arXiv subjects

M. Colpi

Publications and source records attributed to M. Colpi.

At least 73 records · Page 4Linked to original sources

arXiv2006

Variability of ultraluminous X-ray sources in the Cartwheel Ring

The Cartwheel is one of the most outstanding examples of a dynamically perturbed galaxy where star formation is occurring inside the ring--like structure. In previous studies with Chandra, we detected 16 Ultra Luminous X-ray sources lying along the southern portion of the ring. Their Luminosity Function is consistent with them being in the high luminosity tail of the High Mass X-ray Binaries distribution, but with one exception: source N.10. This source, detected with Chandra at L_X = 1.x 10^(41) ergs, is among the brightest non--nuclear sources ever seen in external galaxies. Recently, we have observed the Cartwheel with XMM-Newton in two epochs, six months apart. After having been at its brightest for at least 4 years, the source has dimmed by at least a factor of two between the two observations. This fact implies that the source is compact in nature. Given its extreme isotropic luminosity, there is the possibility that the source hosts an accreting intermediate--mass black hole. Other sources in the ring vary in flux between the different datasets. We discuss our findings in the context of ULX models.

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arXiv2006

The radial distribution of blue straggler stars and the nature of their progenitors

The origin of blue straggler stars (BSS) in globular clusters (GCs) is still not fully understood: they can form from stellar collisions, or through mass transfer in isolated, primordial binaries (PBs). In this paper we use the radial distribution of BSS observed in four GCs (M3, 47Tuc, NGC6752 and omega Cen) to investigate which formation process prevails. We find that both channels co-exist in all the considered GCs. The fraction of mass-transfer (collisional) BSS with respect to the total number of BSS is around ~0.4-0.5 (~0.5-0.6) in M3, 47Tuc, and NGC6752. The case of omega Cen is peculiar with an underproduction of collisional BSS. The relative lack of collisional BSS in omega Cen can be understood if mass segregation has not yet driven to the core a sizeable number of PBs, which dominate stellar collisions through three- and four-body processes. The spatial distribution of BSS provides strong hints to their origin: the BSS in the cluster outskirts form almost exclusively from mass transfer in PBs, whereas the BSS found close to the cluster core most likely have a collisional origin.

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arXiv2006

On the compact nature of the most luminous ULX in the Cartwheel ring

We report the first detection of flux variability in the most luminous X-ray source in the southern ring of the Cartwheel galaxy. XMM--Newton data show that the luminosity has varied over a timescale of six months from L[0.5-10] keV \~1.3 10^{41} erg/s, consistent with the previous Chandra observation, to L[0.5-10] keV < 6.4 x10^{40} erg/s. This fact provides the first evidence that the source is compact in nature and is not a collection of individual fainter sources, such as supernova remnants. The source has been repeatedly observed at the very high luminosity level of L[0.5-10] keV ~1.3 x 10^{41} erg/s, for a period of at least 4 years before dimming at the current level. It represents then the first example of an accreting object revealed in a long lived state of extremely high luminosity.

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arXiv2006

On the search of electromagnetic cosmological counterparts to coalescences of massive black hole binaries

We explore the nature of possible electromagnetic counterparts of coalescences of massive black hole binaries at cosmological distances detectable by the Laser Interferometer Space Antenna (LISA). An electromagnetic precursor, during the last year of gravitational wave (GW)-driven inspiral, or an afterglow within few years after coalescence, may highlight the position in the sky of galaxies hosting LISA sources. We show that observations of precursors and afterglows are mutually exclusive, depending on the mass of the primary black hole. Precursors are expected to occur in binaries where the primary (more massive) black hole is heavier than ~10^7 \Msun. They may correspond to on-off states of accretion, i.e., to a bright X-ray source decaying into quiescence before black hole coalescence, and are likely associated to disturbed galaxies showing signs of ongoing starbursts. Coalescences of lighter binaries, with masses <5x10^6 \Msun, lack of any precursor, as gas is expected to be consumed long before the GW-driven orbital decay. Such events would not be hosted by (massive) galaxies with an associated starburst, given the slow binary inspiral time compared to the typical time scale of starbursts. By contrast, coalescence, for such light binaries, is followed by an electromagnetic afterglow, i.e., an off-on accretion state rising in <20 yrs. Using a cosmological merger tree algorithm, we show that future X-ray missions such as XEUS will be able to identify, in 20 yrs operation, almost all the massive BH binary detectable by LISA, and, in only 5 yrs, all the LISA sources at z>6.

