Some lessons for us scientists (too) from the "Sokal affair"

In this little non-technical piece, I argue that some of the lessons that can be learnt from the bold action carried out in 1996 by the physicist Alan Sokal and typically known as the "Sokal affair" not only apply to some sector of the humanities (which was the original target of the hoax), but also (with much less intensity, but still) to the hardest sciences

The relevance of ontological commitments

In this introductory note, I describe my particular view of the notion of ontological commitments as honest and pragmatic working hypotheses that assume the existence (out there) of certain entities represented by the symbols in our theory. I argue that this is not naive, in the sense that it does not entail the belief that the hypotheses could ever be proved to be true (or false), but it is nevertheless justified by the success and predictive power of the theory that contains the concepts assumed to exist. I also claim that the ontological commitments one holds (even if tacitly so) have a great influence on what kind of science is produced, how it is used, and how it is understood. Not only I justify this claim, but I also propose a sketch of a possible falsification of it. As a natural conclusion, I defend the importance of identifying, clarifying and making explicit one's ontological commitments if fruitful scientific discussions are to be had. Finally, I compare my point of view with that of some philosophers and scientists who have put forward similar notions.Comment: Submitted for peer-revie

Discovery of periodic dips in the light curve of GX 13+1: the X-ray orbital ephemeris of the source

The bright low-mass X-ray binary (LMXB) GX 13+1 is one of the most peculiar Galactic binary systems. A periodicity of 24.27 d with a formal statistical error of 0.03 d was observed in its power spectrum density obtained with RXTE All Sky Monitor (ASM) data spanning 14 years. Starting from a recent study, indicating GX 13+1 as a possible dipping source candidate, we systematically searched for periodic dips in the X-ray light curves of GX 13+1 from 1996 up to 2013 using RXTE/ASM, and MAXI data to determine for the first time the X-ray orbital ephemeris of GX 13+1. We searched for a periodic signal in the ASM and MAXI light curves, finding a common periodicity of 24.53 d. We folded the 1.3-5 keV and 5-12.1 keV ASM light curves and the 2-4 and 4-10 keV MAXI light curves at the period of 24.53 d finding a periodic dip. To refine the value of the period we used the timing technique dividing the ASM light curve in eight intervals and the MAXI light curve in two intervals, obtaining four and two dip arrival times from the ASM and MAXI light curves, respectively. We improved the X-ray position of GX 13+1 using a recent Chandra observation. The new X-ray position is discrepant by \sim 7\arcsec from the previous one, while it is compatible with the infrared and radio counterpart positions. We detected an X-ray dip, that is totally covered by the Chandra observation, in the light curve of GX 13+1 and showed, a-posteriori, that it is a periodic dip. We obtained seven dip arrival times from ASM, MAXI, and Chandra light curves. We calculated the delays of the detected dip arrival times with respect to the expected times for a 24.52 d periodicity. Fitting the delays with a linear function we find that the orbital period and the epoch of reference of GX 13+1 are 24.5274(2) days and 50,086.79(3) MJD, respectively.(Abridged)Comment: 12 pages, including 16 figures. Accepted for publication in A&

Chandra X-ray spectroscopy of a clear dip in GX 13+1

The source GX 13+1 is a persistent, bright Galactic X-ray binary hosting an accreting neutron star. It shows highly ionized absorption features, with a blueshift of $\sim$ 400 km s$^{-1}$ and an outflow-mass rate similar to the accretion rate. Many other X-ray sources exhibit warm absorption features, and they all show periodic dipping behavior at the same time. Recently, a dipping periodicity has also been determined for GX 13+1 using long-term X-ray folded light-curves, leading to a clear identification of one of such periodic dips in an archival Chandra observation. We give the first spectral characterization of the periodic dip of GX 13+1 found in this archival Chandra observation performed in 2010. We used Chandra/HETGS data (1.0-10 keV band) and contemporaneous RXTE/PCA data (3.5-25 keV) to analyze the broadband X-ray spectrum. We adopted different spectral models to describe the continuum emission and used the XSTAR-derived warm absorber component to constrain the highly ionized absorption features. The 1.0-25 keV continuum emission is consistent with a model of soft accretion-disk emission and an optically thick, harder Comptonized component. The dip event, lasting $\sim$ 450 s, is spectrally resolved with an increase in the column density of the neutral absorber, while we do not find significant variations in the column density and ionization parameter of the warm absorber with respect to the out-of-dip spectrum. We argue that the very low dipping duty-cycle with respect to other sources of the same class can be ascribed to its long orbital period and the mostly neutral bulge, that is relatively small compared with the dimensions of the outer disk radius.Comment: 13 pages, 15 figures, accepted for publication in Astronomy and Astrophysic