Contact Moishezon threefolds with second Betti number one

We prove that the only contact Moishezon threefold having second Betti number equal to one is the projective space.Comment: 5 pages. v2: exposition improved as suggested by the referee. To appear in Archiv der Mat

Cohomological characterizations of projective spaces and hyperquadrics

We confirm Beauville's conjecture that claims that if the p-th exterior power of the tangent bundle of a smooth projective variety contains the p-th power of an ample line bundle, then the variety is either the projective space or the p-dimensional quadric hypersurface.Comment: Added Lemma 2.8 and slightly changed proof of Lemma 6.2 to make them apply for torsion-free sheaves and not only to vector bundle

The puzzling case of the accreting millisecond X-ray pulsar IGR J00291+5934: flaring optical emission during quiescence

We present an optical (gri) study during quiescence of the accreting millisecond X-ray pulsar IGR J00291+5934 performed with the 10.4m Gran Telescopio Canarias (GTC) in August 2014. Despite the source being in quiescence at the time of our observations, it showed a strong optical flaring activity, more pronounced at higher frequencies (i.e. the g band). Once the flares were subtracted, we tentatively recovered a sinusoidal modulation at the system orbital period in all bands, even if a significant phase shift with respect to an irradiated star, typical of accreting millisecond X-ray pulsars is detected. We conclude that the observed flaring could be a manifestation of the presence of an accretion disc in the system. The observed light curve variability could be explained by the presence of a superhump, which might be another proof of the formation of an accretion disc. In particular, the disc at the time of our observations was probably preparing to the new outburst of the source, that happened just a few months later, in 2015.Comment: 6 pages, 2 figures, 1 table. Accepted for publication in A&

On the bolometric quiescent luminosity and luminosity swing of black hole candidate and neutron star low mass X-ray transients

Low mass X-ray transients (LMXRTs) hosting black hole candidates (BHCs) display on average a factor of ~100 larger swing in the minimum (quiescent) to maximum (outburst) X-ray luminosity than neutron stars (NSs), despite the fact that the swing in the mass inflow rate is likely in the same range. Advection dominated accretion flows (ADAFs) were proposed to interpret such a difference. The residual optical/UV emission of quiescent LMXRTs, after subtraction of the companion star spectrum, is produced by synchrotron radiation in the (latest version) of ADAF and therefore is part of the ADAF's luminosity budget. We demonstrate that, once the residual optical/UV emission is taken into account, the bolometric luminosity swing of BHCs is consistent with that of NSs. We explore here an alternative scenario to ADAFs in which very little mass accretion onto the collapsed star takes place in the quiescence intervals. The residual optical/UV emission of BHCs are expected to derive from the energy released by the matter transferred from the companion star at radii comparable to the circularisation radius. The quiescent X-ray luminosity originates either from accretion onto the BH at very low rates and/or from coronal activity in the companion star or in the outer disk. For comparably small mass inflow rates, the NSs in these systems are likely in the radio pulsar regime. In the interaction of the radio pulsar relativistic wind with matter transferred from the companion star, a shock forms, the power law-like emission of which powers both the harder X-ray emission and most of the residual optical/UV. The soft, thermal-like X-ray component may arise from the cooling of the NS surface. This scenario matches well both the X-ray and bolometric luminosity swing of LMXRTs. (ABRIDGED).Comment: 13 pages (including 2 postscript figures - use emulateapj macro). Accepted for publication in Ap

