arXiv subjects

E. A. Saavedra

Publications and source records attributed to E. A. Saavedra.

4 recordsLinked to original sources

arXiv2026

The IACOB project XVIII. Prevalence of short-period binaries among Galactic helium rich O-type stars

For decades, the origin of helium enrichment in O-type stars has remained an open question. In this study, we investigate the correlation between surface helium abundance and orbital parameters for a sample of 45 O-type SB1 systems --including Cyg X-1. We find seven He-rich systems, all of which are concentrated at short orbital periods (P$\lesssim 15$ days) and are classified as runaways. In addition, four of them present relatively high eccentricities, while the other three have ellipsoidal variations. We argue that these properties are the result of binary interaction. These findings provide strong observational evidence that binary interaction is the dominant origin of helium enrichment in O-type stars. This result has important implications for the treatment of chemical mixing and surface abundances in massive-star evolutionary models, and establishes helium enrichment as a promising observational tracer for identifying post-interaction binary systems.

astro-ph.SR↗

arXiv2025

The hard ultraluminous state of NGC 5055 ULX X-1

We present the results of the first broadband X-ray analysis of the ultraluminous X-ray source NGC 5055 ULX X-1, combining simultaneous data from XMM$-$Newton and NuSTAR missions, with a combined exposure time of $\sim$100 ks across the $0.3-20$ keV energy range. The source exhibits a stable flux across the entire exposure with no detectable pulsations by any instrument on their X-ray light curves, placing pulsed-fraction upper limits of 10% and 32% for XMM$-$Newton and NuSTAR, respectively. The X-ray spectrum is dominated by two thermal components consistent with the emission from an accretion disk, and shows a weak high-energy tail above 10 keV, with no statistical requirement for an additional nonthermal component. The unabsorbed $0.3-20$ keV luminosity is ${\sim}2\times10^{40}$ erg s$^{-1}$, evidencing the ULX nature of the source. The parameters obtained from spectral modeling are consistent with the hard ultraluminous state. Despite the fact that a neutron-star accretor cannot be ruled out by the available data, under the assumption that the compact object in NGC 5055 ULX X-1 is a black hole accreting through a geometrically thick, radiation-pressure-supported disk that drives an optically thick wind, we constrained its putative mass to $11-26$ M$_{\odot}$.

astro-ph.HE↗

arXiv2025

The orbital period and inclination of the neutron star X-ray transient MAXI J1807+132

The neutron star X-ray transient MAXI J1807+132 has undergone outbursts in 2017, 2019, and 2023. We conducted an $R$-band time series photometry campaign using the Isaac Newton Telescope during the 2022 quiescent state. We detected a periodic variation in the light curve, consistent with ellipsoidal modulation, which allowed us to determine an orbital period of $P_{\rm orb} = 4.258 \pm 0.008$ hr. By modelling the light curve, we obtained a binary inclination of $ i = 72\pm5 \, °$ and a mass ratio $q = 0.24^{+0.19}_{-0.14}$ ($68$ per cent confidence level). Furthermore, our analysis supports an early M-dwarf companion that contributes between 30 and 50 per cent to the total flux in the $R$-band. We extend the previously established absolute magnitude versus orbital period correlation for black hole X-ray transients to neutron star systems. We applied the correlation to MAXI J1807+132, estimating its distance as $6.3 \pm 0.7$ kpc and its height above the Galactic plane to be $1.6 \pm 0.2$ kpc.

astro-ph.HE↗

arXiv2025

GX 340+0: In and Out of Focus

The Nuclear Spectroscopic Telescope Array (NuSTAR) enables detailed high-energy X-ray observations from 3--79 keV, but its performance can be constrained by telemetry saturation when observing bright sources, leading to reduced effective exposure times. In this study, we investigate the use of serendipitous stray light (SL) observations to infer properties of an X-ray bright source in comparison to focused data. Our case study is performed on the neutron star (NS) low-mass X-ray binary (LMXB) GX 340+0, a prominent Z source, where we execute a spectral analysis comparing 25 SL and 7 focused NuSTAR observations. Our findings demonstrate that SL observations can significantly enhance long-term temporal coverage; detecting variations in the thermal components of the system across the baseline of the mission, which could not be inferred from focused observations alone.

astro-ph.HE↗