Late-time Observations Research Articles

Hubble parameter estimation via dark sirens with the LISA-Taiji network.

The Hubble parameter is one of the central parameters in modern cosmology, and describes the present expansion rate of the universe. The values of the parameter inferred from late-time observations are systematically higher than those inferred from early-time measurements by about n}{}10%. To reach a robust conclusion, independent probes with accuracy at percent levels are crucial. Gravitational waves from compact binary coalescence events can be formulated into the standard siren approach to provide an independent Hubble parameter measurement. The future space-borne gravitational wave observatory network, such as the LISA-Taiji network, will be able to measure the gravitational wave signals in the millihertz bands with unprecedented accuracy. By including several statistical and instrumental noises, we show that, within a five-year operation time, the LISA-Taiji network is able to constrain the Hubble parameter within n}{}1% accuracy, and possibly beats the scatters down to n}{}0.5% or even better.

Observational constraints on complex quintessence with attractive self-interaction

ABSTRACT In this paper, we consider that dark energy could be described solely by a complex scalar field with a Bose–Einstein condensate-like potential (denoted as CSFDE), that is, with a self-interaction and a mass term. In particular, we analyse a solution that in a fast oscillation regime at late times behaves as a cosmological constant. Our proposal adequately describes the standard homogeneous and flat Fridman dynamics. Furthermore, in this quintessence–complex scalar field scenario, it is possible to mimic the dynamics related to dark energy. However, when the precision cosmological tests are implemented in this landscape, the generic equation of state derived for this model in a restricted regime of ai (which corresponds to the scale factor at which the scalar field turns on) cannot be constrained by late-time current observations, since the analysis constraints solely the scalar field parameters within values ruled out by the theoretical model. This result is a clear hint to consider future CSFDE models with, for instance, two scalar fields in order to study the early-time dynamics of the Universe.

On the time evolution of cosmological correlators

Developing our understanding of how correlations evolve during inflation is crucial if we are to extract information about the early Universe from our late-time observables. To that end, we revisit the time evolution of scalar field correlators on de Sitter spacetime in the Schrödinger picture. By direct manipulation of the Schrödinger equation, we write down simple “equations of motion” for the coefficients which determine the wavefunction. Rather than specify a particular interaction Hamiltonian, we assume only very basic properties (unitarity, de Sitter invariance and locality) to derive general consequences for the wavefunction’s evolution. In particular, we identify a number of “constants of motion” — properties of the initial state which are conserved by any unitary dynamics — and show how this can be used to partially fix the cubic and quartic wavefunction coefficients at weak coupling. We further constrain the time evolution by deriving constraints from the de Sitter isometries and show that these reduce to the familiar conformal Ward identities at late times. Finally, we show how the evolution of a state from the conformal boundary into the bulk can be described via a number of “transfer functions” which are analytic outside the horizon for any local interaction. These objects exhibit divergences for particular values of the scalar mass, and we show how such divergences can be removed by a renormalisation of the boundary wavefunction — this is equivalent to performing a “Boundary Operator Expansion” which expresses the bulk operators in terms of regulated boundary operators. Altogether, this improved understanding of the wavefunction in the bulk of de Sitter complements recent advances from a purely boundary perspective, and reveals new structure in cosmological correlators.

Search for Radio Remnants of Nearby Off-axis Gamma-Ray Bursts in a Sample of Swift/BAT Events

Abstract The multi-messenger discovery of gravitational waves (GWs) and light from the binary neutron star (NS) merger GW170817, associated with gamma-ray burst (GRB) 170817A and kilonova AT2017gfo, has marked the start of a new era in astrophysics. GW170817 has confirmed that binary NS mergers are progenitors of at least some short GRBs. The peculiar properties of the GRB 170817A radio afterglow, characterized by a delayed onset related to the off-axis geometry, have also demonstrated how some nearby short GRBs may not be identified as such with standard short-timescale electromagnetic follow-up observations. Building upon this new information, we performed late-time radio observations of a sample of four short GRBs with unknown redshift and no previously detected afterglow in the Swift/BAT sample in order to identify nearby ( Mpc) off-axis GRB candidates via their potential late-time radio signatures. We find a previously uncatalogued radio source within the error region of GRB 130626 with a flux density consistent with an NS radio flare at a distance of ∼100 Mpc. An origin related to a persistent radio source unrelated to the GRB cannot be excluded nor confirmed given the high chance of false positives in error regions as large as those considered here, and the limited time baseline of our observations. Further radio (and X-ray) follow-up observations are needed to better understand the origin of this source.

