arXiv subjects

George A. Younes

Publications and source records attributed to George A. Younes.

7 recordsLinked to original sources

arXiv2026

NICER Magnetar Burst Catalog

In this paper, we present a comprehensive catalog of short bursts from magnetars based on eight years of NICER observations. A total of 1130 bursts were identified from 14 sources, with the sample dominated by SGR 1935+2154, which accounts for 76% of all detected bursts. We analyzed burst durations, spectral properties, and their correlations across multiple sources. Bursts from SGR 1935+2154 exhibit significantly longer durations, with a distribution peak at 316 ms, compared to a peak of 23 ms for bursts from other magnetars. Two μs-scale bursts were detected for the first time, originating from 1E 1048.1-5937 and CXOU J010043.1-721134. Spectral analysis in the 0.5-8 keV range using both blackbody and power-law models shows that bursts with higher fluences have harder spectra. In contrast, correlations between burst duration and spectral parameters are weak or absent. This catalog provides a valuable dataset for studying magnetar short bursts, enabling future modeling efforts and improving our understanding of the diversity and physical mechanisms of magnetar bursts.

astro-ph.HE↗

arXiv2025

Pulsed, Polarized X-ray Emission from Neutron Star Surfaces: the Effects of Vacuum Birefringence in the Magnetosphere

Intense magnetic fields in the atmospheres of neutron stars render non-trivial angular dependence of intensity and polarization of soft X-ray emission originating from their surfaces. By tracking the complex electric field vector for each photon during its atmospheric transport and propagation in general relativistic and birefringent magnetospheres, our Monte Carlo simulation, named MAGTHOMSCATT, allows for capturing the complete polarization properties, including the intricate interplay between linearity and circularity. The new inclusion in MAGTHOMSCATT of quantum electrodynamical influences on polarization in the magnetosphere is presented. We simulate the pulsed and polarized X-ray emission from the outer layers of optically thick, fully ionized atmospheres of neutron stars, with a focus on the radiation emitted from extended polar caps of magnetars, which are the most highly magnetized neutron stars. Using the recent intensity pulse profile data for the magnetar 1RXS J11708-4009, we constrain the geometric parameters, namely the angles between the magnetic axis and the observer's viewing direction relative to the spin axis, as well as the sizes of emission regions. The distributions of these parameters and the best-fit configuration are provided. In addition, we discuss the important impacts of vacuum birefringence in the magnetosphere on increasing the linear polarization degree. A comparison with the case of a weakly magnetized neutron star, RX J0822.0-4300, is also discussed. Our simulation still needs further development, particularly to incorporate the vacuum resonance effect. Nevertheless, the formalism presented here can be employed to constrain geometric parameters for various types of neutron stars.

astro-ph.HE↗

arXiv2025

Rapid Spectral Evolution of SGR 1935+2154 During its 2022 Outburst

During the 2022 outburst of SGR 1935+2154, a Fast-Radio-Burst-like event (FRB 20221014A) and X-ray activities occurred between two spin-up glitches, suggesting these glitches may connect to multiwavelength phenomenology. However, the mechanisms altering the magnetar's magnetosphere to enable radio emission remain unclear. This study presents high-cadence NICER and NuSTAR observations revealing spectral changes in burst and persistent emission. Hardness ratio and spectral analysis reveal significant changes during an "intermediate flare" 2.5 hours before FRB 20221014A. This 80-second flare, releasing $>(6.3\pm0.2)\times10^{40}$ erg, coincides with a rapid spectral softening in both burst and persistent emission and a notable decrease in burst occurrence rate. The intermediate flare is bright enough to be detected if placed at a few Mpc, and would appear as a fast X-ray transient. This implies that the connection between magnetar X-ray activity and FRBs can be observed in the local Universe. Post-flare burst spectra peak near 5 keV, resembling the characteristics of the FRB-associated X-ray burst of 2020. Such change persisted for a few hours, implying magnetospheric evolution on similar timescales. However, no radio emission was detected from post-flare bursts, suggesting that FRB emission requires conditions beyond peculiar short bursts. The burst waiting times exhibit a broken power-law distribution, likely resulting from contamination by enhanced persistent emission. Although the bursts appear randomly distributed in the spin phase, the hardness ratio profile as a function of spin phase follows that of the persistent emission, indicating that X-ray bursts originate at low altitudes.

