Monica Colpi

Low-frequency gravitational-wave science with eLISA/NGO

We review the expected science performance of the New Gravitational-Wave Observatory (NGO, a.k.a. eLISA), a mission under study by the European Space Agency for launch in the early 2020s. eLISA will survey the low-frequency gravitational-wave sky (from 0.1 mHz to 1 Hz), detecting and characterizing a broad variety of systems and events throughout the Universe, including the coalescences of massive black holes brought together by galaxy mergers; the inspirals of stellar-mass black holes and compact stars into central galactic black holes; several millions of ultra-compact binaries, both detached and mass transferring, in the Galaxy; and possibly unforeseen sources such as the relic gravitational-wave radiation from the early Universe. eLISAs high signal-to-noise measurements will provide new insight into the structure and history of the Universe, and they will test general relativity in its strong-field dynamical regime.

Rapid Formation of Supermassive Black Hole Binaries in Galaxy Mergers with Gas

Supermassive black holes (SMBHs) are a ubiquitous component of the nuclei of galaxies. It is normally assumed that after the merger of two massive galaxies, a SMBH binary will form, shrink because of stellar or gas dynamical processes, and ultimately coalesce by emitting a burst of gravitational waves. However, so far it has not been possible to show how two SMBHs bind during a galaxy merger with gas because of the difficulty of modeling a wide range of spatial scales. Here we report hydrodynamical simulations that track the formation of a SMBH binary down to scales of a few light years after the collision between two spiral galaxies. A massive, turbulent, nuclear gaseous disk arises as a result of the galaxy merger. The black holes form an eccentric binary in the disk in less than 1 million years as a result of the gravitational drag from the gas rather than from the stars.

Low metallicity and ultra-luminous X-ray sources in the Cartwheel galaxy

Low-metallicity (Z≲ 0.05xa0Z⊙) massive (≳40xa0M⊙) stars might end their life by directly collapsing into massive black holes (BHs, 30 ≲mBH/M⊙≲ 80). More than ∼105 massive BHs might have been generated via this mechanism in the metal-poor ring galaxy Cartwheel, during the last ∼107xa0yr. We show that such BHs might power most of the ultra-luminous X-ray sources (ULXs) observed in the Cartwheel. We also consider a sample of ULX-rich galaxies and we find a possible anticorrelation between the number of ULXs per galaxy and the metallicity in these galaxies. However, the data are not sufficient to draw any robust conclusions about this anticorrelation, and further studies are required.

Ultra-luminous X-ray sources and remnants of massive metal-poor stars

Massive metal-poor stars might form massive stellar black holes (BHs), with mass 25 ≤ m BH /M ⊙ ≤ 80, via direct collapse. We derive the number of massive BHs (N BH ) that are expected to form per galaxy through this mechanism. Such massive BHs might power most of the observed ultra-luminous X-ray sources (ULXs). We select a sample of 64 galaxies with X-ray coverage, measurements of the star formation rate (SFR) and of the metallicity. We find that N BH correlates with the number of observed ULXs per galaxy (N ULX ) in this sample. We discuss the dependence of our model on the SFR and on the metallicity. The SFR is found to be crucial, consistently with previous studies. The metallicity plays a role in our model, since a lower metallicity enhances the formation of massive BHs. Consistently with our model, the data indicate that there might be an anticorrelation between N ULX , normalized to the SFR, and the metallicity. A larger and more homogeneous sample of metallicity measurements is required, in order to confirm our results.

The relation between broad lines and γ-ray luminosities in Fermi blazars

We study the relation between the mass accretion rate, the jet power and the black hole mass of blazars. With this aim, we make use of the Sloan Digital Sky Survey and the 11-month catalogue of blazars detected at energies larger than 100 MeV by the Large Area Telescope onboard the Fermi satellite. This allows us to construct a relatively large sample of blazars with information about both the luminosity (or upper limits) of their emission lines (used as a proxy for the strength of the disc luminosity) and the luminosity of the high-energy emission (used as a proxy for the jet power). We find a good correlation between the luminosity of the broad lines and the γ-ray luminosities as detected by Fermi, both using the absolute values of the luminosities and normalizing them to the Eddington value. The data we have analysed confirm that the division of blazars into BL Lacertae objects (BL Lacs) and flat spectrum radio quasars (FSRQs) is controlled by the line luminosity in Eddington units. For small values of this ratio, the object is a BL Lac, while it is a FSRQ for large values. The transition appears to be smooth, but a much larger number of objects is needed to confirm this point.

Are ring galaxies the ancestors of giant low surface brightness galaxies

We simulate the collisional formation of a ring galaxy and we integrate its evolution up to 1.5 Gyr after the interaction. About 100-200 Myr after the collision, the simulated galaxy is very similar to observed ring galaxies (e.g. Cartwheel). After this stage, the ring keeps expanding and fades. Approximately 0.5-1 Gyr after the interaction, the disc becomes very large (similar to 100kpc) and flat. Such extended discs have been observed only in giant low surface brightness galaxies (GLSBs). We compare various properties of our simulated galaxies (surface brightness profile, morphology, H I spectrum and rotation curve) with the observations of four well-known GLSBs (UGC 6614, Malin 1, Malin 2 and NGC 75 89). The simulations match quite well the observations, suggesting that ring galaxies could be the progenitors of GLSBs. This result is crucial for the cold dark matter (CDM) model, as it was very difficult, so far, to explain the formation of GLSBs within the CDM scenario.

