Compact Dark Objects and Gravitational Microlensing towards the Large Magellanic Cloud

We estimate the long-lasting gravitational wave (GW) emission of compact dark objects following a binary neutron-star (NS) merger. We consider compact dark objects, which initially reside in the centers of NSs and which may consist of self-interacting dark matter (DM). By approximating the compact dark objects as test particles, we model the merging of NS binaries hosting DM components with three-dimensional relativistic simulations. Our simulation results suggest that the DM components remain gravitationally bound and orbit inside the merger remnant with orbital separations of typically a few km. The subsequent orbital motion of the DM components generates a GW signal with frequencies in the range of a few kHz. When considering a range of different binary masses and high-density equations of state (EOS), we find that the GW frequency of the orbiting DM components scales with the compactness of NSs. Similarly, we find relations between the DM GW frequency and the dominant postmerger GW frequency of the stellar fluid or the tidal deformability, which quantifies EOS effects during the binary inspiral. Hence, a measurement of these quantities can be used to specify the frequency range of the GW emission by DM. Under the assumption that GW back reaction is the only relevant dissipative process, the GW signal may last between seconds and years depending on the mass of the DM component. We estimate the detectibility of the GW signals and find that DM components in NS mergers may only be detectable with existing and projected GW instruments if the dark objects are as massive as about 0.01 to $0.1{M}_{\ensuremath{\bigodot}}$. We emphasize that the GW emission is limited by the lifetime of the remnant. A forming black hole will immediately swallow the DM objects because their orbits are smaller than the innermost stable circular orbit of the black hole.

Introduction Gravitational lensing effects arise from the modification of space-time metric produced by mass concentrations. Following an early prediction of General Relativity, gravitational fields deflect the light path of photons and modify the apparent flux and shape of astronomical sources. By observing gravitationally lensed images, cosmologists can probe dark matter almost ‘directly’. They can in principle examine it without the need to speculate on the distribution of matter inside the gravitational potential responsible for the light deflection, nor on its dynamical state and on its thermodynamical properties. A most attractive application of gravitational lensing concerns the quest for dark matter candidates and for the properties of dark matter particles. Successful examples are the microlensing experiments (see [798] and references therein) carried out over the past decade to detect sub-stellar dark compact objects in the Galaxy. Similar searches for invisible cosmologically distributed compact objects have been carried out using microlensing on quasars [609]. These experiments set limits on compact dark objects (CDO) in the mass range 10 −6 M ⊙ m 6 M ⊙ [1918] and prove they represent a fraction Ω M (CDO) Beside microlensing, other extragalactic gravitational lensing effects on quasars and extended sources (galaxies) can also probe dark matter. Although they are less frequent and more complex phenomena than microlensing, these lensing configurations provide much more detail on the lens and on the amount and distribution of matter inside.

It was proposed that the massive compact halo objects (MCHOs) would be produced during an earlier epoch of cosmology if the density perturbations are between $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ and 0.3. Then these objects can accrete dark matter particles onto them due to their high density. If the dark matter is in the form of the weakly interacting massive particles, the MCHOs can have a significant effect on the evolution of cosmology due to the dark matter annihilation within them. Using the WMAP-7 years data, we investigated the constraints on the current abundance of MCHOs (${f}_{\mathrm{MCHOs}}$) formed during the ${e}^{+}{e}^{\ensuremath{-}}$ annihilation phase transition. We have found that the $2\ensuremath{\sigma}$ constraint is ${f}_{\mathrm{MCHOs}}\ensuremath{\lesssim}{10}^{\ensuremath{-}4}$ for dark matter masses in the range between 1 GeV and 1 TeV.