XTE J1946+274 = GRO J1944+26: An Enigmatic Be/X-ray Binary

XTE J1946+274 = GRO J1944+26 is a 15.8 s Be/X-ray pulsar discovered simultaneously in 1998 September with the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO) and the All-Sky Monitor (ASM) on the Rossi X-ray Timing Explorer (RXTE). Here we present new results from BATSE and {\em RXTE} including a pulse timing analysis, spectral analysis, and evidence for an accretion disk. Our pulse timing analysis yielded an orbital period of 169.2 days, a moderate eccentricity of 0.33, and implied a mass function of 9.7 M_sun. We observed evidence for an accretion disk, a correlation between measured spin-up rate and flux, which was fitted to obtain a distance estimate of 9.5 +/- 2.9 kpc. XTE J1946+274 remained active from 1998 September - 2001 July, undergoing 13 outbursts that were not locked in orbital phase. Comparing RXTE PCA observations from the initial bright outburst in 1998 and the last pair of outbursts in 2001, we found energy and intensity dependent pulse profile variations in both outbursts and hardening spectra with increasing intensity during the fainter 2001 outbursts. In 2001 July, optical Halpha observations indicate a density perturbation appeared in the Be disk as the X-ray outbursts ceased. We propose that the equatorial plane of the Be star is inclined with respect to the orbital plane in this system and that this inclination may be a factor in the unusual outburst behavior of the system.Comment: 18 pages, 15 figures, To appear in ApJ v584, Feb 20, 2003 issu

X-ray iron line variability for the model of an orbiting flare above a black hole accretion disc

The broad X-ray iron line, detected in many active galactic nuclei, is likely to be produced by fluorescence from the X-ray illuminated central parts of an accretion disc close to a supermassive black hole. The time-averaged shape of the line can be explained most naturally by a combination of special and general relativistic effects. Such line profiles contain information about the black hole spin and the accretion disc as well as the geometry of the emitting region and may help to test general relativity in the strong gravity regime. In this paper we embark on the computation of the temporal response of the line to the illuminating flux. Previous studies concentrated on the calculation of reverberation signatures from static sources illuminating the disc. In this paper we focus on the more physically justified case of flares located above the accretion disc and corotating with it. We compute the time dependent iron line taking into account all general relativistic effects and show that its shape is of very complex nature, and also present light curves accompanying the iron line variability. We suggest that future X-ray satellites like XMM or Constellation-X may be capable of detecting features present in the computed reverberation maps.Comment: Accepted for publication in MNRAS, 11 pages, 12 figure

Dense conjugate initialization for deterministic PSO in applications: ORTHOinit+

This paper describes a class of novel initializations in Deterministic Particle Swarm Optimization (DPSO) for approximately solving costly unconstrained global optimization problems. The initializations are based on choosing specific dense initial positions and velocities for particles. These choices tend to induce in some sense orthogonality of particles’ trajectories, in the early iterations, in order to better explore the search space. Our proposal is inspired by both a theoretical analysis on a reformulation of PSO iteration, and by possible limits of the proposals reported in Campana et al. (2010); Campana et al. (2013). We explicitly show that, in comparison with other initializations from the literature, our initializations tend to scatter PSO particles, at least in the first iterations. The latter goal is obtained by imposing that the initial choice of particles’ position/velocity satisfies specific conjugacy conditions, with respect to a matrix depending on the parameters of PSO. In particular, by an appropriate condition on particles’ velocities, our initializations also resemble and partially extend a general paradigm in the literature of exact methods for derivative-free optimization. Moreover, we propose dense initializations for DPSO, so that the final approximate global solution obtained is possibly not too sparse, which might cause troubles in some applications. Numerical results, on both Portfolio Selection and Computational Fluid Dynamics problems, validate our theory and prove the effectiveness of our proposal, which applies also in case different neighborhood topologies are adopted in DPSO

Tracing day-zero and forecasting the COVID-19 outbreak in Lombardy, Italy: A compartmental modelling and numerical optimization approach