Shedding light on dark matter-dark energy interactions

The emergence of an increasingly strong tension between the Hubble rate inferred from early- and late-time observations has reinvigorated interest in nonstandard scenarios, with the aim of reconciling these measurements. One such model involves interactions between Dark Matter and Dark Energy. Here we consider a specific form of the coupling between these two fluids proportional to the Dark Energy energy density, which has been studied extensively in the literature and claimed to substantially alleviate the Hubble tension. We complement the work already discussed in several previous analyses and show that, once all relevant cosmological probes are included simultaneously, the value of the Hubble parameter in this model is $H_0=69.82_{-0.76}^{+0.63}$ km/(s Mpc), which reduces the Hubble tension to $2.5\sigma$. Furthermore, we also perform a statistical model comparison, finding a $\Delta\chi^2$ of $-2.15$ (corresponding to a significance of 1.5$\sigma$) with the inclusion of one additional free parameter, showing no clear preference for this model with respect to $\Lambda$CDM, which is further confirmed with an analysis of the Bayes ratio.

Caltech–NRAO Stripe 82 Survey (CNSS). III. The First Radio-discovered Tidal Disruption Event, CNSS J0019+00

Abstract We present the discovery of a nuclear transient with the Caltech–NRAO Stripe 82 Survey (CNSS), a dedicated radio transient survey carried out with the Karl G. Jansky Very Large Array (VLA). This transient, CNSS J001947.3+003527, exhibited a turn-on over a timescale of ≲1 yr, increasing in flux density at 3 GHz from <0.14 mJy in 2014 February to 4.4 ± 0.1 mJy in 2015 March, reaching a peak luminosity of around 2015 October. The association of CNSS J0019+00 with the nucleus (Gaia and our very-long baseline interferometry positions are consistent to within 1 pc) of a nearby S0 Seyfert galaxy at 77 Mpc, together with the radio spectral evolution, implies that this transient is most likely a tidal disruption event (TDE). Our equipartition analysis indicates the presence of a ∼15,000 km s−1 outflow, having energy ∼1049 erg. We derive the radial density profile for the circumnuclear material in the host galaxy to be proportional to R −2.5. All of these properties suggest resemblance with radio-detected thermal TDEs like ASASSN-14li and XMMSL1 J0740-85. No significant X-ray or optical emission is detected from CNSS J0019+00, although this may simply be due to the thermal emission being weak during our late-time follow-up observations. From the CNSS survey we have obtained the first unbiased measurement of the rate of radio TDEs, R(>500μJy) of about 2 × 10−3 deg−2, or equivalently a volumetric rate of about 10 Gpc−3 yr−1. This rate implies that all-sky radio surveys such as the VLA Sky Survey and those planned with ASKAP, will find many tens of radio TDEs over the next few years.

Cosmic dissonance: are new physics or systematics behind a short sound horizon?

Context. Persistent tension between low-redshift observations and the cosmic microwave background radiation (CMB), in terms of two fundamental distance scales set by the sound horizon rd and the Hubble constant H0, suggests new physics beyond the Standard Model, departures from concordance cosmology, or residual systematics. Aims. The role of different probe combinations must be assessed, as well as of different physical models that can alter the expansion history of the Universe and the inferred cosmological parameters. Methods. We examined recently updated distance calibrations from Cepheids, gravitational lensing time-delay observations, and the tip of the red giant branch. Calibrating the baryon acoustic oscillations and type Ia supernovae with combinations of the distance indicators, we obtained a joint and self-consistent measurement of H0 and rd at low redshift, independent of cosmological models and CMB inference. In an attempt to alleviate the tension between late-time and CMB-based measurements, we considered four extensions of the standard ΛCDM model. Results. The sound horizon from our different measurements is rd = (137 ± 3stat. ± 2syst.) Mpc based on absolute distance calibration from gravitational lensing and the cosmic distance ladder. Depending on the adopted distance indicators, the combined tension in H0 and rd ranges between 2.3 and 5.1 σ, and it is independent of changes to the low-redshift expansion history. We find that modifications of ΛCDM that change the physics after recombination fail to provide a solution to the problem, for the reason that they only resolve the tension in H0, while the tension in rd remains unchanged. Pre-recombination extensions (with early dark energy or the effective number of neutrinos Neff = 3.24 ± 0.16) are allowed by the data, unless the calibration from Cepheids is included. Conclusions. Results from time-delay lenses are consistent with those from distance-ladder calibrations and point to a discrepancy between absolute distance scales measured from the CMB (assuming the standard cosmological model) and late-time observations. New proposals to resolve this tension should be examined with respect to reconciling not only the Hubble constant but also the sound horizon derived from the CMB and other cosmological probes.

