new near-infrared (NIR) line imaging observations of the HH80-81 jet in the H2 (2.122 μm) and [Fe ii] (1.644 μm) lines. These lines trace not only the jet close to the exciting source but also the knots located farther away. We have detected nine groups of knot-like structures in the jet including HH80 and HH81 spaced 0.2–0.9 pc apart. The knots in the northern arm of the jet show only [Fe ii] emission closer to the exciting source, a combination of [Fe ii] and H2 at intermediate distances, and solely H2 emission farther outwards. Toward the southern arm, all the knots exhibit both H2 and [Fe ii] emission. The nature of the shocks is inferred by combining the NIR observations with radio and X-ray observations from the literature. In the northern arm, we infer the presence of strong dissociative shocks, in the knots located close to the exciting source. The knots in the southern arm that include HH80 and HH81 are explicable as a combination of strong and weak shocks. The mass-loss rates of the knots determined from [Fe ii] luminosities are in the range ∼3.0 × 10−7–5.2 × 10−5 M ⊙ yr−1, consistent with those from massive protostars. Toward the central region, close to the driving source of the jet, we have observed various arcs in H2 emission that resemble bow shocks, and strings of H2 knots that reveal traces of multiple outflows.

Protostellar jets and outflows are pointers of star-formation and serve as important sources of momentum and energy transfer to the interstellar medium. Radio emission from ionized jets have been detected towards a number of protostellar objects. In few cases, negative spectral indices and polarized emission have also been observed suggesting the presence of synchrotron emission from relativistic electrons. In this work, we develop a numerical model that incorporates both thermal free–free and non-thermal synchrotron emission mechanisms in the jet geometry. The flux densities include contribution from an inner thermal jet, and a combination of emission from thermal and non-thermal distributions along the edges and extremities, where the jet interacts with the interstellar medium. We also include the effect of varying ionization fraction laterally across the jet. An investigation of radio emission and spectra along the jet shows the dependence of the emission process and optical depth along the line of sight. We explore the effect of various parameters on the turnover frequencies and the radio spectral indices (between 10 MHz and 300 GHz) associated with them.

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Electromagnetic observations of the first binary Neutron Star (BNS) merger detected in gravitational waves, GW170817, has established that relativistic jets can be successfully launched in BNS mergers. Typically, such jets produce emission in two phases: γ-ray prompt emission and multi-wavelength afterglow. Due to relativistic beaming and the jet’s angular structure, the detectability of both these counterparts are dependent on the angle (θv) between the observer’s line of sight and the jet axis. We compare the detectability of prompt and afterglow emission from off-axis jets, assuming standard detector thresholds such as that of FERMI GBM, Chandra, and Jansky VLA. We find that for top-hat jets, afterglow is a more likely counterpart than the prompt emission even with unfavourable afterglow parameters. For structured jets with a Gaussian profile, prompt emission is more promising than the afterglows at extreme viewing angles, under the assumption that the total energy emitted in the prompt phase equals the kinetic energy of the outflow. Assuming a Gaussian jet profile, we forecast the population of γ-ray detections and find that extreme viewing angle events like GRB170817A will be rare. In our simulated sample, the observed isotropic equivalent energy in γ-rays is moderately correlated with the viewing angle, such that a low Eiso, γ is almost always associated with a high off-axis viewing angle.