Constraining the formation of black-holes in short-period Black-Hole Low-Mass X-ray Binaries

The formation of stellar mass black holes is still very uncertain. Two main uncertainties are the amount of mass ejected in the supernova event (if any) and the magnitude of the natal kick the black hole receives at birth (if any). Repetto et al. (2012), studying the position of Galactic X-ray binaries containing black holes, found evidence for black holes receiving high natal kicks at birth. In this Paper we extend that study, taking into account the previous binary evolution of the sources as well. The seven short-period black-hole X-ray binaries that we use, are compact binaries consisting of a low-mass star orbiting a black hole in a period less than $1$ day. We trace their binary evolution backwards in time, from the current observed state of mass-transfer, to the moment the black hole was formed, and we add the extra information on the kinematics of the binaries. We find that several systems could be explained by no natal kick, just mass ejection, while for two systems (and possibly more) a high kick is required. So unless the latter have an alternative formation, such as within a globular cluster, we conclude that at least some black holes get high kicks. This challenges the standard picture that black hole kicks would be scaled down from neutron star kicks. Furthermore, we find that five systems could have formed with a non-zero natal kick but zero mass ejected (i.e. no supernova) at formation, as predicted by neutrino-driven natal kicks.Comment: 15 pages, 14 figures, accepted for publication in MNRA

Investigating stellar-mass black hole kicks

We investigate whether stellar-mass black holes have to receive natal kicks in order to explain the observed distribution of low-mass X-ray binaries containing black holes within our Galaxy. Such binaries are the product of binary evolution, where the massive primary has exploded forming a stellar-mass black hole, probably after a common envelope phase where the system contracted down to separations of order 10-30 Rsun. We perform population synthesis calculations of these binaries, applying both kicks due to supernova mass-loss and natal kicks to the newly-formed black hole. We then integrate the trajectories of the binary systems within the Galactic potential. We find that natal kicks are in fact necessary to reach the large distances above the Galactic plane achieved by some binaries. Further, we find that the distribution of natal kicks would seem to be similar to that of neutron stars, rather than one where the kick velocities are reduced by the ratio of black hole to neutron-star mass (i.e. where the kicks have the same momentum). This result is somewhat surprising; in many pictures of stellar-mass black-hole formation, one might have expected black holes to receive kicks having the same momentum (rather than the same speed) as those given to neutron stars.Comment: 13 pages, 8 figures, 4 tables. Accepted for publication in MNRA

The formation of black-holes derived from X-ray binaries

This Thesis revolves around the topic of black holes (BHs) in X-ray binaries (XRBs). The trigger of this work was to understand how stellar-mass BHs form, a question which we tackled both with theoretical as well as observational studies. The formation mechanism of BHs is an unsolved problem in high-energy astrophysics, in particular when it comes to the amount of mass ejected at formation and to how large is the velocity BHs acquire at birth (the so-called natal kick), if they acquire a velocity at all. One way of tackling these uncertainties is to perform physically-motivated simulations of the core-collapse supernova, which is computationally challenging. Another way is to study the birth and evolution of XRBs hosting a BH accreting from a stellar companion, which is the method we use in this Thesis. On the theoretical side, we combine an approach typical of binary population synthesis calculations with a more detailed approach which solves for the binary evolution of each source. When possible, we describe the different evolutionary paths of the binary via analytical calculations and timescale arguments. On the observational side, we combine the techniques of optical spectroscopy and photometry, with the goal of increasing the sample of Galactic BHs

Virtual reality as a potential tool to face frailty challenges

The aging population and the corresponding increase in age-related diseases present scientific community and public health authorities with imminent challenges. One of these challenges deals with a deeper understanding of functional status of elderly in order to prevent and/or delay the onset of late-life disability (Rodr\uedguez-Artalejo and Rodr\uedguez-Ma\uf1as, 2014). The syndrome of \u201cfrailty\u201d has been recently introduced in literature to specifically characterize the health of older individuals who deserve special attention because of their increased vulnerability to adverse health outcomes (Afilalo et al., 2010). Although there is not a unique definition of frailty (Morley et al., 2013), the majority of studies refers to the five operational criteria (Fried et al., 2001): decreased gait speed, reduced grip strength, prolonged and unmotivated exhaustion, low physical activity, unintended weight loss. The problem of different definitions leads also to a large variation in reported prevalence rates, which range approximately from 5 to 60% (Collard et al., 2012). However, this multifaceted decline in different physiological systems make frail older individuals progressively more exposed to stressors (Clegg et al., 2013), making urgent the need for better care intervention

Investigating Black Hole Kicks

It has been known for some time that neutron stars receive kicks (so-called natal kicks) when they are formed in core-collapse supernovae. Whether black holes receive these kicks too is still a matter of debate. We study Galactic low-mass X-ray binaries containing a black hole as the accreting object and look at their position within the Galaxy: some systems are almost coplanar, while others are found in the halo. Starting from sensible guesses on the initial binary properties and assuming the objects to be originated in the plane of the Galaxy, we perform a series of Monte Carlo simulations in which we calculate the trajectories of low-mass X-ray binary systems that receive a kick when the progenitor of the black hole explodes as a supernova, and determine their resulting location in the Galaxy. The comparison between the simulated distribution and the observed one leads us to conclude that a natal kick is indeed required for the formation of the systems

Antiretroviral treatment in pregnancy: a six-year perspective on recent trends in prescription patterns, viral load suppression, and pregnancy outcomes

The aim of the study was to describe the recent trends in antiretroviral treatment in late pregnancy and the sociodemographic changes among pregnant women with HIV over the last 6 years. Data from the National Program on Surveillance on Antiretroviral Treatment in Pregnancy in Italy were grouped per calendar year, and changes in antiretroviral treatment, population characteristics, maternal immunovirologic status and newborn clinical parameters were analyzed. A total of 981 HIV-infected mothers who delivered between 2002 and 2008 were evaluated. The proportion of women receiving at least three antiretroviral drugs at delivery increased significantly from 63.0% in 2002 to 95.5% in 2007-2008, paralleled by a similar upward trend in the proportion of women who achieved complete viral suppression at third trimester (from 37.3 in 2002 to 80.9 in 2007-2008; p < 0.001). The co-formulation of zidovudine plus lamivudine remained the most common nucleoside backbone in pregnancy, even if a significant increase in the use of tenofovir plus emtricitabine was observed in more recent years. Starting from 2003, nevirapine prescription declined, paralleled by a significant rise in the use of protease inhibitors (PI), which were present in more than 60% of regimens administered in 2007-2008. Nelfinavir was progressively replaced by ritonavir-boosted PIs, mainly lopinavir. No significant changes in preterm delivery, Apgar score, birth weight, and birth defects were observed during the study period, and the rate of HIV transmission remained below 2%. These data demonstrate a significant evolution in the treatment of HIV in pregnancy. Constant improvements in the rates of HIV suppression were observed, probably driven by the adoption of stronger and more effective regimens and by the increasing options available for combination treatmen