arXiv subjects

Sophia Sánchez-Maes

Publications and source records attributed to Sophia Sánchez-Maes.

5 recordsLinked to original sources

arXiv2026

The Demographics of Sagittarius A* X-ray Flares over 25 Years with Chandra

We present the Chandra 25-year Sagittarius A* (Sgr A*) X-ray flare catalog: a systematic analysis of 6.8 Ms of Sgr A* monitoring spanning the Chandra X-ray Observatory's mission lifetime. This is the most complete Chandra Sgr A* X-ray flare catalog to date, consisting of 100 flares with 2$-$10 keV unabsorbed luminosities ranging from $\sim$ 4$-$575 $\times 10^{33}$ erg s$^{-1}$. 18 flares are reported for the first time, including the second brightest Sgr A* flare observed by Chandra. The expanded dataset supports previous indications of a correlation between X-ray flare hardness and luminosity. Spectral modeling corroborates this finding, showing a change in the X-ray spectral index, from $Γ\sim 3$ to 2 with increasing flare brightness. Previously-established correlations between flare duration, fluence, and maximum count rate are strengthened via the greater sample size. These results likely reflect variations in the underlying particle distribution that produce weak and strong flares, and the new catalog serves as a rich archive for ongoing observational and numerical investigations into the physical mechanisms responsible for producing Sgr A*'s X-ray flares.

astro-ph.HE↗

arXiv2026

Detection and Evolution of Linear Polarization of the Galactic Center Transient MAXI J1744-294

MAXI J1744$-$294, likely a low-mass X-ray binary system, is a Galactic-center transient source, detected at radio and X-ray wavelengths, located approximately $19''$ southeast of Sgr A*. We report the first detection of its variable linear polarization in four epochs spanning 2025 Apr 04--09. The normalized 33 and 43 GHz Stokes parameters $q$ and $u$ over the four epochs imply a common Faraday rotation screen with a rotation measure RM $=-63\,606^{+844}_{-861}$ radians m$^{-2}$, the third largest RM detected within the Galaxy. The RM is consistent with that of the Galactic center magnetar PSR J1745$-$2900, giving the first direct evidence that MAXI J1744 lies within the Galactic center region, is bound to Sgr A*, and therefore, is part of the nuclear star cluster. The uniformity in the Galactic center Faraday screen suggests that Sgr A*'s $\approx-10^5$ rad m$^{-2}$ RM is intrinsic rather than originating from an unrelated line-of-sight source. On 2025 Apr 06, we detected a secondary polarized component with an additional RM $\approx-6000$ rad m$^{-2}$, which was not seen at any other epoch. Assuming this secondary component primarily cools by synchrotron radiation, the implied local magnetic field strength is $\sim$15--30 gauss. In the context of a jetted X-ray binary progenitor, the additional RM screen and magnetic field strength are explainable with a short-lived knot in a putative jet.

astro-ph.HE↗

arXiv2025

First Mid-infrared Detection and Modeling of a Flare from Sgr A*. II. Mid-IR Spectral Energy Distribution and Millimeter Polarimetry

S. D. von Fellenberg et al. (2025a, Paper I) reported the first mid-infrared detection of a flare from Sgr A*. The JWST/MIRI/MRS observations were consistent with an orbiting hotspot undergoing electron injection with a spectrum that subsequently breaks from synchrotron cooling. However, mid-infrared extinction measurements appropriate for these data were not yet determined, and therefore the temporal evolution of the absolute spectral index remained unknown. This work applies new Galactic Center extinction measurements to the flare observations. The evolution of the spectral index after the peak is fully consistent with that reported in Paper I with a maximum absolute mid-infrared spectral index $α_{\rm{MIR}}=0.45\pm0.01_{\rm{stat}}\pm0.08_{\rm{sys}}$ during the second mid-infrared flare peak, matching the known near-infrared spectral index during bright states ($α_{\rm{NIR}}\approx0.5$). There was a near-instantaneous change in the mid-infrared spectral index of $Δα_{\rm{MIR}}=0.33\pm0.06_{\rm{stat}}\pm0.11_{\rm{sys}}$ at the flare onset. We propose this as a quantitative definition for this infrared flare's beginning, physically interpreted as the underlying electron distribution's transition into a hard power-law distribution. This paper also reports the SMA millimeter polarization during the flare, which shows a small, distorted, but overall clockwise-oriented Stokes Q--U loop during the third mid-infrared peak. Extrapolating the mid-infrared flux power law to the millimeter yields a variable flux consistent with the observed 220 GHz emission. These results, together with the Paper I modeling, plausibly suggest a single hotspot produced both the mid-infrared and millimeter variability during this event. However, additional flares are required to make a general statement about the millimeter and mid-infrared connection.

astro-ph.HE↗

arXiv2024

An Audit of the Light Curves of RV Tau Variable Stars in the ASAS-SN Database

RV Tauri variable stars are pulsating evolved stars that are identified by a characterizing feature in their light curves: alternating deep and shallow minima. Many RV Tauri variable stars were originally classified decades ago using visual observations and photographic plates. However, recent studies suggest that there are imposters, or misclassified variables, amongst the sample of RV Tau stars. In this study, we examine 84 known and suspected RV Tau stars that appear in the variable star catalog of the All Sky Automated Survey for Supernova (ASAS-SN). For each ASAS-SN light curve, we performed a period analysis and compared our results with those of the General Catalog of Variables Stars (GCVS) and the automatic classification algorithm used by ASAS-SN. We found that the pattern of alternating minima present in RV Tau variable stars often confuses automatic classification. Our results include updated periods and classifications for our sample. Our study provides an important step towards obtaining a robust sample of RV Tau variable stars to better understand the pulsation mechanism and the evolutionary pathway of these variable stars.

astro-ph.SR↗

arXiv2024

Improving Radial Velocities by Marginalizing over Stars and Sky: Achieving 30 m/s RV Precision for APOGEE in the Plate Era

The radial velocity catalog from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) is unique in its simultaneously large volume and high precision as a result of its decade-long survey duration, multiplexing (600 fibers), and spectral resolution of $R \sim 22,500$. However, previous data reductions of APOGEE have not fully realized the potential radial velocity (RV) precision of the instrument. Here we present an RV catalog based on a new reduction of all 2.6 million visits of APOGEE DR17 and validate it against improved estimates for the theoretical RV performance. The core ideas of the new reduction are the simultaneous modeling of all components in the spectra, rather than a separate subtraction of point estimates for the sky, and a marginalization over stellar types, rather than a grid search for an optimum. We show that this catalog, when restricted to RVs measured with the same fiber, achieves noise-limited precision down to 30 m/s and delivers well-calibrated uncertainties. We also introduce a general method for calibrating fiber-to-fiber constant RV offsets and demonstrate its importance for high RV precision work in multi-fiber spectrographs. After calibration, we achieve 47 m/s RV precision on the combined catalog with RVs measured with different fibers. This degradation in precision relative to measurements with only a single fiber suggests that refining line spread function models should be a focus in SDSS-V to improve the fiber-unified RV catalog.

astro-ph.IM↗