Sara Motta

Bio: Sara Motta is an academic researcher from University of Oxford. The author has contributed to research in topics: Black hole & Neutron star. The author has an hindex of 31, co-authored 117 publications receiving 2925 citations. Previous affiliations of Sara Motta include University of Palermo & INAF.

XIPE: the x-ray imaging polarimetry explorer

Abstract: XIPE, the X-ray Imaging Polarimetry Explorer, is a mission dedicated to X-ray Astronomy. At the time of writing XIPE is in a competitive phase A as fourth medium size mission of ESA (M4). It promises to reopen the polarimetry window in high energy Astrophysics after more than 4 decades thanks to a detector that efficiently exploits the photoelectric effect and to X-ray optics with large effective area. XIPE uniqueness is time-spectrally-spatially- resolved X-ray polarimetry as a breakthrough in high energy astrophysics and fundamental physics. Indeed the payload consists of three Gas Pixel Detectors at the focus of three X-ray optics with a total effective area larger than one XMM mirror but with a low weight. The payload is compatible with the fairing of the Vega launcher. XIPE is designed as an observatory for X-ray astronomers with 75 % of the time dedicated to a Guest Observer competitive program and it is organized as a consortium across Europe with main contributions from Italy, Germany, Spain, United Kingdom, Poland, Sweden.

eXTP: Enhanced X-ray Timing and Polarization mission

Abstract: eXTP is a science mission designed to study the state of matter under extreme conditions of density, gravity and magnetism. Primary goals are the determination of the equation of state of matter at supra-nuclear density, the measurement of QED effects in highly magnetized star, and the study of accretion in the strong-field regime of gravity. Primary targets include isolated and binary neutron stars, strong magnetic field systems like magnetars, and stellar-mass and supermassive black holes. The mission carries a unique and unprecedented suite of state-of-the-art scientific instruments enabling for the first time ever the simultaneous spectral-timing-polarimetry studies of cosmic sources in the energy range from 0.5-30 keV (and beyond). Key elements of the payload are: the Spectroscopic Focusing Array (SFA) - a set of 11 X-ray optics for a total effective area of similar to 0.9 m(2) and 0.6 m(2) at 2 keV and 6 keV respectively, equipped with Silicon Drift Detectors offering < 180 eV spectral resolution; the Large Area Detector (LAD) - a deployable set of 640 Silicon Drift Detectors, for a total effective area of similar to 3.4 m(2), between 6 and 10 keV, and spectral resolution better than 250 eV; the Polarimetry Focusing Array (PFA) - a set of 2 X-ray telescope, for a total effective area of 250 cm(2) at 2 keV, equipped with imaging gas pixel photoelectric polarimeters; the Wide Field Monitor (WFM) - a set of 3 coded mask wide field units, equipped with position-sensitive Silicon Drift Detectors, each covering a 90 degrees x 90 degrees field of view. The eXTP international consortium includes major institutions of the Chinese Academy of Sciences and Universities in China, as well as major institutions in several European countries and the United States. The predecessor of eXTP, the XTP mission concept, has been selected and funded as one of the so-called background missions in the Strategic Priority Space Science Program of the Chinese Academy of Sciences since 2011. The strong European participation has significantly enhanced the scientific capabilities of eXTP. The planned launch date of the mission is earlier than 2025.

The faint “heartbeats” of igr j17091−3624: an exceptional black hole candidate

Abstract: We report on the first 180 days of Rossi X-Ray Timing Explorer observations of the outburst of the black hole candidate IGR J17091-3624. This source exhibits a broad variety of complex light curve patterns including periods of strong flares alternating with quiet intervals. Similar patterns in the X-ray light curves have been seen in the (up to now) unique black hole system GRS 1915+105. In the context of the variability classes defined by Belloni et al. for GRS 1915+105, we find that IGR J17091-3624 shows the ν, ρ, α, λ, β, and μ classes as well as quiet periods which resemble the χ class, all occurring at 2-60 keV count rate levels which can be 10-50 times lower than observed in GRS 1915+105. The so-called ρ class "heartbeats" occur as fast as every few seconds and as slow as ~100 s, tracing a loop in the hardness-intensity diagram which resembles that previously seen in GRS 1915+105. However, while GRS 1915+105 traverses this loop clockwise, IGR J17091-3624 does so in the opposite sense. We briefly discuss our findings in the context of the models proposed for GRS 1915+105 and find that either all models requiring near Eddington luminosities for GRS 1915+105-like variability fail, or IGR J17091-3624 lies at a distance well in excess of 20 kpc, or it harbors one of the least massive black holes known (<3 M ☉).