Recently the Polish-American collaboration OGLE has reported the observation of four possible microlensing events by monitoring, over several months, the brightness of millions of stars in the region of the galactic bulge. If these events are due to microlensing, the most accurate way to get information on the mass of the dark compact objects, that acted as gravitational lenses, is to use the method of the mass moments. Here I apply this method to the analysis of the events detected by OGLE. The average mass turns out to be 0.28$M_{\odot}$, suggesting that the lens objects are faint disk stars. The same method applied to the five microlensing events detected so far by the EROS and MACHO collaborations, which monitor stars in the Large Magellanic Cloud, leads to an average value of 0.08$M_{\odot}$ for the dark compact halo objects.

The nature of the dark matter in the halo of our galaxy is still largely unknown. The microlensing events found so far towards the Large Magellanic Cloud suggest that at most about 20% of the halo dark matter is in form of MACHOs (Massive Astrophysical Compact Halo Objects). The dark matter could also, at least partially, be made of cold molecular clouds (mainly H2). We proposed a model for baryonic dark matter, according to which dark clusters of brown dwarfs and cold self-gravitating H2 clouds populate the outer galactic halo. A signature would be a diffuse gray emission from the galactic halo. Basically, cosmic-ray protons in the galactic halo scatter on the clouds clumped into dark clusters, giving rise to a γ-ray flux. An analysis of EGRET data has led to the discovery of a statistically significant diffuse γ-ray emission from the galactic halo, which turns out to be in remarkably good agreement with our prediction.

We use the dynamical heating of stars in ultrafaint dwarf (UFD) galaxies to set limits on Massive Compact Halo Objects (MACHOs). In our analysis we study the robustness of the bounds under uncertainties in key UFD parameters, such as the half-light radius, stellar velocity dispersion, total halo mass and dark matter and stellar density profiles. We apply this framework to both well-established UFD candidates, as well as the recently discovered UFD candidate Ursa Major III/UNIONS 1. We find that multiple UFDs yield consistently strong limits in the mass range 10 M ⊙ ≲ M MACHO ≲ 10 9 M ⊙ , underscoring the robustness of a previous analysis solely based on Segue I. We also demonstrate that Ursa Major III, if confirmed as an UFD, would improve the constraints significantly, providing the strongest constraints on MACHO dark matter in the mass range 1 M ⊙ ≲ M MACHO ≲ 10 5 M ⊙ .

Aims. It has been shown that the ultra-faint dwarf galaxy Eridanus 2 may host a stellar cluster in its centre. If this cluster is shown to exist, it can be used to set constraints on the mass and abundance of massive astrophysical compact halo objects (MACHOs) as a form of dark matter. Previous research has shown promising expectations in the mass range of 10−100 M⊙, but lacked spectroscopic measurements of the cluster. We aim to provide spectroscopic evidence regarding the nature of the putative star cluster in Eridanus 2 and to place constraints on MACHOs as a constituent of dark matter. Methods. We present spectroscopic observations of the central square arcminute of Eridanus 2 from MUSE-Faint, a survey of ultra-faint dwarf galaxies with the Multi Unit Spectroscopic Explorer on the Very Large Telescope. We derived line-of-sight velocities for possible member stars of the putative cluster and for stars in the centre of Eridanus 2. We discuss the existence of the cluster and determine new constraints for MACHOs using the Fokker–Planck diffusion approximation. Results. Out of 182 extracted spectra, we identify 26 member stars of Eridanus 2, seven of which are possible cluster members. We find intrinsic mean line-of-sight velocities of 79.7+3.1−3.8 km s−1 and 76.0+3.2−3.7 km s−1 for the cluster and the bulk of Eridanus 2, respectively, as well as intrinsic velocity dispersions of < 7.6 km s−1 (68% upper limit) and 10.3+3.9−3.2 km s−1, respectively. This indicates that the cluster most likely exists as a distinct dynamical population hosted by Eridanus 2 and that it does not have a surplus of dark matter over the background distribution. Among the member stars in the bulk of Eridanus 2, we find possible carbon stars, alluding to the existence of an intermediate-age population. We derived constraints on the fraction of dark matter that can consist of MACHOs with a given mass between 1 and 105 M⊙. For dark matter consisting purely of MACHOs, the mass of the MACHOs must be less than ∼7.6 M⊙ and ∼44 M⊙ at a 68- and 95% confidence level, respectively.

