We report our observation of the short gamma-ray burst (GRB) GRB 170817A, associated to the binary neutron star merger gravitational wave (GW) event GW 170817, performed in the X-ray band with ...XMM-Newton 135 d after the event (on 29 December, 2017). We find evidence for a flattening of the X-ray light curve with respect to the previously observed brightening. This is also supported by a nearly simultaneous optical Hubble Space Telescope observation and successive X-ray Chandra and low-frequency radio observations recently reported in the literature. Since the optical-to-X-ray spectral slope did not change with respect to previous observations, we exclude that the change in the temporal evolution of the light curve is due to the passage of the cooling frequency: its origin must be geometric or dynamical. We interpret all the existing afterglow data with two models: i) a structured jet and ii) a jet-less isotropic fireball with some stratification in its radial velocity structure. Both models fit the data and predict that the radio flux must decrease simultaneously with the optical and X-ray emission, making it difficult to distinguish between them at the present stage. Polarimetric measurements and the rate of short GRB-GW associations in future LIGO/Virgo runs will be key to disentangle these two geometrically different scenarios.
Abstract
The joint detection of gravitational waves (GWs) and electromagnetic radiation from the binary neutron star (BNS) merger GW170817 has provided unprecedented insight into a wide range of ...physical processes: heavy element synthesis via the
r
-process; the production of relativistic ejecta; the equation of state of neutron stars and the nature of the merger remnant; the binary coalescence timescale; and a measurement of the Hubble constant via the “standard siren” technique. In detail, all of these results depend on the distance to the host galaxy of the merger event, NGC 4993. In this Letter we measure the surface brightness fluctuation (SBF) distance to NGC 4993 in the F110W and F160W passbands of the Wide Field Camera 3 Infrared Channel (WFC3/IR) on the
Hubble Space Telescope
(
HST
). For the preferred F110W passband we derive a distance modulus of
(
m
−
M
)
=
33.05
±
0.08
±
0.10
mag, or a linear distance
d
= 40.7 ± 1.4 ± 1.9 Mpc (random and systematic errors, respectively); a virtually identical result is obtained from the F160W data. This is the most precise distance to NGC 4993 available to date. Combining our distance measurement with the corrected recession velocity of NGC 4993 implies a Hubble constant
H
0
= 71.9 ± 7.1 km s
−1
Mpc
−1
. A comparison of our result to the GW-inferred value of
H
0
indicates a binary orbital inclination of
i
≳ 137°. The SBF technique can be applied to early-type host galaxies of BNS mergers to ∼100 Mpc with
HST
and possibly as far as ∼300 Mpc with the
James Webb Space Telescope
, thereby helping to break the inherent distance-inclination degeneracy of the GW data at distances where many future BNS mergers are likely to be detected.
ABSTRACT
Multimessenger astronomy received a great boost following the discovery of kilonova (KN) AT2017gfo, the optical counterpart of the gravitational wave source GW170817 associated with the ...short gamma-ray burst GRB 170817A. AT2017gfo was the first KN that could be extensively monitored in time using both photometry and spectroscopy. Previously, only few candidates have been observed against the glare of short GRB afterglows. In this work, we aim to search the fingerprints of AT2017gfo-like KN emissions in the optical/NIR light curves of 39 short GRBs with known redshift. For the first time, our results allow us to study separately the range of luminosity of the blue and red components of AT2017gfo-like kilonovae in short GRBs. In particular, the red component is similar in luminosity to AT2017gfo, while the blue KN can be more than 10 times brighter. Finally, we exclude a KN as luminous as AT2017gfo in GRBs 050509B and 061201.
Context.
Coalescences of binary white dwarfs represent a copious source of information for gravitational wave interferometers operating in the decihertz band. Moreover, according to the double ...degenerate scenario, they have been suggested to be possible progenitors of supernovae (SNe) Type Ia events.
Aims.
In this paper we discuss the detectability of gravitational waves emitted by the inspiral of double white dwarfs. We focus on the constraints that can be derived on the source’s luminosity distance, and on other binary’s parameters, such as the angular momentum orientation.
Methods.
We explore the possibility of coincident detections of gravitational and electromagnetic signals; the latter comes from the observation of the supernova counterpart. Confirmation of the double degenerate scenario would allow one to use distances inferred in the gravitational wave channel to consistently calibrate SNe as standard candles.