Intermediate-mass black holes and ultraluminous X-ray sources in the Cartwheel ring galaxy

Chandra and XMM-Newton observations of the Cartwheel galaxy show similar to 17 bright X-ray sources (greater than or similar to 5 x 10(38) erg s(-1)), all within the gas-rich outer ring. We explore the hypothesis that these X-ray sources are powered by intermediate-mass black holes (IMBHs) accreting gas or undergoing mass transfer from a stellar companion. To this purpose, we run N-body/smoothed particle hydrodynamics simulations of the galaxy interaction which might have led to the formation of Cartwheel, tracking the dynamical evolution of two different IMBH populations: halo and disc IMBHs. Halo IMBHs cannot account for the observed X-ray sources, as only a few of them cross the outer ring. Instead, more than half of the disc IMBHs are pulled in the outer ring as a consequence of the galaxy collision. However, also in the case of disc IMBHs, accretion from surrounding gas clouds cannot account for the high luminosities of the observed sources. Finally, more than 500 disc IMBHs are required to produce less than or similar to 15 X-ray sources via mass transfer from very young stellar companions. Such number of IMBHs is very large and implies extreme assumptions. Thus, the hypothesis that all the observed X-ray sources in Cartwheel are associated with IMBHs is hardly consistent with our simulations, even if it is still possible that IMBHs account for the few (less than or similar to 1-5) brightest ultraluminous X-ray sources.

1E 161348–5055 in the Supernova Remnant RCW 103: A Magnetar in a Young Low-Mass Binary System?

We suggest that the unique X-ray source 1E 161348?5055 at the center of the supernova remnant RCW 103 consists of a neutron star in close orbit with a low-mass main-sequence star. The time signature of 6.67 hr is interpreted as the neutron stars spin period. This requires the neutron star to be endowed with a high surface magnetic field of ~1015 G. Magnetic or/and material (propeller) torques are able to rapidly spin the young neutron star down to an asymptotic, equilibrium spin period in close synchronism with the orbital period, similar to what happens in the Polar cataclysmic variables. 1E 161348?5055 could be the first case of a magnetar born in a young low-mass binary system.

Dynamics of massive stellar black holes in young star clusters and the displacement of ultra-luminous X-ray sources

In low-metallicity environments, massive stars might avoid supernova explosions and directly collapse, forming massive (∼25–80xa0M⊙) stellar black holes (MSBHs), at the end of their life. MSBHs, when hosted in young massive clusters, are expected to form binaries and to strongly interact with stars, mainly via three-body encounters. We simulate various realizations of young star clusters hosting MSBHs in hard binaries with massive stars. We show that a large fraction (∼44xa0per cent) of MSBH binaries are ejected on a short time-scale (≤10xa0Myr). The offset of the ejected MSBHs with respect to the parent cluster is consistent with observations of X-ray binaries and ultra-luminous X-ray sources. Furthermore, three-body encounters change the properties of MSBH binaries: the semimajor axis changes by ≤50xa0per cent and the eccentricity of the system generally increases. We shortly discuss the implications of our simulations on the formation of high-mass X-ray binaries hosting MSBHs.

Two-temperature model of spherical accretion onto a black hole

Spherical accretion onto a black hole is studied, considering a two-temperature plasma model. The ion and electron temperature profiles are described by two energy balance equations coupled by an energy exchange term including only Coulomb interactions between the two populations. For accretion rates smaller than the Eddington one, the proton temperature profile is closely adiabAtic. at the Schwarzschild radius the temperature approaches 10/sup 12/ K. The electron temperature at large radii is close to that of protons. Approaching the black hole, the cooling mechanisms, namely opaque synchrotron radiation and multiple Compton scattering, limit the electron temperature to approx.10/sup 9/ K. For large accretion rates (tau/sub T/roughly-equal1) the protons deviate significantly from adiabatiity, because of the energy transfer to the electrons. The temperature at the Schwarzschild radius decreases by a factor approx.2. The threshold temperature for pion production which was just reached in the very thin cases is not met for tau/sub T/roughly-equal1. The maximum achievable ..gamma..-ray flux from ..pi../sup 0/-decay is approx.2 x 10/sup 33/ ergs s/sup -1/ for a 10 M/sub sun/ hole. The electron component always contributes most (by many orders of magnitude) to the overall luminosity with an efficiency epsilon = L/Mc/sup 2/roughly-equal5 x 10/sup -3/. Themorexa0» spectrum is basically a power law extending from the synchrotron transparency frequency at the inner radius (10/sup 12/--10/sup 13/ Hz) to 3kT (approx.1 MeV) with spectral index varying from 1 to 0.5 for tau/sub T/ = 10/sup -2/ and tau/sub T/ = 1, respectively. All results are essentially independent of the black hole mass.«xa0less