arXiv subjects

T. Salmi

Publications and source records attributed to T. Salmi.

6 recordsLinked to original sources

arXiv2026

Mass and radius measurements of the neutron star 47~Tuc X7 -- A new bias-free method

Neutron star (NS) radius measurements provide precious information to constrain the dense matter equation of state (EOS). Quiescent low-mass X-ray binaries (qLMXBs) have been used for this purpose, but a number of sources of systematic biases were uncovered, making other sources more favored for EOS studies. We aim to reintroduce qLMXBs as reliable sources of NS mass and radius measurements with a new method, free of systematic biases. We test our implementation on the qLMXB X7 in the globular cluster 47 Tucanae. We used X-PSI to perform the spectral analysis of the 47Tuc X7 observations. X-PSI accurately models the effects of the unknown NS rotation and possible surface anisotropies (two sources of biases in qLMXBs) on the NS spectra. The most significant source of bias on the radius is usually the chemical composition of the NS atmosphere, which, in the case of 47Tuc X7, is known to be hydrogen-rich. A broad range of masses and radii was explored. We obtain a NS radius at 1.4 $M_\odot$ of $R_{1.4} = 12.9\pm0.4$ km (68% credible interval). A shift of the radius by less than a % is measured compared to the model where these sources of systematic uncertainties are neglected. More importantly, including rotation and surface anisotropies in the modeling does not significantly broaden the radius posteriors. We also place strong constraints on the X-ray pulsed fraction (upper limit of 6.0% at a 99.97% credible level) caused by the possible presence of a hot spot. This suggests that, for 47Tuc X7, robust radius constraints can be obtained even without considering systematics, likely because of the deep exposures. We use the resulting M-R constraints from this NS to quantify the improvement on an EOS inference when combined with other measurements. We show that, using recently developed tools, qLMXBs can be exploited to infer reliable NS masses and radii, which can in turn constrain the EOS.

astro-ph.HE↗

arXiv2019

Effects of Compton scattering on the neutron star radius constraints in rotation-powered millisecond pulsars

The aim of this work is to study the possible effects and biases on the radius constraints for rotation-powered millisecond pulsars when using Thomson approximation to describe electron scattering in the atmosphere models, instead of using exact formulation for Compton scattering. We compare the differences between the two models in the energy spectrum and angular distribution of the emitted radiation. We also analyse a self-generated synthetic phase-resolved energy spectrum, based on Compton atmosphere and the most X-ray luminous rotation-powered millisecond pulsars observed by the Neutron star Interior Composition ExploreR (NICER). We derive constraints for the neutron star parameters using both the Compton and Thomson models. The results show that the method works by reproducing the correct parameters with the Compton model. However, biases are found in size and the temperature of the emitting hot spot, when using the Thomson model. The constraints on the radius are still not significantly changed, and therefore the Thomson model seems to be adequate if we are interested only in the radius measurements using NICER.

astro-ph.HE↗

arXiv2018

Bayesian parameter constraints for neutron star masses and radii using X-ray timing observations of accretion-powered millisecond pulsars

We present a Bayesian method to constrain the masses and radii of neutron stars (NSs) using the information encoded in the X-ray pulse profiles of accreting millisecond pulsars. We model the shape of the pulses using "oblate Schwarzschild" approximation, which takes into account the deformed shape of the star together with the special and general relativistic corrections to the photon trajectories and angles. The spectrum of the radiation is obtained from an empirical model of Comptonization in a hot slab in which a fraction of seed blackbody photons is scattered into a power-law component. By using an affine-invariant Markov chain Monte Carlo ensemble sampling method, we obtain posterior probability distributions for the different model parameters, especially for the mass and the radius. To test the robustness of our method, we first analyzed self-generated synthetic data with known model parameters. Similar analysis was then applied for the observations of SAX J1808.4-3658 by the Rossi X-ray Timing Explorer (RXTE). The results show that our method can reproduce the model parameters of the synthetic data, and that accurate constraints for the radius can be obtained using the RXTE pulse profile observations if the mass is a priori known. For a mass in the range 1.5-1.8 Msun, the radius of the NS in SAX J1808.4-3658 is constrained between 9 and 13 km. If the mass is accurately known, the radius can be determined with an accuracy of 5% (68% credibility). For example, for the mass of 1.7 Msun the equatorial radius is Req = 11.9+0.5 -0.4 km. Finally, we show that further improvements can be obtained when the X-ray polarization data from the Imaging X-ray Polarimeter Explorer will become available.