Results.
We find that decihertz gravitational wave interferometers can measure the luminosity distance with relative accuracy better than 1% for binaries at 100 Mpc. We show how multimessenger observations can put strong constraints on the Hubble constant, which are tighter than current bounds at low redshift, and how they can potentially shed new light on the differences with early-universe measurements.
The binary neutron star merger event GW170817 was detected through both electromagnetic radiation and gravitational waves. Its afterglow emission may have been produced by either a narrow ...relativistic jet or an isotropic outflow. High-spatial-resolution measurements of the source size and displacement can discriminate between these scenarios. We present very-long-baseline interferometry observations, performed 207.4 days after the merger by using a global network of 32 radio telescopes. The apparent source size is constrained to be smaller than 2.5 milli-arc seconds at the 90% confidence level. This excludes the isotropic outflow scenario, which would have produced a larger apparent size, indicating that GW170817 produced a structured relativistic jet. Our rate calculations show that at least 10% of neutron star mergers produce such a jet.
ABSTRACT
Short gamma‐ray bursts (SGRBs) observed by Swift potentially reveal the first insight into cataclysmic compact object mergers. To ultimately acquire a fundamental understanding of these ...events requires pan‐spectral observations and knowledge of their spatial distribution to differentiate between proposed progenitor populations. Up to 2012 April, there are only some 30 per cent of SGRBs with reasonably firm redshifts, and this sample is highly biased by the limited sensitivity of Swift to detect SGRBs. We account for the dominant biases to calculate a realistic SGRB rate density out to z ≈ 0.5 using the Swift sample of peak fluxes, redshifts and those SGRBs with a beaming angle constraint from X‐ray/optical observations. We find an SGRB lower rate density of 8−3+5 Gpc −3 yr −1 (assuming isotropic emission) and a beaming corrected upper limit of 1100−470+700 Gpc −3 yr −1. Assuming a significant fraction of binary neutron star mergers produce SGRBs, we calculate lower and upper detection rate limits of (1–180) yr−1 by an Advanced LIGO (aLIGO) and Virgo coincidence search. Our detection rate is similar to the lower and realistic rates inferred from extrapolations using Galactic pulsar observations and population synthesis.
The X-ray emission of
γ
-ray burst (GRBs) is often characterized by an initial steep decay followed by a nearly constant emission phase (so-called “plateau”) which can extend up to thousands of ...seconds. While the steep decay is usually interpreted as the tail of the prompt
γ
-ray flash, the long-lasting plateau is commonly associated to the emission from the external shock sustained by energy injection from a long-lasting central engine. A recent study proposed an alternative interpretation, ascribing both the steep decay and the plateau to high-latitude emission (HLE) from a “structured jet” whose energy and bulk Lorentz factor depend on the angular distance from the jet symmetry axis. In this work we expand on this idea and explore more realistic conditions: (a) the finite duration of the prompt emission, (b) the angular dependence of the optical depth, and (c) the dependence of the light curve on the observer viewing angle. We find that, when viewed highly off-axis, the structured jet HLE light curve is smoothly decaying with no clear distinction between the steep and flat phases, as opposed to the on-axis case. For a realistic choice of physical parameters, the effects of a latitude-dependent Thomson opacity and finite duration of the emission have a marginal effect on the overall light-curve evolution. We discuss the possible HLE of GW170817, showing that the emission would have faded away long before the first
Swift
-XRT observations. Finally, we discuss the prospects for the detection of HLE from off-axis GRBs by present and future wide-field X-ray telescopes and X-ray surveys, such as eROSITA and the mission concept THESEUS.
