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Register a new userWhat if LIGOs gravitational wave detections are strongly lensed by massive galaxy clusters?
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Motivated by the preponderance of so-called heavy black holes in the binary black hole (BBH) gravitational wave (GW) detections to date, and the role that gravitational lensing continues to play in discovering new galaxy populations, we explore the possibility that the GWs are strongly-lensed by massive galaxy clusters. For example, if one of the GW sources were actually located at $z=1$, then the rest-frame mass of the associated BHs would be reduced by a factor $sim2$. Based on the known populations of BBH GW sources and strong-lensing clusters, we estimate a conservative lower limit on the number of BBH mergers detected per detector year at LIGO/Virgos current sensitivity that are multiply-imaged, of $R_{rm detect}simeq10^{-5}{rm yr}^{-1}$. This is equivalent to rejecting the hypothesis that one of the BBH GWs detected to date was multiply-imaged at $<sim4sigma$. It is therefore unlikely but not impossible that one of the GWs is multiply-imaged. We identify three spectroscopically confirmed strong-lensing clusters with well constrained mass models within the $90%$ credible sky localisations of the BBH GWs from LIGOs first observing run. In the event that one of these clusters multiply-imaged one of the BBH GWs, we predict that $20-60%$ of the putative next appearances of the GWs would be detectable by LIGO, and that they would arrive at Earth within three years of first detection.
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A number of merging galaxy clusters shows the presence of shocks and cold fronts, i.e. sharp discontinuities in surface brightness and temperature. The observation of these features requires an X-ray telescope with high spatial resolution like Chandra, and allows to study important aspects concerning the physics of the intra-cluster medium (ICM), such as its thermal conduction and viscosity, as well as to provide information on the physical conditions leading to the acceleration of cosmic rays and magnetic field amplification in the cluster environment. In this work we search for new discontinuities in 15 merging and massive clusters observed with Chandra by using different imaging and spectral techniques of X-ray observations. Our analysis led to the discovery of 22 edges: six shocks, eight cold fronts and eight with uncertain origin. All the six shocks detected have $mathcal{M} < 2$ derived from density and temperature jumps. This work contributed to increase the number of discontinuities detected in clusters and shows the potential of combining diverse approaches aimed to identify edges in the ICM. A radio follow-up of the shocks discovered in this paper will be useful to study the connection between weak shocks and radio relics.
Though theoretically expected, the charge exchange emission from galaxy clusters has not yet been confidently detected. Accumulating hints were reported recently, including a rather marginal detection with the Hitomi data of the Perseus cluster. As suggested in Gu et al. (2015), a detection of charge exchange line emission from galaxy clusters would not only impact the interpretation of the newly-discovered 3.5 keV line, but also open up a new research topic on the interaction between hot and cold matter in clusters. We aim to perform the most systematic search for the O VIII charge exchange line in cluster spectra using the RGS on board XMM. We introduce a sample of 21 clusters observed with the RGS. The dominating thermal plasma emission is modeled and subtracted with a two-temperature CIE component, and the residuals are stacked for the line search. The systematic uncertainties in the fits are quantified by refitting the spectra with a varying continuum and line broadening. By the residual stacking, we do find a hint of a line-like feature at 14.82 A, the characteristic wavelength expected for oxygen charge exchange. This feature has a marginal significance of 2.8 sigma, and the average equivalent width is 2.5E-4 keV. We further demonstrate that the putative feature can be hardly affected by the systematic errors from continuum modelling and instrumental effects, or the atomic uncertainties of the neighbouring thermal lines. Assuming a realistic temperature and abundance pattern, the physical model implied by the possible oxygen line agrees well with the theoretical model proposed previously to explain the reported 3.5 keV line. If the charge exchange source indeed exists, we would expect that the oxygen abundance is potentially overestimated by 8-22% in previous X-ray measurements which assumed pure thermal lines.
It has been recently shown that quadruply lensed gravitational-wave (GW) events due to coalescing binaries can be localized to one or just a few galaxies, even in the absence of an electromagnetic counterpart. We discuss how this can be used to extract information on modified GW propagation, which is a crucial signature of modifications of gravity at cosmological scales. We show that, using quadruply lensed systems, it is possible to constrain the parameter $Xi_0$ that characterizes modified GW propagation, without the need of imposing a prior on $H_0$. A LIGO/Virgo/Kagra network at target sensitivity might already get a significant measurement of $Xi_0$, while a third generation GW detector such as the Einstein Telescope could reach a very interesting accuracy.
Recently, the optical counterpart of a gravitational wave source GW170817 has been identified in NGC 4993 galaxy. Together with evidence from observations in electromagnetic waves, the event has been suggested as a result of a merger of two neutron stars. We analyze the multi-wavelength data to characterize the host galaxy property and its distance to examine if the properties of NGC 4993 are consistent with this picture. Our analysis shows that NGC 4993 is a bulge-dominated galaxy with reff ~ 2-3 kpc and the Sersic index of n = 3-4 for the bulge component. The spectral energy distribution from 0.15 to 24 micron indicates that this galaxy has no significant ongoing star formation, the mean stellar mass of (0.3 - 1.2) times 10^11 Msun,the mean stellar age greater than ~3 Gyr, and the metallicity of about 20% to 100% of solar abundance. Optical images reveal dust lanes and extended features that suggest a past merging activity. Overall, NGC 4993 has characteristics of normal, but slightly disturbed elliptical galaxies. Furthermore, we derive the distance to NGC 4993 with the fundamental plane relation using 17 parameter sets of 7 different filters and the central stellar velocity dispersion from literature, finding an angular diameter distance of 37.7 +- 8.7 Mpc. NGC 4993 is similar to some host galaxies of short gamma-ray bursts but much different from those of long gamma-ray bursts, supporting the picture of GW170817 as a result of a merger of two NSs.
Recent work has demonstrated the potential of gravitationally lensed quasars to extend measurements of black hole spin out to high-redshift with the current generation of X-ray observatories. Here we present an analysis of a large sample of 27 lensed quasars in the redshift range 1.0<z<4.5 observed with Chandra, utilizing over 1.6 Ms of total observing time, focusing on the rest-frame iron K emission from these sources. Although the X-ray signal-to-noise (S/N) currently available does not permit the detection of iron emission from the inner accretion disk in individual cases in our sample, we find significant structure in the stacked residuals. In addition to the narrow core, seen almost ubiquitously in local AGN, we find evidence for an additional underlying broad component from the inner accretion disk, with a clear red wing to the emission profile. Based on simulations, we find the detection of this broader component to be significant at greater than the 3-sigma level. This implies that iron emission from the inner disk is relatively common in the population of lensed quasars, and in turn further demonstrates that, with additional observations, this population represents an opportunity to significantly extend the sample of AGN spin measurements out to high-redshift.
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