We propose a method to remove the mass-sheet degeneracy that arises when the mass of galaxy clusters is inferred from gravitational shear. The method utilizes high-redshift standard candles that undergo weak lensing. Natural candidates for such standard candles are type Ia supernovae (SNe Ia). When corrected with the light-curve shape (LCS), the peak magnitude of SNe Ia provides a standard candle with an uncertainty in apparent magnitude of Δm≃0.1–0.2. Gravitational magnification of a background SN Ia by an intervening cluster would cause a mismatch between the observed SN Ia peak magnitude compared with that expected from its LCS and redshift. The average detection rate for SNe Ia with a significant mismatch of ≥2Δm behind a cluster at z≃0.05–0.15 is about 1–2 supernovae per cluster per year at J, I, R≲25–26. Since SNe are point-like sources for a limited period, they can experience significant microlensing by massive compact halo objects (MACHOs) in the intracluster medium. Microlensing events caused by MACHOs of ∼10−4 M⊙ are expected to have time-scales similar to that of the SN light curve. Both the magnification curve by a MACHO and the light curve of a SN Ia have characteristic shapes that allow us to separate them. Microlensing events caused by MACHOs of smaller mass can unambiguously be identified in the SN light curve if the latter is continuously monitored. The average number of identifiable microlensing events per nearby cluster (z≲0.05) per year is ∼0.02 (f/0.01), where f is the fraction of the cluster mass in MACHOs of masses 10−7<Mmacho/M⊙<10−4.

Two spacecraft separated by approximately 1 AU and equipped with gamma-ray burst (GRB) detectors could detect or rule out a cosmological density of massive compact halo objects (MACHOs) in the mass range 10−15 M☉ M 10−7 M☉, provided that GRBs prove to be cosmological. Previously devised methods for detecting MACHOs have spanned the mass range 10−16 M☉ M 107 M☉, but with a gap of several orders of magnitude near 10−9 M☉. For MACHOs and sources both at a cosmological distance, the Einstein radius is approximately 1 AU(M/10−7 M☉)1/2. Hence, if a GRB lies within the Einstein ring of a MACHO of mass M 10−7 M☉ as seen by one detector, it will not lie in the Einstein ring as seen by a second detector approximately 1 AU away. This implies that if GRBs are measured to have significantly different fluxes by the two detectors, this would signal the presence of a MACHO 10−7 M☉. By the same token, if the two detectors measured similar fluxes for several hundred events, a cosmological abundance of such low-mass MACHOs would be ruled out. The lower limit is set by the time resolution tres of the detectors: M 10−15 M☉ corresponds to tres 10-2 s. If low-mass MACHOs are detected, there are tests that can discriminate among events generated by MACHOs in the three mass ranges M 10−12 M☉, 10−12 M☉ M 10−7 M☉, and M 10−7 M☉. Further experiments would then be required to make more accurate mass measurements.