Introduction Italy became the second epicenter of the novel coronavirus disease 2019 (COVID-19) pandemic after China, surpassing by far China’s death toll. The disease swept through Lombardy, which remained in lockdown for about two months, starting from the 8th of March. As of that day, the isolation measures taken in Lombardy were extended to the entire country. Here, assuming that effectively there was one case “zero” that introduced the virus to the region, we provide estimates for: (a) the day-zero of the outbreak in Lombardy, Italy; (b) the actual number of asymptomatic infected cases in the total population until March 8; (c) the basic (R0)and the effective reproduction number (Re) based on the estimation of the actual number of infected cases. To demonstrate the efficiency of the model and approach, we also provide a tentative forecast two months ahead of time, i.e. until May 4, the date on which relaxation of the measures commenced, on the basis of the COVID-19 Community Mobility Reports released by Google on March 29. Methods To deal with the uncertainty in the number of the actual asymptomatic infected cases in the total population Volpert et al. (2020), we address a modified compartmental Susceptible/ Exposed/ Infectious Asymptomatic/ Infected Symptomatic/ Recovered/ Dead (SEIIRD) model with two compartments of infectious persons: one modelling the cases in the population that are asymptomatic or experience very mild symptoms and another modelling the infected cases with mild to severe symptoms. The parameters of the model corresponding to the recovery period, the time from the onset of symptoms to death and the time from exposure to the time that an individual starts to be infectious, have been set as reported from clinical studies on COVID-19. For the estimation of the day-zero of the outbreak in Lombardy, as well as of the “effective” per-day transmission rate for which no clinical data are available, we have used the proposed SEIIRD simulator to fit the numbers of new daily cases from February 21 to the 8th of March. This was accomplished by solving a mixed-integer optimization problem. Based on the computed parameters, we also provide an estimation of the basic reproduction number R0 and the evolution of the effective reproduction number Re. To examine the efficiency of the model and approach, we ran the simulator to “forecast” the epidemic two months ahead of time, i.e. from March 8 to May 4. For this purpose, we considered the reduction in mobility in Lombardy as released on March 29 by Google COVID-19 Community Mobility Reports, and the effects of social distancing and of the very strict measures taken by the government on March 20 and March 21, 2020. Results Based on the proposed methodological procedure, we estimated that the expected day-zero was January 14 (min-max rage: January 5 to January 23, interquartile range: January 11 to January 18). The actual cumulative number of asymptomatic infected cases in the total population in Lombardy on March 8 was of the order of 15 times the confirmed cumulative number of infected cases, while the expected value of the basic reproduction number R0 was found to be 4.53 (min-max range: 4.40- 4.65). On May 4, the date on which relaxation of the measures commenced the effective reproduction number was found to be 0.987 (interquartiles: 0.857, 1.133). The model approximated adequately two months ahead of time the evolution of reported cases of infected until May 4, the day on which the phase I of the relaxation of measures was implemented over all of Italy. Furthermore the model predicted that until May 4, around 20% of the population in Lombardy has recovered (interquartile range: *10% to *30%)

Pulsating in unison at optical and X-ray energies: simultaneous high-time resolution observations of the transitional millisecond pulsar PSR J1023+0038

PSR J1023+0038 is the first millisecond pulsar discovered to pulsate in the visible band; such a detection took place when the pulsar was surrounded by an accretion disk and also showed X-ray pulsations. We report on the first high time resolution observational campaign of this transitional pulsar in the disk state, using simultaneous observations in the optical (TNG, NOT, TJO), X-ray (XMM-Newton, NuSTAR, NICER), infrared (GTC) and UV (Swift) bands. Optical and X-ray pulsations were detected simultaneously in the X-ray high intensity mode in which the source spends $\sim$ 70% of the time, and both disappeared in the low mode, indicating a common underlying physical mechanism. In addition, optical and X-ray pulses were emitted within a few km, had similar pulse shape and distribution of the pulsed flux density compatible with a power-law relation $F_{\nu} \propto \nu^{-0.7}$ connecting the optical and the 0.3-45 keV X-ray band. Optical pulses were detected also during flares with a pulsed flux reduced by one third with respect to the high mode; the lack of a simultaneous detection of X-ray pulses is compatible with the lower photon statistics. We show that magnetically channeled accretion of plasma onto the surface of the neutron star cannot account for the optical pulsed luminosity ($\sim 10^{31}$ erg/s). On the other hand, magnetospheric rotation-powered pulsar emission would require an extremely efficient conversion of spin-down power into pulsed optical and X-ray emission. We then propose that optical and X-ray pulses are instead produced by synchrotron emission from the intrabinary shock that forms where a striped pulsar wind meets the accretion disk, within a few light cylinder radii away, $\sim$ 100 km, from the pulsar.Comment: 26 pages, 14 figures, first submitted to ApJ on 2019, January 1