Stability analysis for cosmological models in f(T,\xa0B) gravity

In this paper we study cosmological solutions of the f(T, B) gravity using dynamical system analyses. For this purpose, we consider cosmological viable functions of f(T, B) that are capable of reproducing the dynamics of the Universe. We present three specific models of f(T, B) gravity which have a general form of their respective solutions by writing the equations of motion as an autonomous system. Finally, we study its hyperbolic critical points and general trajectories in the phase space of the resulting dynamical variables which turn out to be compatible with the current late-time observations.

Signatures of bimodality in nebular phase Type Ia supernova spectra

ABSTRACT One observational prediction for Type Ia supernovae (SNe Ia) explosions produced through white dwarf–white dwarf collisions is the presence of bimodal velocity distributions for the 56Ni decay products, although this signature can also be produced by an off-centre ignition in a delayed detonation explosion. These bimodal velocity distributions can manifest as double-peaked or flat-topped spectral features in late-time spectroscopic observations for favourable viewing angles. We present nebular-phase spectroscopic observations of 17 SNe Ia obtained with the Large Binocular Telescope. Combining these observations with an extensive search of publicly available archival data, we collect a total sample of 48 SNe Ia and classify them based on whether they show compelling evidence for bimodal velocity profiles in three features associated with 56Ni decay products: the [Fe ii] and [Fe iii] feature at ∼5300 Å, the [Co iii] λ5891 feature, and the [Co iii] and [Fe ii] feature at ∼6600 Å. We identify nine bimodal SNe in our sample, and we find that these SNe have average peak MV about 0.3 mag fainter than those that do not. This is consistent with theoretical predictions for explosions created by nearly head-on collisions of white dwarfs due to viewing angle effects and 56Ni yields.

Looking for interactions in the cosmological dark sector

We study observational signatures of non-gravitational interactions between the dark components of the cosmic fluid, which can be either due to creation of dark particles from the expanding vacuum or an effect of the clustering of a dynamical dark energy. In particular, we analyse a class of interacting models (Λ(t)CDM), characterised by the parameter α, that behaves at background level like cold matter at early times and tends to a cosmological constant in the asymptotic future. Our analysis improves previous works, where only the position of the Cosmic Microwave Background (CMB) first peak and supernova and LSS data where used, considering for the first time both background and primordial perturbations evolutions of the model. We use full spectra of the CMB Planck data together with late time observations, such as the Joint Light-curve Analysis (JLA) supernovae data, the Hubble Space Telescope (HST) measurement of the local value of the Hubble-Lemaȋtre parameter, and primordial deuterium abundance from Lyα systems to test the observational viability of the model and some of its extensions, adapting an Einstein-Boltzmann solver code for our purpose. We found that there is no preference for values of α different from zero (characterising interaction), even if there are some indications for positive values when the minimal Λ(t)CDM model is analysed. When extra degrees of freedom in the relativistic component of the cosmic fluid are considered, the data favour negative values of α, which means an energy flux from dark energy to dark matter.

Dark sectors with dynamical coupling

Coupled dark matter-dark energy scenarios are modeled via a dimensionless parameter $\xi$, which controls the strength of their interaction. While this coupling is commonly assumed to be constant, there is no underlying physical law or symmetry that forbids a time-dependent $\xi$ parameter. The most general and complete interacting scenarios between the two dark sectors should therefore allow for such a possibility, and it is the main purpose of this study to constrain two possible and well-motivated coupled cosmologies by means of the most recent and accurate early and late-time universe observations. We find that CMB data alone prefers $\xi(z) >0$ and therefore a smaller amount of dark matter, alleviating some crucial and well-known cosmological data tensions. An objective assessment of the Bayesian evidence for the coupled models explored here shows no particular preference for the presence of a dynamical dark sector coupling.