astro-ph.HE↗

arXiv2014

The nature of the late achromatic bump in GRB 120326A

The long ${\it Swift}$ gamma-ray burst GRB 120326A at redshift $z=1.798$ exhibited a multi-band light curve with a striking feature: a late-time, long-lasting achromatic rebrightening, rarely seen in such events. Peaking in optical and X-ray bands $\sim 35$ ks ($\sim 12.5$ ks in the GRB rest frame) after the 70-s GRB prompt burst, the feature brightens nearly two orders of magnitude above the underlying optical power-law decay. Modelling the multiwavelength light curves, we investigate possible causes of the rebrightening in the context of the standard fireball model. We exclude a range of scenarios for the origin of this feature: reverse-shock flash, late-time forward shock peak due to the passage of the maximal synchrotron frequency through the optical band, late central engine optical/X-ray flares, interaction between the expanding blast wave and a density enhancement in the circumburst medium and gravitational microlensing. Instead we conclude that the achromatic rebrightening may be caused by a refreshed forward shock or a geometrical effect. In addition, we identify an additional component after the end of the prompt emission, that shapes the observed X-ray and optical light curves differently, ruling out a single overall emission component to explain the observed early time emission.

astro-ph.HE↗

arXiv2014

Modeling heat transfer from quench protection heaters to superconducting cables in Nb3Sn magnets

We use a recently developed quench protection heater modeling tool for an analysis of heater delays in superconducting high-field Nb3Sn accelerator magnets. The results suggest that the calculated delays are consistent with experimental data, and show how the heater delay depends on the main heater design parameters.

physics.acc-ph↗

arXiv2012

Panchromatic observations of the textbook GRB 110205A: constraining physical mechanisms of prompt emission and afterglow

We present a comprehensive analysis of a bright, long duration (T90 ~ 257 s) GRB 110205A at redshift z= 2.22. The optical prompt emission was detected by Swift/UVOT, ROTSE-IIIb and BOOTES telescopes when the GRB was still radiating in the gamma-ray band. Nearly 200 s of observations were obtained simultaneously from optical, X-ray to gamma-ray, which makes it one of the exceptional cases to study the broadband spectral energy distribution across 6 orders of magnitude in energy during the prompt emission phase. By fitting the time resolved prompt spectra, we clearly identify, for the first time, an interesting two-break energy spectrum, roughly consistent with the standard GRB synchrotron emission model in the fast cooling regime. Although the prompt optical emission is brighter than the extrapolation of the best fit X/gamma-ray spectra, it traces the gamma-ray light curve shape, suggesting a relation to the prompt high energy emission. The synchrotron + SSC scenario is disfavored by the data, but the models invoking a pair of internal shocks or having two emission regions can interpret the data well. Shortly after prompt emission (~ 1100 s), a bright (R = 14.0) optical emission hump with very steep rise (alpha ~ 5.5) was observed which we interpret as the emission from the reverse shock. It is the first time that the rising phase of a reverse shock component has been closely observed. The full optical and X-ray afterglow lightcurves can be interpreted within the standard reverse shock (RS) + forward shock (FS) model. In general, the high quality prompt emission and afterglow data allow us to apply the standard fireball shock model to extract valuable information about the GRB including the radiation mechanism, radius of prompt emission R, initial Lorentz factor of the outflow, the composition of the ejecta, as well as the collimation angle and the total energy budget.

astro-ph.HE↗