The cosmic merger rate of neutron stars and black holes

Six gravitational wave detections have been reported so far, providing crucial insights on the merger rate of double compact objects. We investigate the cosmic merger rate of double neutron stars (DNSs), neutron star-black hole binaries (NSBHs) and black hole binaries (BHBs) by means of population-synthesis simulations coupled with the Illustris cosmological simulation. We have performed six different simulations, considering different assumptions for the efficiency of common envelope (CE) ejection and exploring two distributions for the supernova (SN) kicks. The current BHB merger rate derived from our simulations spans from $\sim{}150$ to $\sim{}240$ Gpc$^{-3}$ yr$^{-1}$ and is only mildly dependent on CE efficiency. In contrast, the current merger rates of DNSs (ranging from $\sim{}20$ to $\sim{}600$ Gpc$^{-3}$ yr$^{-1}$) and NSBHs (ranging from $\sim{}10$ to $\sim{}100$ Gpc$^{-3}$ yr$^{-1}$) strongly depend on the assumptions on CE and natal kicks. The merger rate of DNSs is consistent with the one inferred from the detection of GW170817 only if a high efficiency of CE ejection and low SN kicks (drawn from a Maxwellian distribution with one dimensional root mean square $\sigma{}=15$ km s$^{-1}$) are assumed.Comment: 9 pages, 6 figures, 2 tables, accepted for publication in MNRA

The progenitors of compact-object binaries: impact of metallicity, common envelope and natal kicks

Six gravitational wave events have been reported by the LIGO-Virgo collaboration (LVC), five of them associated with black hole binary (BHB) mergers and one with a double neutron star (DNS) merger, while the coalescence of a black hole-neutron star (BHNS) binary is still missing. We investigate the progenitors of double compact object binaries with our population-synthesis code MOBSE. MOBSE includes advanced prescriptions for mass loss by stellar winds (depending on metallicity and on the Eddington ratio) and a formalism for core-collapse, electron-capture and (pulsational) pair instability supernovae. We investigate the impact of progenitor's metallicity, of the common-envelope parameter $\alpha{}$ and of the natal kicks on the properties of DNSs, BHNSs and BHBs. We find that neutron-star (NS) masses in DNSs span from 1.1 to 2.0 M$_\odot$, with a preference for light NSs, while NSs in merging BHNSs have mostly large masses ($1.3-2.0$ M$_\odot$). BHs in merging BHNSs are preferentially low mass ($5-15$ M$_\odot$). BH masses in merging BHBs strongly depend on the progenitor's metallicity and span from $\sim{}5$ to $\sim{}45$ M$_\odot$. The local merger rate density of both BHNSs and BHBs derived from our simulations is consistent with the values reported by the LVC in all our simulations. In contrast, the local merger rate density of DNSs matches the value inferred from the LVC only if low natal kicks are assumed. This result adds another piece to the intricate puzzle of natal kicks and DNS formation.Comment: 22 pages, 15 figures, 2 tables, published in MNRAS, We corrected a bug in the script for producing table 2 and figure 1

The impact of electron-capture supernovae on merging double neutron stars

Natal kicks are one of the most debated issues about double neutron star (DNS) formation. Several observational and theoretical results suggest that some DNSs have formed with low natal kicks ($\lesssim{}50$ km s$^{-1}$), which might be attributed to electron-capture supernovae (ECSNe). We investigate the impact of ECSNe on the formation of DNSs by means of population synthesis simulations. In particular, we assume a Maxwellian velocity distribution for the natal kick induced by ECSNe with one dimensional root-mean-square $\sigma_{\rm ECSN} = 0,7,15,26,265$ km s$^{-1}$. The total number of DNSs scales inversely with $\sigma_{\rm ECSN}$ and the number of DNS mergers is higher for relatively low kicks. This effect is particularly strong if we assume low efficiency of common-envelope ejection (described by the parameter $\alpha=1$), while it is only mild for high efficiency of common-envelope ejection ($\alpha{}=5$). In most simulations, more than 50 per cent of the progenitors of merging DNSs undergo at least one ECSN and the ECSN is almost always the first SN occurring in the binary system. Finally, we have considered the extreme case in which all neutron stars receive a low natal kick ($\lesssim{}50$~km~s$^{-1}$). In this case, the number of DNSs increases by a factor of ten and the percentage of merging DNSs which went through an ECSN is significantly suppressed ($<40$ per cent).Comment: 11 pages, 7 figures, 1 tables, to appear in MNRA

The High Mass X-ray Binaries in star-forming galaxies

The high mass X-ray binaries (HMXBs) provide an exciting framework to investigate the evolution of massive stars and the processes behind binary evolution. HMXBs have shown to be good tracers of recent star formation in galaxies and might be important feedback sources at early stages of the Universe. Furthermore, HMXBs are likely the progenitors of gravitational wave sources (BH--BH or BH--NS binaries that may merge producing gravitational waves). In this work, we investigate the nature and properties of HMXB population in star-forming galaxies. We combine the results from the population synthesis model MOBSE (Giacobbo et al. 2018) together with galaxy catalogs from EAGLE simulation (Schaye et al. 2015). Therefore, this method describes the HMXBs within their host galaxies in a self-consistent way. We compute the X-ray luminosity function (XLF) of HMXBs in star-forming galaxies, showing that this methodology matches the main features of the observed XLF.Comment: 4 pages, 2 figures. To appear in Proc. IAUS 346: High-mass X-ray binaries: illuminating the passage from massive binaries to merging compact object