Deep late-time observations of the supernova impostors SN 1954J and SN 1961V

ABSTRACT SN 1954J in NGC 2403 and SN 1961V in NGC 1058 were two luminous transients whose definitive classification as either non-terminal eruptions or supernovae remains elusive. A critical question is whether a surviving star can be significantly obscured by dust formed from material ejected during the transient. We use three lines of argument to show that the candidate surviving stars are not significantly optically extinct (τ ≲ 1) by dust formed in the transients. First, we use SED fits to new HST optical and near-IR photometry. Secondly, neither source is becoming brighter as required by absorption from an expanding shell of ejected material. Thirdly, the ejecta masses implied by the Hα luminosities are too low to produce significant dust absorption. The latter two arguments hold independent of the dust properties. The Hα fluxes should also be declining with time as t−3, and this seems not to be observed. As a result, it seems unlikely that recently formed dust can be responsible for the present faintness of the sources compared to their progenitors, although this can be verified with the James Webb Space Telescope. This leaves three possibilities: (1) the survivors were misidentified; (2) they are intrinsically less luminous; (3) SN 1954J and SN 1961V were true supernovae.

Reverse Shock Emission Revealed in Early Photometry in the Candidate Short GRB 180418A

Abstract We present observations of the possible short GRB 180418A in γ-rays, X-rays, and in the optical. Early optical photometry with the TAROT and RATIR instruments shows a bright peak (≈14.2 AB mag) between T + 28 and T + 90 s that we interpret as the signature of a reverse shock. Later observations can be modeled by a standard forward shock model and show no evidence of a jet break, allowing us to constrain the jet collimation to θ j > 7°. Using deep late-time optical observations, we place an upper limit of r > 24 AB mag on any underlying host galaxy. The detection of the afterglow in the Swift UV filters constrains the GRB redshift to z < 1.3 and places an upper bound on the γ-ray isotropic equivalent energy E γ,iso < 3 × 1051 erg. The properties of this GRB (e.g., duration, hardness ratio, energy, and environment) lie at the intersection between short and long bursts, and we cannot conclusively identify its type. We estimate that the probability that it is drawn from the population of short GRBs is 10%–30%.

Late-time UV Observations of Tidal Disruption Flares Reveal Unobscured, Compact Accretion Disks∗

Abstract The origin of thermal optical and UV emission from stellar tidal disruption flares (TDFs) remains an open question. We present Hubble Space Telescope far-UV (FUV) observations of eight optical/UV-selected TDFs 5–10 yr post-peak. Six sources are cleanly detected, showing point-like FUV emission ( ) from the centers of their host galaxies. We discover that the light curves of TDFs from low-mass black holes (<106.5 M ⊙) show significant late-time flattening. Conversely, FUV light curves from high-mass black hole TDFs are generally consistent with an extrapolation from the early-time light curve. The observed late-time emission cannot be explained by existing models for early-time TDF light curves (i.e., reprocessing or circularization shocks), but is instead consistent with a viscously spreading, unobscured accretion disk. These disk models can only reproduce the observed FUV luminosities, however, if they are assumed to be thermally and viscously stable, in contrast to the simplest predictions of α-disk theory. For one TDF in our sample, we measure an upper limit to the UV luminosity that is significantly lower than expectations from theoretical modeling and an extrapolation of the early-time light curve. This dearth of late-time emission could be due to a disk instability/state change absent in the rest of the sample. The disk models that explain the late-time UV detections solve the TDF “missing energy problem” by radiating a rest-mass energy of ∼0.1 M ⊙ over a period of decades, primarily in extreme UV wavelengths.