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arXiv2005

LISA double black holes: Dynamics in gaseous nuclear discs

We study the inspiral of double black holes, with masses in the LISA window of detectability, orbiting inside a massive circum-nuclear disc. Using high-resolution SPH simulations, we follow the black hole dynamics in the early phase when gas-dynamical friction acts on the black holes individually, and continue our simulation until they form a close binary. We find that in the early sinking the black holes lose memory of their initial orbital eccentricity if they co-rotate with the gaseous disc, forming a binary with a low eccentricity, consistent with zero within our numerical resolution limit. The cause of circularization resides in the rotation present in the gaseous background where dynamical friction operates. Circularization may hinder gravitational waves from taking over and leading the binary to coalescence. In the case of counter-rotating orbits the initial eccentricity does not decrease, and the black holes may bind forming an eccentric binary. When dynamical friction has subsided, for equal mass black holes and regardless their initial eccentricity, angular momentum loss, driven by the gravitational torque exerted on the binary by surrounding gas, is nevertheless observable down to the smallest scale probed. In the case of unequal masses, dynamical friction remains efficient down to our resolution limit, and there is no sign of formation of any ellipsoidal gas distribution that may further harden the binary. During inspiral, gravitational capture of gas by the black holes occurs mainly along circular orbits: eccentric orbits imply high relative velocities and weak gravitational focusing. Thus, AGN activity may be excited during the black hole pairing process and double active nuclei may form when circularization is completed, on distance-scales of tens of pcs.

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arXiv2005

The fingerprint of binary intermediate mass black holes in globular clusters: supra-thermal stars and angular momentum alignment

We explore the signatures that a binary intermediate mass black hole (IMBH) imprints on the velocity and on the angular momentum of globular cluster stars. Simulating 3-body encounters between a star and a binary IMBH, we find that the binary IMBH generates a family of few hundreds of stars (~100-300) which remain bound to the globular cluster (GC) and have velocity significantly higher than the dispersion velocity. For this reason we term them ''supra-thermal'' stars. We also notice that, after the interaction, a considerable fraction (55-70%) of stars tend to align their orbital angular momentum with the angular momentum of the binary IMBH, introducing an anisotropy in the angular momentum distribution of cluster stars. We simulate the dynamical evolution of these supra-thermal stars before thermalization, and find that these stars tend to cluster at a distance of few core radii from the GC center. We conclude that the detectability of such signatures appears problematic with present telescopes.

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arXiv2005

Radio pulsars around intermediate mass black holes in super stellar clusters

We study accretion in binaries hosting an intermediate mass black hole (IMBH) of 1000 solar masses, and a donor star more massive than 15 solar masses. These systems experience an active X-ray phase characterized by luminosities varying over a wide interval, from <10^36 erg/s up to a few 10^40 erg/s typical of the ultra luminous X-ray sources (ULXs). Roche lobe overflow on the zero-age main sequence and donor masses above 20 solar masses can maintain a long-lived accretion phase at the level required to feed a ULX source. In wide systems, wind transfer rates are magnified by the focusing action of the IMBH yielding wind luminosities around 10^38 erg/s. These high mass-IMBH binaries can be identified as progenitors of IMBH-radio pulsar (PSR) binaries. We find that the formation of an IMBH-PSR binary does not necessarely require the transit through a ULX phase, but that a ULX can highlight a system that will evolve into an IMBH-PSR, if the mass of the donor star is constrained to lie within 15 to 30 solar masses. We show that binary evolution delivers the pre-exploding helium core in an orbit such that after explosion, the neutron star has a very high probability to remain bound to the IMBH, at distances of 1-10 AU. The detection of an IMBH-PSR binary in the Milky Way has suffered, so far, from the same small number of statistics limit affecting the population of ULXs in our Galaxy. Ongoing deeper surveys or next generation radio telescopes like SKA will have an improved chance to unveil such intriguing systems. Timing analysis of a pulsar orbiting around an IMBH would weigh the black hole in the still uncharted interval of mass around 1000 solar masses

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arXiv2005

The dynamical fingerprint of intermediate mass black holes in globular clusters

A number of observations hints for the presence of an intermediate mass black hole (IMBH) in the core of three globular clusters: M15 and NGC 6752 in the Milky Way, and G1, in M31. However the existence of these IMBHs is far form being conclusive. In this paper, we review their main formation channels and explore possible observational signs that a single or binary IMBH can imprint on cluster stars. In particular we explore the role played by a binary IMBH in transferring angular momentum and energy to stars flying by.