Stars are the most fundamental components of the visible universe. They provide reference points in the sky, distance markers for cosmologists and tracers of dark matter; serve as the basis for understanding the composition and origin of our solar system; and are companions used to find planets, brown dwarfs, and black holes. Our ability to understand and to use stars effectively for these and other applications depends on knowledge of stellar life cycles. Our view of stellar evolution has changed and progressed rapidly in the past 5 to 10 years, due in part to technological advances, including bigger and better charge-coupled devices, improved ground-based telescopes such as Keck I and II, and continued spectacular results from orbiting satellites such as the Hubble Space Telescope, the Rontgen X-ray Satellite, and the Solar Heliospheric Observatory. Given this progress, the recent discoveries of extrasolar planets (see News story on p. [1336][1]) and brown dwarfs, and the possibility that we may soon identify a black hole and a source for gamma ray bursts using the characteristics of star-like objects near these phenomena, it seems fitting to highlight some advances in stellar research in the nine Articles in this special issue. A star is born by the gravitational collapse of a cloud of dust and gas. If the condensing spherical mass cannot grow large enough, then it slowly gets colder and fades, becoming a brown dwarf. If it grows enough to sustain hydrogen fusion, a main sequence star, like our sun, is formed. Eventually hydrogen is exhausted, the star expands to a red giant as it feeds off other nuclear reactions, and finally the red giant collapses into a small, dense white dwarf, when these reactions cease. In some cases, the collapsing star throws off excess mass as a gaseous envelope, called a planetary nebula. More massive stars tend to die in rapid energetic outbursts called supernovae explosions that produce white dwarfs; or if the remnant core is too massive, collapse continues until most energetic particles are converted to neutrons that stabilize the core into a neutron star. The most massive stars, with the shortest lifetimes and the most energetic supernovae explosions, cannot collapse into white dwarfs or neutron stars; instead, they collapse into an even smaller, denser core from which not even light can escape: a black hole. The first five Articles discuss the early part of the stellar cycle. Kulkarni (p. [1350][2]) focuses on the direct detection of a brown dwarf and discusses how brown dwarfs may be a link between planets and stars. O'Dell and Beckwith (p. [1355][3]) describe the dynamics of stellar birth, focusing on star-forming regions in the Orion Nebula, which show that young stars do not just collapse into spheres of gas but produce jets, winds, and possible planet-forming disks of dust. Arnett and Bazan (p. [1359][4]) discuss the latest progress in understanding nucleosynthetic processes, including the identification of presolar grains in meteorites and the use of high-energy density laser experiments to simulate thermonuclear reactions. Neuhauser (p. [1363][5]) focuses on the x-ray emissions from protostars, which can be seen through molecular clouds that surround protostars. Many protostars are binaries and interact with the circumstellar disk, possibly allowing planet formation within the disks. Burgarella (p. [1370][6]) reviews globular clusters, which are regions of colliding galaxies that produce bursts of star-forming regions encased in dense envelopes of gas. Globular clusters help us understand galaxy formation and the evolution and age of the universe. The death of stars is perhaps most obvious in supernovae explosions. Supernova 1987A, because it was discovered early in its explosive history and was relatively close to Earth, has provided a wealth of information about the chemical and physical evolution of supernovae. Chevalier (p. [1374][7]) reviews the progress made in understanding Type II supernovae, particularly 1987A and 1993 J. Nomoto, Iwamoto, and Kishimoto (p. [1378][8]) discuss Type Ia supernovae, which are believed to represent the thermonuclear explosions of accreting white dwarfs and are used to estimate the age of the universe and distance to its edge. On the quieter side of stellar death, Weinberger and Kerber (p. [1382][9]) describe the diverse morphologies of and applications for planetary nebulae. Spectroscopic observations of planetary nebulae in the halos of galaxies confirm that we cannot account for most of the dark matter in the universe. Finally, Kaaret and Ford (p. [1386][10]) review x-ray binaries, which consist of a neutron star or black hole and a compact star. Understanding how material flows from the star to the black hole will help to identify the event horizon of a black hole. [1]: /lookup/doi/10.1126/science.276.5317.1336 [2]: /lookup/doi/10.1126/science.276.5317.1350 [3]: /lookup/doi/10.1126/science.276.5317.1355 [4]: /lookup/doi/10.1126/science.276.5317.1359 [5]: /lookup/doi/10.1126/science.276.5317.1363 [6]: /lookup/doi/10.1126/science.276.5317.1370 [7]: /lookup/doi/10.1126/science.276.5317.1374 [8]: /lookup/doi/10.1126/science.276.5317.1378 [9]: /lookup/doi/10.1126/science.276.5317.1382 [10]: /lookup/doi/10.1126/science.276.5317.1386