Constraining the fraction of binary black holes formed in isolation and young star clusters with gravitational-wave data

Ten binary black-hole mergers have already been detected during the first two observing runs of advanced LIGO and Virgo, and many more are expected to be observed in the near future. This opens the possibility for gravitational-wave astronomy to better constrain the properties of black hole binaries, not only as single sources, but as a whole astrophysical population. In this paper, we address the problem of using gravitational-wave measurements to estimate the proportion of merging black holes produced either via isolated binaries or binaries evolving in young star clusters. To this end, we use a Bayesian hierarchical modeling approach applied to catalogs of merging binary black holes generated using state-of-the-art population synthesis and N-body codes. In particular, we show that, although current advanced LIGO/Virgo observations only mildly constrain the mixing fraction $f \in [0,1]$ between the two formation channels, we expect to narrow down the fractional errors on $f$ to $10-20\%$ after a few hundreds of detections.Comment: 17 pages, 4 figure

Fingerprints of Binary Black Hole Formation Channels Encoded in the Mass and Spin of Merger Remnants

Binary black holes (BBHs) are thought to form in different environments, including the galactic field and (globular, nuclear, young, and open) star clusters. Here, we propose a method to estimate the fingerprints of the main BBH formation channels associated with these different environments. We show that the metallicity distribution of galaxies in the local universe along with the relative amount of mergers forming in the field or in star clusters determine the main properties of the BBH population. Our fiducial model predicts that the heaviest merger to date, GW170729, originated from a progenitor that underwent 2–3 merger events in a dense star cluster, possibly a galactic nucleus. The model predicts that at least one merger remnant out of a hundred BBH mergers in the local universe has mass , and one in a thousand can reach a mass as large as . Such massive black holes would bridge the gap between stellar-mass and intermediate-mass black holes. The relative number of low- and high-mass BBHs can help us unravel the fingerprints of different formation channels. Based on the assumptions of our model, we expect that isolated binaries are the main channel of BBH merger formation if of the whole BBH population has remnants with masses , whereas % of remnants having masses points to a significant subpopulation of dynamically formed BBH binaries

The cosmic merger rate density of compact objects: impact of star formation, metallicity, initial mass function and binary evolution

We evaluate the redshift distribution of binary black hole (BBH), black hole - neutron star binary (BHNS) and binary neutron star (BNS) mergers, exploring the main sources of uncertainty: star formation rate (SFR) density, metallicity evolution, common envelope, mass transfer via Roche lobe overflow, natal kicks, core-collapse supernova model and initial mass function. Among binary evolution processes, uncertainties on common envelope ejection have a major impact: the local merger rate density of BNSs varies from $\sim{}10^3$ to $\sim{}20$ Gpc$^{-3}$ yr$^{-1}$ if we change the common envelope efficiency parameter from $\alpha_{\rm CE}=7$ to 0.5, while the local merger rates of BBHs and BHNSs vary by a factor of $\sim{}2-3$. The BBH merger rate changes by one order of magnitude, when $1 \sigma$ uncertainties on metallicity evolution are taken into account. In contrast, the BNS merger rate is almost insensitive to metallicity. Hence, BNSs are the ideal test bed to put constraints on uncertain binary evolution processes, such as common envelope and natal kicks. Only models assuming values of $\alpha_{\rm CE}\gtrsim{}2$ and moderately low natal kicks (depending on the ejected mass and the SN mechanism), result in a local BNS merger rate density within the 90% credible interval inferred from the second gravitational-wave transient catalogue.Comment: 14 pages, 12 figures, 2 tables, accepted for publication in MNRA

Merging black hole binaries with the SEVN code

Studying the formation and evolution of black hole binaries (BHBs) is essential for the interpretation of current and forthcoming gravitational wave (GW) detections. We investigate the statistics of BHBs that form from isolated binaries, by means of a new version of the SEVN population-synthesis code. SEVN integrates stellar evolution by interpolation over a grid of stellar evolution tracks. We upgraded SEVN to include binary stellar evolution processes and we used it to evolve a sample of 1.5 x 10(8) binary systems, with metallicity in the range [10(-4); 4 x 10(-2)]. From our simulations, we find that the mass distribution of black holes (BHs) in double compact-object binaries is remarkably similar to the one obtained considering only single stellar evolution. The maximum BH mass we obtain is similar to 30, 45, and 55 M-circle dot at metallicity Z = 2 x 10(-2), 6 x 10(-3), and 10(-4), respectively. A few massive single BHs may also form (less than or similar to 0.1 per cent of the total number of BHs), with mass up to similar to 65, 90, and 145 M-circle dot at Z = 2 x 10(-2), 6 x 10(-3), and 10(-4), respectively. These BHs fall in the mass gap predicted from pair-instability supernovae. We also show that the most massive BHBs are unlikely to merge within a Hubble time. In our simulations, merging BHs like GW151226 and GW170608, form at all metallicities, the high-mass systems (like GW150914, GW170814, and GW170104) originate from metal-poor (Z less than or similar to 6 x 10(-3)) progenitors, whereas GW170729-like systems are hard to form, even at Z = 10(-4). The BHB merger rate in the local Universe obtained from our simulations is similar to 90Gpc(-3)yr(-1), consistent with the rate inferred from LIGO-Virgo data