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arXiv2004

Young close-by neutron stars: the Gould Belt vs. the Galactic disc

We present new population synthesis calculations of close young neutron stars. In comparison with our previous investigation we use a different neutron star mass spectrum and different initial spatial and velocity distributions. The results confirm that most of ROSAT dim radioquiet isolated neutron stars had their origin in the Gould Belt. Several tens of young neutron stars can be identified in future in ROSAT data at low galactic latitudes and some of them also can be EGRET unidentified sources.

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arXiv2004

The contribution of primordial binaries to the blue straggler population in 47 Tucanae

The recent observation (Ferraro et al. 2003b) of the blue straggler population in 47 Tucanae gives the first detailed characterization of their spatial distribution in the cluster over its entire volume. Relative to the light distribution, blue stragglers appear to be overabundant in the core and at large radii. The observed surface density profile shows a central peak, a zone of avoidance and a rise beyond twenty core radii. In light of these findings we explored the evolution of blue stragglers mimicking their dynamics in a multi-mass King model for 47 Tucanae. We find that the observed spatial distribution can not be explained within a purely collisional scenario in which blue stragglers are generated exclusively in the core through direct mergers. An excellent fit is obtained if we require that a sizable fraction of blue stragglers is generated in the peripheral regions of the cluster inside primordial binaries that evolve in isolation experiencing mass-transfer.

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arXiv2003

Log N -- Log S distributions of accreting and cooling isolated neutron stars

We model populations of isolated neutron stars in the Galaxy following their orbital and magneto-rotational evolution. It is shown that accretors become more abundant than coolers at fluxes below $\sim 10^{-13}$ erg cm$^{-2}$ s$^{-1},$ and one can predict that about one accreting neutron star per square degree should be observed at the {\it Chandra} and {\it Newton} flux limits of $\sim 10^{-16}$ erg cm$^{-2}$ s$^{-1}.$ The soft ROSAT sources associated with isolated neutron stars can be relatively young cooling objects only if the neutron star birth rate in the Solar vicinity during the last $\sim 10^6$ yr is higher than that inferred from radiopulsar observations.

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arXiv2003

The X-ray spectrum of the black hole candidate GX339-4 in a low state

We report on four observations of the black hole candidate GX 339-4 collected in the spectral range 0.4-140 keV with the BeppoSAX satellite during 1997. All our spectra are typical of a black hole candidate in its low-hard state, i.e. are dominated by the hard tail of the emission from the corona. Although the direct emission from the accretion disk in the soft X-ray range is weak, spectra are well described only with models that take into consideration the Compton reflection of hard X-rays from the corona onto the surface of an accretion disk of non negligible extent.

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arXiv2003

Young isolated neutron stars from the Gould Belt

The origin of the local population of young, cooling neutron stars is investigated with a population synthesis model taking into account the contribution of neutron stars born in the Gould Belt, in addition to those originating in the Galactic disk. We estimate their emission in the soft X-ray band as a function of distance and age and construct the Log N -- Log S distribution. It is shown that the inclusion of neutron stars from the Gould Belt provides a good fit to the observed Log N -- Log S distribution. As the Sun is situated inside the Gould Belt, one can naturally explain the comparative local overabundance of massive progenitors and can remove the difficulty of the deficit of relatively bright ($\ga 0.1$ ROSAT PSPC cts s$^{-1}$) cooling neutron stars previously reported from models where only neutron stars from the Galactic disk were accounted for.

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arXiv2002

Young compact objects in the Solar vicinity

We present Log N -- Log S distribution for close-by young isolated neutron stars. On the basis of this distribution it is shown that the seven ROSAT isolated neutron stars (if they are young cooling objects) are genetically related to the Gould Belt. We predict, that there are about few tens unidentified close-by young isolated neutron stars in the ROSAT All-Sky Survey. The possibility that these seven peculiar sources contain a neutron star less massive and more magnetized then in ordinary radiopulsars is also discussed. In the aftermath of relatively close recent supernova explosions (1 kpc around the Sun, a few Myrs ago), a few black holes might have been formed, according to the local initial mass function. We thus discuss the possibility of determining approximate positions of close-by isolated black holes using data on runaway stars and simple calculations of binary evolution and disruption.

