ABSTRACT
A new window is opening in high-energy astronomy: X-ray polarimetry. With many missions currently under development and scheduled to launch as early as 2021, observations of the X-ray ...polarization of accreting X-ray pulsars will soon be available. As polarization is particularly sensitive to the geometry of the emission region, the upcoming polarimeters will shed new light on the emission mechanism of these objects, provided that we have sound theoretical models that agree with current spectroscopic and timing observation and that can make predictions of the polarization parameters of the emission. We here present a new model for the polarized emission of accreting X-ray pulsars in the accretion column scenario that for the first time takes into account the macroscopic structure and dynamics of the accretion region and the propagation of the radiation towards the observer, including relativistic beaming, gravitational lensing, and quantum electrodynamics. In this paper, we present all the details of the model, while in a companion paper, we apply our model to predict the polarization parameters of the bright X-ray pulsar Hercules X-1.
Abstract
We present a model to account for the observed debris discs around young white dwarfs and the presence of metal lines in their spectra. Stellar evolution models predict that the mass-loss on ...the AGB will be pulsed; furthermore, observations indicate that the bulk of the mass-loss occurs on the AGB. In this case, if the progenitors of the white dwarfs had remnants of planetary formation like the Sun’s Oort cloud or the Kuiper Belt and a planet lying within that cloud or nearby, we find that up to 2 per cent of the planetesimals will fall either into planet-crossing orbits or into chaotic regions after the mass-loss, depending on the location and mass of the planet (from Mars to Neptune). This yields a sufficient mass of comets that can be scattered towards the star, form a debris disc and pollute the atmosphere.
ABSTRACT
We employ our new model for the polarized emission of accreting X-ray pulsars to describe the emission from the luminous X-ray pulsar Hercules X-1. In contrast with previous works, our model ...predicts the polarization parameters independently of spectral formation, and considers the structure and dynamics of the accretion column, as well as the additional effects on propagation due to general relativity and quantum electrodynamics. We find that our model can describe the observed pulse fraction and the pulse shape of the main peak, as well as the modulation of the cyclotron line with phase. We pick two geometries, assuming a single accretion column or two columns at the magnetic poles, that can describe current observations of pulse shape and cyclotron modulation with phase. Both models predict a high polarization fraction, between 60 and 80 per cent in the 1–10 keV range, that is phase and energy dependent, and that peaks at the same phase as the intensity. The phase and energy dependence of the polarization fraction and of the polarization angle can help discern between the different geometries.
NASA’s Imaging X-ray Polarimetry Explorer (IXPE) was launched in December 2021. It is 100 times more sensitive to polarized X-ray emission than any preceding mission and it is opening a new ...observational window into high-energy astrophysics. I outline Yury N. Gnedin’s many contributions to understanding polarization from neutron stars and present new simulations of observations that IXPE will perform of the X-ray pulsar Hercules X-1 and the magnetar 4U 0141+561 in February 2022. These observations highlight and test particular models that Gnedin and collaborators first proposed. I outline how IXPE will provide unique constraints on the structure and kinematics of the boundary region between the accretion flow and the neutron star surface of Hercules X-1 and how IXPE will verify the predictions of vacuum birefringence for the magnetar 4U 0142+561.
Unbinned likelihood analysis for X-ray polarization González-Caniulef, Denis; Caiazzo, Ilaria; Heyl, Jeremy
Monthly notices of the Royal Astronomical Society,
03/2023, Volume:
519, Issue:
4
Journal Article
Peer reviewed
Open access
ABSTRACT
We present a systematic study of the unbinned, photon-by-photon likelihood technique which can be used as an alternative method to analyse phase-dependent, X-ray spectro-polarimetric ...observations obtained with IXPE and other photoelectric polarimeters. We apply the unbinned technique to models of the luminous X-ray pulsar Hercules X-1, for which we produce simulated observations using the ixpeobssim package. We consider minimal knowledge about the actual physical process responsible for the polarized emission from the accreting pulsar and assume that the observed phase-dependent polarization angle can be described by the rotating vector model. Using the unbinned technique, the detector’s modulation factor, and the polarization information alone, we found that both the rotating vector model and the underlying spectro-polarimetry model can reconstruct equally well the geometric configuration angles of the accreting pulsar. However, the measured polarization fraction becomes biased with respect to the underlying model unless the energy dispersion and effective area of the detector are also taken into account. To this end, we present an energy-dispersed likelihood estimator that is proved to be unbiased. For different analyses, we obtain posterior distributions from multiple ixpeobssim realizations and show that the unbinned technique yields $\sim 10{{\ \rm per\ cent}}$ smaller error bars than the binned technique. We also discuss alternative sources, such as magnetars, in which the unbinned technique and the rotating vector model might be applied.