The X-shooter GRB afterglow legacy sample (XS-GRB)

In this work we present spectra of all γ-ray burst (GRB) afterglows that have been promptly observed with the X-shooter spectrograph until 31/03/2017. In total, we have obtained spectroscopic observations of 103 individual GRBs observed within 48 hours of the GRB trigger. Redshifts have been measured for 97 per cent of these, covering a redshift range from 0.059 to 7.84. Based on a set of observational selection criteria that minimise biases with regards to intrinsic properties of the GRBs, the follow-up effort has been focused on producing a homogeneously selected sample of 93 afterglow spectra for GRBs discovered by the Swift satellite. We here provide a public release of all the reduced spectra, including continuum estimates and telluric absorption corrections. For completeness, we also provide reductions for the 18 late-time observations of the underlying host galaxies. We provide an assessment of the degree of completeness with respect to the parent GRB population, in terms of the X-ray properties of the bursts in the sample and find that the sample presented here is representative of the full Swift sample. We have constrained the fraction of dark bursts to be <28 per cent and confirm previous results that higher optical darkness is correlated with increased X-ray absorption. For the 42 bursts for which it is possible, we have provided a measurement of the neutral hydrogen column density, increasing the total number of published HI column density measurements by ∼33 per cent. This dataset provides a unique resource to study the ISM across cosmic time, from the local progenitor surroundings to the intervening Universe.

A multiwavelength analysis of a collection of short-duration GRBs observed between 2012 and 2015

AbstractWe investigate the prompt emission and the afterglow properties of short-duration gamma-ray burst (sGRB) 130603B and another eight sGRB events during 2012–2015, observed by several multiwavelength facilities including the Gran Canarias Telescope 10.4 m telescope. Prompt emission high energy data of the events were obtained by INTEGRAL-SPI-ACS, Swift-BAT, and Fermi-GBM satellites. The prompt emission data by INTEGRAL in the energy range of 0.1–10 MeV for sGRB 130603B, sGRB 140606A, sGRB 140930B, sGRB 141212A, and sGRB 151228A do not show any signature of the extended emission or precursor activity and their spectral and temporal properties are similar to those seen in case of other short bursts. For sGRB 130603B, our new afterglow photometric data constrain the pre-jet-break temporal decay due to denser temporal coverage. For sGRB 130603B, the afterglow light curve, containing both our new and previously published photometric data is broadly consistent with the ISM afterglow model. Modeling of the host galaxies of sGRB 130603B and sGRB 141212A using the LePHARE software supports a scenario in which the environment of the burst is undergoing moderate star formation activity. From the inclusion of our late-time data for eight other sGRBs we are able to: place tight constraints on the non-detection of the afterglow, host galaxy, or any underlying ‘kilonova’ emission. Our late-time afterglow observations of the sGRB 170817A/GW170817 are also discussed and compared with the sub-set of sGRBs.

Fingerprints of Heavy-Element Nucleosynthesis in the Late-Time Lightcurves of Kilonovae.

The kilonova emission observed following the binary neutron star merger event GW170817 provided the first direct evidence for the synthesis of heavy nuclei through the rapid neutron capture process (r process). The late-time transition in the spectral energy distribution to near-infrared wavelengths was interpreted as indicating the production of lanthanide nuclei, with atomic mass number A≳140. However, compelling evidence for the presence of even heavier third-peak (A≈195) r-process elements (e.g., gold, platinum) or translead nuclei remains elusive. At early times (∼days) most of the r-process heating arises from a large statistical ensemble of β decays, which thermalize efficiently while the ejecta is still dense, generating a heating rate that is reasonably approximated by a single power law. However, at later times of weeks to months, the decay energy input can also possibly be dominated by a discrete number of α decays, ^{223}Ra (half-life t_{1/2}=11.43 d), ^{225}Ac (t_{1/2}=10.0 d, following the β decay of ^{225}Ra with t_{1/2}=14.9 d), and the fissioning isotope ^{254}Cf (t_{1/2}=60.5 d), which liberate more energy per decay and thermalize with greater efficiency than β-decay products. Late-time nebular observations of kilonovae which constrain the radioactive power provide the potential to identify signatures of these individual isotopes, thus confirming the production of heavy nuclei. In order to constrain the bolometric light to the required accuracy, multiepoch and wideband observations are required with sensitive instruments like the James Webb Space Telescope. In addition, by comparing the nuclear heating rate obtained with an abundance distribution that follows the solar r abundance pattern, to the bolometric lightcurve of AT2017gfo, we find that the yet-uncertain r abundance of ^{72}Ge plays a decisive role in powering the lightcurve, if one assumes that GW170817 has produced a full range of the solar r abundances down to mass number A∼70.