Low-frequency oscillations in black holes: a spectral-timing approach to the case of GX 339-4

Abstract: We analysed Rossi X-ray Timing Explorer (RXTE)/PCA and HEXTE data of the transient black hole binary GX 339-4, collected over a time-span of 8 years. We studied the properties and the behaviour of low-frequency quasi-periodic oscillations (QPOs) as a function of the integrated broad-band variability and the spectral parameters during four outbursts (2002, 2004, 2007 and 2010). Most of the QPOs could be classified following the ABC classification which has been proposed before. Our results show that the ABC classification can be extended to include spectral dependencies and that the three QPO types have indeed intrinsically different properties. In terms of the relation between QPO frequency and power-law flux, types A and C QPOs may follow the same relation, whereas the type B QPOs trace out a very different relation. Type B QPO frequencies clearly correlate with the power-law flux and are connected to local increases of the count rate. The frequencies of all QPOs observed in the rising phase of the 2002, 2007 and 2010 outbursts correlate with the disc flux. Our results can be interpreted within the framework of the recently proposed QPO models involving Lense–Thirring precession. We suggest that types C and A QPOs might be connected and could be interpreted as being the result of the same phenomenon observed at different stages of the outburst evolution, while a different physical process produces type B QPOs.

The evolution of the high-energy cut-off in the X-ray spectrum of GX 339−4 across a hard-to-soft transition

Abstract: We report on X-ray observations of the black hole candidate GX 339−4 during its 2006/2007 outburst. The hardness–intensity diagram (HID) of all RXTE/Proportional Counter Array data combined shows a q-shaped track similar to that observed in previous outbursts. The evolution through the HID suggests that in the early phase of the outburst the source underwent a sequence of state transitions, from the hard to the soft state, which is supported by our timing analysis. Broad-band (4–200 keV) spectra, fitted with an exponentially cut-off power law, show that the hard spectral component steepens during the transition from the hard to the soft state. The high-energy cut-off decreased monotonically from 120 to 60 keV during the brightening of the hard state, but again increased to 100 keV during the softening in the hard intermediate state. In the short-lived soft intermediate state the cut-off energy was ∼130 keV, but was no longer detected in the soft state. This is one of the first times that the high-energy cut-off has been followed in such detail across several state transitions. We find that in comparison to several other spectral parameters, the cut-off energy changes more rapidly, just like the timing properties. The observed behaviour of the high-energy cut-off of GX 339−4 is also similar to that observed with RXTE–INTEGRAL–Swift during the 2005 outburst of GRO J1655−40. These results constitute a valuable reference to be considered when testing theoretical models for the production of the hard component in these systems.

Parameter estimation in astronomy through application of the likelihood ratio. [satellite data analysis techniques

Abstract: Many problems in the experimental estimation of parameters for models can be solved through use of the likelihood ratio test. Applications of the likelihood ratio, with particular attention to photon counting experiments, are discussed. The procedures presented solve a greater range of problems than those currently in use, yet are no more difficult to apply. The procedures are proved analytically, and examples from current problems in astronomy are discussed.

Advection-Dominated Accretion: Underfed Black Holes and Neutron Stars

Abstract: We describe new optically thin solutions for rotating accretion flows around black holes and neutron stars. These solutions are advection dominated, so that most of the viscously dissipated energy is advected radially with the flow. We model the accreting gas as a two temperature plasma and include cooling by bremsstrahlung, synchrotron, and Comptonization. We obtain electron temperatures $T_e\sim 10^{8.5}-10^{10}$K. The new solutions are present only for mass accretion rates $\dot M$ less than a critical rate $\dot M_{crit}$ which we calculate as a function of radius $R$ and viscosity parameter $\alpha$. For $\dot M<\dot M_{crit}$ we show that there are three equilibrium branches of solutions. One of the branches corresponds to a cool optically thick flow which is the well-known thin disk solution of Shakura \& Sunyaev (1973). Another branch corresponds to a hot optically thin flow, discovered originally by Shapiro, Lightman \& Eardley (1976, SLE). This solution is thermally unstable. The third branch corresponds to our new advection-dominated solution. This solution is hotter and more optically thin than the SLE solution, but is viscously and thermally stable. It is related to the ion torus model of Rees et al. (1982) and may potentially explain the hard X-ray and $\gamma$-ray emission from X-ray binaries and active galactic nuclei.