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arXiv2002

An XMM-Newton study of the 401 Hz accreting pulsar SAX J1808.4-3658 in quiescence

SAX J1808.4-3658 is a unique source being the first Low Mass X-ray Binary showing coherent pulsations at a spin period comparable to that of millisecond radio pulsars. Here we present an XMM-Newton observation of SAX J1808.4-3658 in quiescence, the first which assessed its quiescent luminosity and spectrum with good signal to noise. XMM-Newton did not reveal other sources in the vicinity of SAX J1808.4-3658 likely indicating that the source was also detected by previous BeppoSAX and ASCA observations, even if with large positional and flux uncertainties. We derive a 0.5-10 keV unabsorbed luminosity of L_X=5x10^{31} erg/s, a relatively low value compared with other neutron star soft X-ray transient sources. At variance with other soft X-ray transients, the quiescent spectrum of SAX J1808.4-3658 was dominated by a hard (Gamma~1.5) power law with only a minor contribution (<10%) from a soft black body component. If the power law originates in the shock between the wind of a turned-on radio pulsar and matter outflowing from the companion, then a spin-down to X-ray luminosity conversion efficiency of eta~10^{-3} is derived; this is in line with the value estimated from the eclipsing radio pulsar PSR J1740-5340. Within the deep crustal heating model, the faintness of the blackbody-like component indicates that SAX J1808.4-3658 likely hosts a massive neutronstar (M>1.7 solar masses).

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arXiv2002

Young close isolated compact objects

We suggest that the seven radio-quiet isolated neutron stars observed with ROSAT are young cooling objects associated to recent near-by supernova explosions which formed runaway stars and the Local Bubble, affecting the topology of the interstellar medium in the vicinity of the Sun (within a few hundred parsecs). In the aftermath of these explosions, a few black holes might have been formed, according to the local initial mass function. We thus discuss the possibility of determining approximate positions of close-by isolated black holes using data on runaway stars and simple calculations of binary evolution and disruption.

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arXiv2001

The Magnificent Seven: Close-by Cooling Neutron Stars?

We model Galactic populations of accreting and cooling isolated neutron stars in the attempt to explore their link with a new class of dim soft X-ray sources revealed by ROSAT. For accretors we follow the magneto-rotational and dynamical evolution in the Galactic potential and a realistic large scale distribution of the interstellar medium is used. Under standard assumptions old neutron stars enter the accretor stage only if their magnetic field exceeds $\approx 10^{11}$--$10^{12}$ G. We predict about 1 source per square degree for fluxes $\approx 10^{-15}$--$10^{-16}$ erg cm$^{-2}$s$^{-1}$ in the energy range 0.5-2 keV. Cooling neutron stars are explored within a simpler model of local sources, including however interstellar absorption. They are found to be significantly less abundant at low fluxes, $<0.1$ sources per square degree, but dominate over accretors at higher flux levels ($\approx 10^{-12}$--$10^{-11}$ erg cm$^{-2}$s$^{-1}$). We suggest that the faint sources observed by ROSAT may be young cooling neutron stars with typical age $\la 10^6$ yrs, if the total number of young neutron stars in the Solar proximity is $\sim 10$ times higher than inferred from radiopulsars statistics.

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arXiv2001

The transient X-ray pulsar 4U 0115+63 from quiescence to outburst through the centrifugal transition

We report on a BeppoSAX observation of the transient X-ray pulsar 4U 0115+63 close to periastron. This led to the discovery of a dramatic luminosity variation from ~2x10^34 erg/s to ~5x10^36 erg/s (factor ~250) in less than 15 hr. The variation was accompanied by only minor (if any) changes in the emitted spectrum and pulse fraction. On the contrary an observation near apastron detected the source in a nearly constant state at a level of ~2x10^33 erg/s. Direct accretion onto the neutron star surface encounters major difficulties in explaining the source variability properties. When the different regimes expected for a rotating magnetic neutron star subject to a variable inflow of matter from its companion are taken into consideration, the results of BeppoSAX observations of 4U 0115+63 can be explained naturally. In particular close to apastron, the regime of centrifugal inhibition of accretion applies, whereas the dramatic source flux variability observed close to periastron is readily interpreted as the transition regime between direct neutron star accretion and the propeller regime. In this centrifugal transition regime small variations of the mass inflow rate give rise to very large luminosity variations. We present a simple model for this transition, which we successfully apply to the X-ray flux and pulse fraction variations measured by BeppoSAX.

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