We report the discovery and monitoring of the near-infrared counterpart (AT2017gfo) of a binary neutron-star merger event detected as a gravitational wave source by Advanced Laser Interferometer ...Gravitational-wave Observatory (LIGO)/Virgo (GW170817) and as a short gamma-ray burst by Fermi Gamma-ray Burst Monitor (GBM) and Integral SPI-ACS (GRB 170817A). The evolution of the transient light is consistent with predictions for the behavior of a "kilonova/macronova" powered by the radioactive decay of massive neutron-rich nuclides created via r-process nucleosynthesis in the neutron-star ejecta. In particular, evidence for this scenario is found from broad features seen in Hubble Space Telescope infrared spectroscopy, similar to those predicted for lanthanide-dominated ejecta, and the much slower evolution in the near-infrared K s -band compared to the optical. This indicates that the late-time light is dominated by high-opacity lanthanide-rich ejecta, suggesting nucleosynthesis to the third r-process peak (atomic masses A 195 ). This discovery confirms that neutron-star mergers produce kilo-/macronovae and that they are at least a major-if not the dominant-site of rapid neutron capture nucleosynthesis in the universe.
Using the Very Long Baseline Array, we have measured a trigonometric parallax for the microquasar GRS 1915+105, which contains a black hole and a K-giant companion. This yields a direct distance ...estimate of 8.6 super(+2.0) sub(-1.6) kpc and a revised estimate for the mass of the black hole of 12.4 super(+2.0) sub(-1.8) M sub(middot in circle). GRS 1915+105 is at about the same distance as some H II regions and water masers associated with high-mass star formation in the Sagittarius spiral arm of the Galaxy. The absolute proper motion of GRS 1915+105 is -3.19 + or - 0.03 mas yr super(-1) and -6.24 + or - 0.05 mas yr super(-1) toward the east and north, respectively, which corresponds to a modest peculiar speed of 22 + or - 24 km s super(-1) at the parallax distance, suggesting that the binary did not receive a large velocity kick when the black hole formed. On one observational epoch, GRS 1915+105 displayed superluminal motion along the direction of its approaching jet. Considering previous observations of jet motions, the jet in GRS 1915+105 can be modeled with a jet inclination to the line of sight of 60degrees + or - 5degrees and a variable flow speed between 0.65c and 0.81c, which possibly indicates deceleration of the jet at distances from the black hole gap2000 AU. Finally, using our measurements of distance and estimates of black hole mass and inclination, we provisionally confirm our earlier result that the black hole is spinning very rapidly.
We present observations of the optical afterglow of GRB 170817A, made by the Hubble Space Telescope, between 2018 February and August, up to one year after the neutron star merger GW170817. The ...afterglow shows a rapid decline beyond 170 days, and confirms the jet origin for the observed outflow, in contrast to more slowly declining expectations for "failed-jet" scenarios. We show here that the broadband (radio, optical, X-ray) afterglow is consistent with a structured outflow where an ultra-relativistic jet, with a Lorentz factor of Γ 100, forms a narrow core (∼5°) and is surrounded by a wider angular component that extends to ∼15°, which is itself relativistic (Γ 5). For a two-component model of this structure, the late-time optical decline, where F ∝ t− , is = 2.20 0.18, and for a Gaussian structure the decline is = 2.45 0.23. We find the Gaussian model to be consistent with both the early ∼10 days and late 290 days data. The agreement of the optical light curve with the evolution of the broadband spectral energy distribution, and its continued decline, indicates that the optical flux is arising primarily from the afterglow and not any underlying host system. This provides the deepest limits on any host stellar cluster with a luminosity 4000 L (MF606W −4.3).
We present Hubble Space Telescope (HST) and Chandra imaging, combined with Very Large Telescope MUSE integral field spectroscopy of the counterpart and host galaxy of the first binary neutron star ...merger detected via gravitational-wave emission by LIGO and Virgo, GW170817. The host galaxy, NGC 4993, is an S0 galaxy at z = 0.009783. There is evidence for large, face-on spiral shells in continuum imaging, and edge-on spiral features visible in nebular emission lines. This suggests that NGC 4993 has undergone a relatively recent ( 1 Gyr) "dry" merger. This merger may provide the fuel for a weak active nucleus seen in Chandra imaging. At the location of the counterpart, HST imaging implies there is no globular or young stellar cluster, with a limit of a few thousand solar masses for any young system. The population in the vicinity is predominantly old with 1% of any light arising from a population with ages < 500 Myr . Both the host galaxy properties and those of the transient location are consistent with the distributions seen for short-duration gamma-ray bursts, although the source position lies well within the effective radius ( r e ∼ 3 kpc), providing an re-normalized offset that is closer than ∼ 90 % of short GRBs. For the long delay time implied by the stellar population, this suggests that the kick velocity was significantly less than the galaxy escape velocity. We do not see any narrow host galaxy interstellar medium features within the counterpart spectrum, implying low extinction, and that the binary may lie in front of the bulk of the host galaxy.
