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Register a new userDetectability of compact binary merger macronovae
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We study the optical and near-infrared luminosities and detectability of radioactively powered electromagnetic transients (macronovae) occuring in the aftermath of binary neutron star and neutron star black hole mergers. We explore the transients that result from the dynamic ejecta and those from different types of wind outflows. Based on full nuclear network simulations we calculate the resulting light curves in different wavelength bands. We scrutinize the robustness of the results by comparing a) two different nuclear reaction networks and b) two macronova models. We explore in particular how sensitive the results are to the production of alpha-decaying trans-lead nuclei. We compare two frequently used mass models: the Finite-Range Droplet Model (FRDM) and the nuclear mass model of Duflo and Zuker (DZ31). We find that the abundance of alpha-decaying trans-lead nuclei has a significant impact on the observability of the resulting macronovae. For example, the DZ31 model yields considerably larger abundances resulting in larger heating rates and thermalization efficiencies and therefore predicts substantially brighter macronova transients. We find that the dynamic ejecta from NSNS models can reach peak K-band magnitudes in excess of -15 while those from NSBH cases can reach beyond -16. Similar values can be reached by some of our wind models. Several of our models (both wind and dynamic ejecta) yield properties that are similar to the transient that was observed in the aftermath of the short GRB 130603B. We further explore the expected macronova detection frequencies for current and future instruments such as VISTA, ZTF and LSST.
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Using neural networks, we integrate the ability to account for Doppler smearing due to a pulsars orbital motion with the pulsar population synthesis package psrpoppy to develop accurate modeling of the observed binary pulsar population. As a first application, we show that binary neutron star systems where the two components have highly unequal mass are, on average, easier to detect than systems which are symmetric in mass. We then investigate the population of ultra-compact ($1.5 , {rm min} leq P_{rm b} leq 15,rm min$) neutron star--white dwarf (NS--WD) and double neutron star (DNS) systems which are promising sources for the Laser Interferometer Space Antenna gravitational-wave detector. Given the non-detection of these systems in radio surveys thus far, we estimate a 95% confidence upper limit of $sim$1450 and $sim$1100 ultra-compact NS--WD and DNS systems in the Milky Way that are beaming towards the Earth respectively. We also show that using survey integration times in the range 20~s to 200~s with time-domain resampling will maximize the signal-to-noise ratio as well as the probability of detection of these ultra-compact binary systems. Among all the large scale radio pulsar surveys, those that are currently being carried out at the Arecibo radio telescope have $sim$50--80% chance of detecting at least one of these systems using current integration integration times and $sim$80--95% using optimal integration times in the next several years.
Electromagnetic (EM) follow-up of gravitational wave (GW) candidates is important for verifying their astrophysical nature and studying their physical properties. While the next generation of GW detectors will have improved sensitivities to make the first detection of GW events, their ability to localize these events will remain poor during the early days of their operation. This makes EM follow-up challenging for most telescopes. Many new low frequency radio instruments have come online recently or will come online over the next few years, and their wide fields of view allow them to cover large areas of the sky in a short amount of time. This paper studies comprehensively the detectability of radio afterglows from compact binary coalescence (CBC), a predicted GW source and the most promising progenitor of short gamma-ray bursts. We explore the properties of simulated afterglow lightcurves from the forward shock for a range of source and observer parameters, then we use these lightcurves to estimate the expected rates of detection for different radio instruments and survey methods. Detecting radio afterglows and constraining their properties and rates are feasible with the current and upcoming widefield radio instruments. As a result, widefield radio instruments will play an important role in the EM follow-up of GW events.
VLBI and JVLA observations revealed that GW170817 involved a narrow jet ($ theta_j approx 4^circ $) that dominated the afterglow peak at our viewing angle, $ theta_{rm obs} approx 20^circ $. This implies that at the time of the afterglow peak, the observed signal behaved like an afterglow of a top-hat jet seen at $ theta_{rm obs} gg theta_j $, and it can be modeled by analytic expressions that describe such jets. We use a set of numerical simulations to calibrate these analytic relations and obtain generic equations for the peak time and flux of such an afterglow as seen from various observing angles. Using the calibrated equations and the estimated parameters of GW170817, we estimate the detectability of afterglows from future double neutron star mergers during the Advanced LIGO/Virgo observation run O3. GW170817 took place at a relatively low-density environment. Afterglows of similar events will be detectable only at small viewing angles, $ theta_{rm obs} lesssim 20^circ $, and only $sim 20% $ of the GW detections of these events will be accompanied by a detectable afterglow. At higher densities, more typical to sGRB sites, up to $ 70% $ of the GW detections are expected to be followed by a detectable afterglow, typically at $ theta_{rm obs} sim 30^circ $. We also provide the latest time one should expect an afterglow detection. We find that for typical parameters, if the jet emission had not been detected within about a year after the merger, it is unlikely to be ever detected.
A macronova (or kilonova) was observed as an infrared excess several days after short gamma-ray burst, GRB 130603B. Although the $r$-process radioactivity is widely discussed as an energy source, it requires huge mass of ejecta from a neutron star (NS) binary merger. We propose a new model that the X-ray excess gives rise to the simultaneously observed infrared excess via thermal re-emission and explore what constraints this would place on the mass and velocity of the ejecta. This X-ray-powered model explains both the X-ray and infrared excesses with a single energy source by the central engine like a black hole, and allows for broader parameter region, in particular smaller ejecta mass $sim10^{-3}-10^{-2}M_{odot}$ with iron mixed as suggested by general relativistic simulations for typical NS-NS mergers, than the previous models. We also discuss the other macronova candidates in GRB 060614 and GRB 080503, and implications for the search of electromagnetic counterparts to gravitational waves.
We report on our observing campaign of the compact binary merger GW190814, detected by the Advanced LIGO and Advanced Virgo detectors on August 14th, 2019. This signal has the best localisation of any observed gravitational wave (GW) source, with a 90% probability area of 18.5 deg$^2$, and an estimated distance of ~ 240 Mpc. We obtained wide-field observations with the Deca-Degree Optical Transient Imager (DDOTI) covering 88% of the probability area down to a limiting magnitude of $w$ = 19.9 AB. Nearby galaxies within the high probability region were targeted with the Lowell Discovery Telescope (LDT), whereas promising candidate counterparts were characterized through multi-colour photometry with the Reionization and Transients InfraRed (RATIR) and spectroscopy with the Gran Telescopio de Canarias (GTC). We use our optical and near-infrared limits in conjunction with the upper limits obtained by the community to constrain the possible electromagnetic counterparts associated with the merger. A gamma-ray burst seen along its jets axis is disfavoured by the multi-wavelength dataset, whereas the presence of a burst seen at larger viewing angles is not well constrained. Although our observations are not sensitive to a kilonova similar to AT2017gfo, we can rule out high-mass (> 0.1 M$_{odot}$) fast-moving (mean velocity >= 0.3c) wind ejecta for a possible kilonova associated with this merger.
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