To set the stage for our study of ultralight bosonic dark matter (UBDM), we review the evidence for the existence of dark matter: galactic and stellar dynamics, gravitational lensing studies, measurements of the cosmic microwave background radiation (CMB), surveys of the large-scale structure of the universe, and the observed abundance of light elements. This diverse array of observational evidence informs what we know about dark matter: its universal abundance, its spatial and velocity distribution, and that its explanation involves physics beyond the Standard Model. But what we know about dark matter is far outweighed by what we do not know. We examine UBDM in the context of several of the most prominent alternative hypotheses for the nature of dark matter: weakly interacting massive particles (WIMPs), sterile neutrinos, massive astrophysical compact halo objects (MACHOs), and primordial black holes (PBHs). Finally we examine some of the key general characteristics of UBDM, including its wavelike nature, coherence properties, and couplings to Standard Model particles and fields.

Baryonic matter, in the form of Machos (Massive Compact Halo Objects), might be a significant constituent of the dark matter that dominates the Milky Way. This article describes the experimental searches for Machos that exploit the gravitational microlens magnification of extragalactic stars. These surveys monitor millions of stars, in some cases every night, looking for magnification events. The early results from the surveys have yielded some spectacular events, and pose a significant new puzzle for galactic structure: toward the Large Magellanic Cloud we see fewer events than anticipated for a standard dark halo dominated by Machos, but toward the galactic bulge, the event rate is much higher than anticipated. This is a field of research that is ripe with opportunities for beginning (and senior) scientists.

The MACHO project 2nd year LMC microlensing results and dark matter implications

Observations have found black holes spanning 10 orders of magnitude in mass across most of cosmic history. The Kerr black hole solution is, however, provisional as its behavior at infinity is incompatible with an expanding universe. Black hole models with realistic behavior at infinity predict that the gravitating mass of a black hole can increase with the expansion of the universe independently of accretion or mergers, in a manner that depends on the black hole’s interior solution. We test this prediction by considering the growth of supermassive black holes in elliptical galaxies over 0 < z ≲ 2.5. We find evidence for cosmologically coupled mass growth among these black holes, with zero cosmological coupling excluded at 99.98% confidence. The redshift dependence of the mass growth implies that, at z ≲ 7, black holes contribute an effectively constant cosmological energy density to Friedmann’s equations. The continuity equation then requires that black holes contribute cosmologically as vacuum energy. We further show that black hole production from the cosmic star formation history gives the value of ΩΛ measured by Planck while being consistent with constraints from massive compact halo objects. We thus propose that stellar remnant black holes are the astrophysical origin of dark energy, explaining the onset of accelerating expansion at z ∼ 0.7.

The detection of old neutron stars and black holes in isolation is one of the cornerstones of compact object astrophysics. Microlensing surveys may help on this purpose since the lensing mechanism is independent of the emission properties of the lens. Indeed, several black hole candidates deriving through microlensing observations have been reported in the literature. The identification of counterparts, especially in the X-rays, would be a strong argument in favor of the compact nature of these lenses. We perform a cross-correlation between the catalogs of microlensing events by the OGLE, MACHO and MOA teams, and those of X-rays sources from XMM-Newton and Chandra satellites. Based on our previous work, we select only microlensing events longer than 100 days, which should contain a large fraction of lenses as compact objects. Our matching criterion takes into account the positional coincidence in the sky. We find a single match between a microlensing event OGLE 2004-BLG-81 and the X-ray source 2XMM J180540.5-273427. The angular separation is \sim 0.5 arc-seconds, well inside the 90% error box of the X-ray source. The hardness ratios reported in the 2XMM catalog suggest a hard spectrum with a peak between 2 and 4.5 keV or a softer but highly absorbed source. Moreover the microlensing event is not fully constrained, and possible association with a flaring CV or a RS CVn star have been suggested as well. The small angular separation is a strong indicator that 2XMM J180540.5-273427 is the X-ray counterpart of the OGLE event. However, the uncertainties on both the nature of lensed system and the lens itself challenge the interpretation of 2XMM J180540.5-273427 as the first confirmed isolated black hole identified so far. In any case, it probes the diagnostic capacity of microlensing surveys and open the path for further identifications of black hole or neutron star candidates.