Late-time observations of the extraordinary Type II supernova iPTF14hls

Aims. We study iPTF14hls, a luminous and extraordinary long-lived Type II supernova, which lately has attracted much attention and disparate interpretation. Methods. We have presented new optical photometry that extends the light curves up to more than three years past discovery. We also obtained optical spectroscopy over this period, and furthermore present additional space-based observations using Swift and HST. Results. After an almost constant luminosity for hundreds of days, the later light curve of iPTF14hls finally fades and then displays a dramatic drop after about 1000 d, but the supernova is still visible at the latest epochs presented. The spectra have finally turned nebular, and our very last optical spectrum likely displays signatures from the deep and dense interior of the explosion. A high-resolution HST image highlights the complex environment of the explosion in this low-luminosity galaxy. Conclusions. We provide a large number of additional late-time observations of iPTF14hls, which are (and will continue to be) used to assess the many different interpretations for this intriguing object. In particular, the very late (+1000 d) steep decline of the optical light curve is difficult to reconcile with the proposed central engine models. The lack of very strong X-ray emission, and the emergence of intermediate-width emission lines including [S II] that we propose originate from dense, processed material in the core of the supernova ejecta, are also key observational tests for both existing and future models.

Late-time Observations of the Type Ia Supernova SN 2014J with the Hubble Space Telescope Wide Field Camera 3

Abstract Recent works have studied the late-time light curves of Type Ia supernovae (SNe Ia) when these were older than 500 days past B-band maximum light. Of these, SN 2014J, which exploded in the nearby galaxy M82, was studied with the Advanced Camera for Surveys onboard the Hubble Space Telescope (HST) by Yang et al. Here, I report complementary photometry of SN 2014J taken with the HST Wide Field Camera 3 when it was ∼360–1300 days old. My F555W measurements are consistent with the F606W measurements of Yang et al., but the F438W measurements are ∼1 mag fainter than their F475W measurements. I corroborate their finding that, even though SN 2014J has spatially resolved light echoes, its photometry is not contaminated by an unresolved echo. Finally, I compare the F438W and F555W light curves of SN 2014J to those of the other late-time SNe Ia observed to date and show that more intrinsically luminous SNe have slower light curve decline rates. This is consistent with the correlation claimed by Graur et al., which was based on a comparison of pseudo-bolometric light curves. By conducting a direct comparison of the late-time light curves in the same filters, I remove any systematic uncertainties introduced by the assumptions that go into constructing the pseudo-bolometric light curves, thus strengthening the Graur et al. claim.

Late-time Observations of ASASSN-14lp Strengthen the Case for a Correlation between the Peak Luminosity of Type Ia Supernovae and the Shape of Their Late-time Light Curves

Abstract Late-time observations of Type Ia supernovae (SNe Ia), >900 days after explosion, have shown that this type of SN does not suffer an “IR catastrophe” at 500 days as previously predicted. Instead, several groups have observed a slow-down in the optical light curves of these SNe. A few reasons have been suggested for this slow-down, from a changing fraction of positrons reprocessed by the expanding ejecta, through a boost of energy from slow radioactive decay chains such as 57Co→57Fe, to atomic “freeze-out.” Discovering which of these (or some other) heating mechanisms is behind the slow-down will directly impact studies of SN Ia progenitors, explosion models, and nebular-stage physics. Recently, Graur et al. suggested a possible correlation between the shape of the late-time light curves of four SNe Ia and their stretch values, which are proxies for their intrinsic luminosities. Here, we present Hubble Space Telescope observations of the SN Ia ASASSN-14lp at ∼850–960 days past maximum light. With a stretch of s = 1.15 ± 0.05, it is the most luminous normal SN Ia observed so far at these late times. We rule out contamination by light echoes and show that the late-time, optical light curve of ASASSN-14lp is flatter than that of previous SNe Ia observed at late times. This result is in line with—and strengthens—the Graur et al. correlation, but additional observations of SNe are needed to verify it.