Scalar-tensor theories of gravity generally violate the strong equivalence principle, namely that compact objects have a suppressed coupling to the scalar force, which causes them to fall slower. A ...black hole is the extreme example where such a coupling vanishes, i.e., black holes have no scalar hair. We explore observational scenarios for detecting strong equivalence principle violation, focusing on galileon gravity as an example. For galaxies infalling toward galaxy clusters, the supermassive black hole can be offset from the galaxy center away from the direction of the cluster. Well-resolved images of galaxies around nearby clusters can therefore be used to identify the displaced black hole via the star cluster bound to it. We show that this signal is accessible with imaging surveys, both ongoing ones such as the Dark Energy Survey and future ground- and space-based surveys. Already, the observation of the central black hole in M 87 places new constraints on the galileon parameters, which we present here. matter couplings are disfavored for a large region of the parameter space. We also find a novel phenomenon whereby the black hole can escape the galaxy completely in less than one billion years.
By examining the locations of central black holes in two elliptical galaxies, M32 and M87, we derive constraints on the violation of the strong equivalence principle for purely gravitational objects, ...i.e. black holes, of less than about two-thirds, ... < 0.68 from the gravitational interaction of M87 with its neighbours in the Virgo cluster. Although M32 appears to be a good candidate for this technique, the high concentration of stars near its centre substantially weakens the constraints. On the other hand, if a central black hole is found in NGC 205 or one of the other satellite ellipticals of M31, substantially better constraints could be obtained. In all cases, the constraints could improve dramatically with better astrometry. (ProQuest: ... denotes formulae/symbols omitted.)
Abstract
Despite their shared origin, members of globular clusters display star-to-star variations in composition. The observed pattern of element abundances is unique to these stellar environments ...and cannot be fully explained by any proposed mechanism. It remains unclear whether stars form with chemical heterogeneity or inherit it from interactions with other members. These scenarios may be differentiated by the dependence of chemical spread on stellar mass; however, obtaining a sufficiently large mass baseline requires abundance measurements on the lower main sequence, which is too faint for spectroscopy even in the nearest globular clusters. We developed a stellar modeling method to obtain precise chemical abundances for stars near the end of the main sequence from multiband photometry, and we applied it to the globular cluster 47 Tucanae. The computational efficiency is attained by matching chemical elements to the model components that are most sensitive to their abundance. We determined O/Fe for ∼5000 members below the main-sequence knee at the level of accuracy, comparable to the spectroscopic measurements of evolved members in the literature. The inferred distribution disfavors stellar interactions as the origin of chemical spread; however, an accurate theory of accretion is required to draw a more definitive conclusion. We anticipate that future observations of 47 Tucanae with the James Webb Space Telescope will extend the mass baseline of our analysis into the substellar regime. Therefore, we present predicted color–magnitude diagrams and mass–magnitude relations for the brown dwarf members of 47 Tucanae.
We examine two trigger mechanisms, one internal and the other external to the neutron star, that give rise to the intense soft gamma-ray repeater (SGR) giant flares. So far, three giant flares have ...been observed from the three out of the seven confirmed SGRs on 1979 March 5, 1998 August 27 and 2004 December 27, respectively. The last two events were found to be much more powerful than the first, and both showcased the existence of a precursor, which we show to have had initiated the main flare. In the internal mechanism, we propose that the strongly wound-up poloidal magnetic field develops tangential discontinuities and dissipates its torsional energy in heating the crust. The time-scale for the instability to develop coincides with the duration of the quiescent state that followed the precursor. Alternatively, we develop a reconnection model based on the hypothesis that shearing motion of the footpoints causes the materialization of a Sweet–Parker current layer in the magnetosphere. The thinning of this macroscopic layer due to the development of an embedded super-hot turbulent current layer switches on the impulsive Hall reconnection, which powers the giant flare. Again, we show that the thinning time is on the order of the pre-flare quiescent time. This model naturally explains the origin of the observed non-thermal radiation during the flares as well as the post-flare radio afterglows.
Radiative corrections of quantum electrodynamics cause a vacuum threaded by a magnetic field to be birefringent. This means that radiation of different polarizations travels at different speeds. Even ...in the strong magnetic fields of astrophysical sources, the difference in speed is small. However, it has profound consequences for the extent of polarization expected from strongly magnetized sources. We demonstrate how the birefringence arises from first principles, show how birefringence affects the polarization state of radiation and present recent calculations for the expected polarization from magnetars and X-ray pulsars.
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