GRB 190114C is the first gamma-ray burst detected at very high energies (VHE, i.e., > 300 GeV) by the MAGIC Cherenkov telescope. The analysis of the emission detected by the Fermi satellite at lower ...energies, in the 10 keV–100 GeV energy range, up to ∼50 s (i.e., before the MAGIC detection) can hold valuable information. We analyze the spectral evolution of the emission of GRB 190114C as detected by the Fermi Gamma-Ray Burst Monitor (GBM) in the 10 keV–40 MeV energy range up to ∼60 s. The first 4 s of the burst feature a typical prompt emission spectrum, which can be fit by a smoothly broken power-law function with typical parameters. Starting on ∼4 s post-trigger, we find an additional nonthermal component that can be fit by a power law. This component rises and decays quickly. The 10 keV–40 MeV flux of the power-law component peaks at ∼6 s; it reaches a value of 1.7 × 10−5 erg cm−2 s−1. The time of the peak coincides with the emission peak detected by the Large Area Telescope (LAT) on board Fermi. The power-law spectral slope that we find in the GBM data is remarkably similar to that of the LAT spectrum, and the GBM+LAT spectral energy distribution seems to be consistent with a single component. This suggests that the LAT emission and the power-law component that we find in the GBM data belong to the same emission component, which we interpret as due to the afterglow of the burst. The onset time allows us to estimate that the initial jet bulk Lorentz factor Γ0 is about 500, depending on the assumed circum-burst density.
We derive the luminosity function φ(L) and redshift distribution Ψ(z) of short gamma-ray bursts (SGRBs) using all the available observer-frame constraints (i.e. peak flux, fluence, peak energy and ...duration distributions) of the large population of Fermi SGRBs and the rest-frame properties of a complete sample of SGRBs detected by Swift. We show that a steep φ(L) ∝ L− α with α ≥ 2.0 is excluded if the full set of constraints is considered. We implement a Markov chain Monte Carlo method to derive the φ(L) and Ψ(z) functions assuming intrinsic Ep−Liso and Ep−Eiso correlations to hold or, alternatively, that the distributions of intrinsic peak energy, luminosity, and duration are independent. To make our results independent from assumptions on the progenitor (NS−NS binary mergers or other channels) and from uncertainties on the star formation history, we assume a parametric form for the redshift distribution of the population of SGRBs. We find that a relatively flat luminosity function with slope ~0.5 below a characteristic break luminosity ~3 × 1052 erg s-1 and a redshift distribution of SGRBs peaking at z ~ 1.5−2 satisfy all our constraints. These results also hold if no Ep−Liso and Ep−Eiso correlations are assumed and they do not depend on the choice of the minimum luminosity of the SGRB population. We estimate, within ~200 Mpc (i.e. the design aLIGO range for the detection of gravitational waves produced by NS−NS merger events), that there should be 0.007−0.03 SGRBs yr-1 detectable as γ-ray events. Assuming current estimates of NS−NS merger rates and that all NS−NS mergers lead to a SGRB event, we derive a conservative estimate of the average opening angle of SGRBs ⟨ θjet ⟩ ~ 3°−6°. The luminosity function implies a prompt emission average luminosity ⟨L⟩ ~ 1.5 × 1052 erg s-1, higher by nearly two orders of magnitude than previous findings in the literature, which greatly enhances the chance of observing SGRB “orphan” afterglows. Effort should go in the direction of finding and identifying such orphan afterglows as counterparts of GW events.
THESEUS, one of the two space mission concepts being studied by ESA as candidates for next M5 mission within its Comsic Vision programme, aims at fully exploiting Gamma-Ray Bursts (GRB) to solve key ...questions about the early Universe, as well as becoming a cornerstone of multi-messenger and time-domain astrophysics. By investigating the first billion years of the Universe through high-redshift GRBs, THESEUS will shed light on the main open issues in modern cosmology, such as the population of primordial low mass and luminosity galaxies, sources and evolution of cosmic re-ionization, SFR and metallicity evolution up to the “cosmic dawn” and across Pop-III stars. At the same time, the mission will provide a substantial advancement of multi-messenger and time-domain astrophysics by enabling the identification, accurate localisation and study of electromagnetic counterparts to sources of gravitational waves and neutrinos, which will be routinely detected in the late ‘20s and early ‘30s by the second and third generation Gravitational Wave (GW) interferometers and future neutrino detectors, as well as of all kinds of GRBs and most classes of other X/gamma-ray transient sources. Under all these respects, THESEUS will provide great synergies with future large observing facilities in the multi-messenger domain. A Guest Observer programme, comprising Target of Opportunity (ToO) observations, will expand the science return of the mission, to include, e.g., solar system minor bodies, exoplanets, and AGN.
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