astro-ph.HE↗

arXiv2025

X-ray polarization of the magnetar 1E 1841-045

We report on IXPE and NuSTAR observations beginning forty days after the 2024 outburst onset of magnetar 1E 1841-045, marking the first IXPE observation of a magnetar in an enhanced state. Our spectropolarimetric analysis indicates that both a blackbody (BB) plus double power-law (PL) and a double blackbody plus power-law spectral model fit the phase-averaged intensity data well, with a hard PL tail ($Γ$=1.19 and 1.35, respectively) dominating above $\approx 5$ keV. For the former model, we find the soft PL (the dominant component at soft energies) exhibits a polarization degree (PD) of $\approx 30\%$ while the hard PL displays a PD of $\approx 40\%$. Similarly, the cool BB of the 2BB+PL model possesses a PD of $\approx 15\%$ and a hard PL PD of $\approx 57\%$. For both models, each component has a polarization angle (PA) compatible with celestial north. Model-independent polarization analysis supports these results, wherein the PD increases from$ \approx 15\%$ to $\approx 70\%$ in the 2-3 keV and 6-8 keV ranges, respectively, while the PA remains nearly constant. We find marginal evidence for phase-dependent variability of the polarization properties, namely a higher PD at phases coinciding with the hard X-ray pulse peak. We compare the hard X-ray PL to the expectation from resonant inverse Compton scattering (RICS) and secondary pair cascade synchrotron radiation from primary high-energy RICS photons; both present reasonable spectropolarimetric agreement with the data, albeit, the latter more naturally. We suggest that the soft PL X-ray component may originate from a Comptonized corona in the inner magnetosphere.

astro-ph.HE↗

arXiv2024

Pulsed and Polarized X-ray Emission from Neutron Star Surfaces

The intense magnetic fields of neutron stars naturally lead to strong anisotropy and polarization of radiation emanating from their surfaces, both being sensitive to the hot spot position on the surface. Accordingly, pulse phase-resolved intensities and polarizations depend on the angle between the magnetic and spin axes and the observer's viewing direction. In this paper, results are presented from a Monte Carlo simulation of neutron star atmospheres that uses a complex electric field vector formalism to treat polarized radiative transfer due to magnetic Thomson scattering. General relativistic influences on the propagation of light from the stellar surface to a distant observer are taken into account. The paper outlines a range of theoretical predictions for pulse profiles at different X-ray energies, focusing on magnetars and also neutron stars of lower magnetization. By comparing these models with observed intensity and polarization pulse profiles for the magnetar 1RXS J1708-40, and the light curve for the pulsar PSR J0821-4300, constraints on the stellar geometry angles and the size of putative polar cap hot spots are obtained.

astro-ph.HE↗

arXiv2023

The High Energy X-ray Probe (HEX-P): Sensitive broadband X-ray observations of transient phenomena in the 2030s

HEX-P will launch at a time when the sky is being routinely scanned for transient gravitational wave, electromagnetic and neutrino phenomena that will require the capabilities of a sensitive, broadband X-ray telescope for follow up studies. These include the merger of compact objects such as neutron stars and black holes, stellar explosions, and the birth of new compact objects. \hexp\ will probe the accretion and ejecta from these transient phenomena through the study of relativistic outflows and reprocessed emission, provide unique capabilities for understanding jet physics, and potentially revealing the nature of the central engine.

astro-ph.HE↗

arXiv2019

Discovery and Identification of MAXI J1621-501 as a Type I X-ray Burster with a Super-Orbital Period

MAXI J1621-501 is the first Swift/XRT Deep Galactic Plane Survey transient that was followed up with a multitude of space missions (NuSTAR, Swift, Chandra, NICER, INTEGRAL, and MAXI) and ground-based observatories (Gemini, IRSF, and ATCA). The source was discovered with MAXI on 2017 October 19 as a new, unidentified transient. Further observations with NuSTAR revealed 2 Type I X-ray bursts, identifying MAXI J1621-501 as a Low Mass X-ray Binary (LMXB) with a neutron star primary. Overall, 24 Type I bursts were detected from the source during a 15 month period. At energies below 10 keV, the source spectrum was best fit with three components: an absorbed blackbody with kT = 2.3 keV, a cutoff power law with index $Γ$ = 0.7, and an emission line centered on 6.3 keV. Timing analysis of the X-ray persistent emission and burst data has not revealed coherent pulsations from the source or an orbital period. We identified, however, a super-orbital period $\sim{}$78 days in the source X-ray light curve. This period agrees very well with the theoretically predicted radiative precession period of $\sim{}$82 days. Thus, MAXI J1621-501 joins a small group of sources characterized with super-orbital periods.

astro-ph.HE↗