AM CVn binaries are hydrogen deficient compact binaries with an orbital period in the 5–65 min range and are predicted to be strong sources of persistent gravitational wave radiation. Using Gaia Data ...Release 2, we present the parallaxes and proper motions of 41 out of the 56 known systems. Compared to the parallax determined using the HST Fine Guidance Sensor we find that the archetype star, AM CVn, is significantly closer than previously thought. This resolves the high luminosity and mass accretion rate which models had difficulty in explaining. Using Pan-STARRS1 data we determine the absolute magnitude of the AM CVn stars. There is some evidence that donor stars have a higher mass and radius than expected for white dwarfs or that the donors are not white dwarfs. Using the distances to the known AM CVn stars we find strong evidence that a large population of AM CVn stars has yet to be discovered. As this value sets the background to the gravitational wave signal of LISA this is of wide interest. We determine the mass transfer rate for 15 AM CVn stars and find that the majority has a rate significantly greater than expected from standard models. This is further evidence that the donor star has a greater size than expected.
We report on a detailed optical spectroscopic follow-up of the black hole (BH) transient MAXI J1820+070 (ASASSN-18ey). The observations cover the main part of the X-ray binary outburst, when the ...source alternated between hard and soft states following the classical pattern widely seen in other systems. We focus the analysis on the He i emission lines at 5876 and 6678 , as well as on H . We detect clear accretion disk wind features (P-Cyg profiles and broad emission line wings) in the hard state, both during outburst rise and decay. These are not witnessed during the several months long soft state. However, our data suggest that the visibility of the outflow might be significantly affected by the ionization state of the accretion disk. The terminal velocity of the wind is above ∼1200 km s−1, which is similar to outflow velocities derived from (hard-state) optical winds and (soft-state) X-ray winds in other systems. The wind signatures, in particular the P-Cyg profiles, are very shallow, and their detection has only been possible thanks to a combination of source brightness and intense monitoring at very high signal-to-noise. This study indicates that cold, optical winds are most likely a common feature of BH accretion, and therefore, that wind-like outflows are a general mechanism of mass and angular momentum removal operating throughout the entire X-ray binary outburst.
ABSTRACT
Neutron stars (NSs) in low-mass X-ray binaries are considered promising candidate sources of continuous gravitational waves. These NSs are typically rotating many hundreds of times a second. ...The process of accretion can potentially generate and support non-axisymmetric distortions to the compact object, resulting in persistent emission of gravitational waves. We present a study of existing optical spectroscopic data for Sco X-1, a prime target for continuous gravitational-wave searches, with the aim of providing revised constraints on key orbital parameters required for a directed search with advanced Laser Interferometer Gravitational-Wave Observatory data. From a circular orbit fit to an improved radial velocity curve of the Bowen emission components, we derived an updated orbital period and ephemeris. Centre of symmetry measurements from the Bowen Doppler tomogram yield a centre of the disc component of 90 km s−1, which we interpret as a revised upper limit to the projected orbital velocity of the NS K1. By implementing Monte Carlo binary parameter calculations, and imposing new limits on K1 and the rotational broadening, we obtained a complete set of dynamical system parameter constraints including a new range for K1 of 40–90 km s−1. Finally, we discussed the implications of the updated orbital parameters for future continuous-wave searches.
We present optical, X-ray and radio observations of the black hole transient (BHT) XTE J1752−223 towards and in quiescence. Optical photometry shows that the quiescent magnitude of XTE J1752−223 is ...fainter than 24.4 mag in the i′ band. A comparison with measurements of the source during its 2009-2010 outburst shows that the outburst amplitude is more than 8 mag in the i′ band. Known X-ray properties of the source combined with the faintness of the quiescence optical counterpart and the large outburst optical amplitude point towards a short orbital-period system (P
orb≲ 6.8 h) with an M type (or later) mass donor, at a distance of 3.5 ≲d≲ 8 kpc. Simultaneous X-ray and radio data were collected with Chandra and the Expanded Very Large Array (EVLA), allowing constraints to be placed on the quiescent X-ray and radio flux of XTE J1752−223. Furthermore, using data covering the final stage of the outburst decay, we investigated the low-luminosity end of the X-ray-radio correlation for this source and compared it with other BHTs. We found that XTE J1752−223 adds to the number of outliers with respect to the 'standard' X-ray-radio luminosity relation. Furthermore, XTE J1752−223 is the second source, after the BHT H1743−322, that shows a transition from the region of the outliers towards the 'standard' correlation at low luminosity. Finally, we report on a faint, variable X-ray source we discovered with Chandra at an angular distance of ∼2.9 arcsec to XTE J1752−223 and at a position angle consistent with that of the radio jets previously observed from the BHT. We discuss the possibility that we detected X-ray emission associated with a jet from XTE J1752−223.
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