arXiv subjects

J. A. J. Alford

Publications and source records attributed to J. A. J. Alford.

9 recordsLinked to original sources

arXiv2026

Evidence of an Energetic Magnetar Powering 1LHAASO J0500$+$4454

We investigate the origin of unidentified, extended TeV source 1LHAASO J0500$+$4454, considering three possible origins: cosmic rays interacting with a molecular cloud (MC), particles accelerated in a currently undetected supernova remnant (SNR), and an energetic outflow powered by a pulsar. Upper limits on the CO and X-ray emission from the $γ$-ray emitting region disfavor the MC and SNR scenarios, respectively. If a nebula of inverse Compton scattering $e^{\pm}$ powers 1LHAASO J0500$+$4454, then SED modeling indicates that the current particle energy in the nebula is $\sim 4 \times 10^{48}$ erg. If the coincident magnetar SGR 0501$+$4516's rotational energy powered 1LHAASO J0500$+$4454, then a conservative energy budget calculation requires an initial magnetar spin period $P_{0} \lesssim 5$ ms and a spin-down timescale $τ_{\rm sd} \lesssim 30$ yr, which has implications for the origins of magnetars.

astro-ph.HE↗

arXiv2025

The Timing Evolution of the Magnetar Swift J1818.0-1607 During a Period of Reduced Activity

We report results from an observational campaign of the radio-loud magnetar Swift J1818.0-1607 using the Green Bank Telescope (GBT) at 2.0 GHz, which began in November 2021 during a period of reduced activity approximately 20 months after its March 2020 outburst. Over the 60-day duration reported here, the integrated pulse profile remained consistently stable, exhibiting a single, narrow peak with with a small precursor component and no evidence of a postcursor one. This pulse profile is in sharp contrast to the double-peaked morphology observed during an observing campaign approximately 120 days preceding ours. Along with this change in the integrated pulse profile shape, we also measure a slower spin-down rate compared to the end of that preceding campaign. Together, these differences suggest that a mode-switching event likely occurred between the end of that campaign and the start of ours. Finally, we derived a phase-connected timing solution from our data, from which we inferred a characteristic age of approximately 2500 years, about 2.5 times older than the most recent published estimate, and a surface dipole magnetic field strength of roughly 1 x 10^14 G, nearly three times weaker. These updated estimates reflect the short-term variations in the magnetar's spin-down rate, from which both its age and magnetic field strength are inferred, rather than intrinsic changes in the magnetar itself.

astro-ph.HE↗

arXiv2025

NuSTAR and XMM-Newton Observations of PSR J1930+1852 and Its Pulsar Wind Nebula

Synchrotron X-ray emission from a pulsar wind nebula (PWN) is a sensitive probe of its magnetic field and high-energy particle population. Here we analyze contemporaneous NuSTAR and XMM-Newton observations of the PWN G54.1+0.3, powered by pulsar PSR J1930+1852. We also present a preliminary timing analysis of the central pulsar PSR J1930+1852 and analyze its X-ray pulse profiles in different energy bands. We detect X-ray emission from the combined pulsar and PWN system up to $\approx70$ keV, while emission from the PWN itself has been detected up to $\approx30$ keV, with a photon index $Γ$ increasing from $\sim 1.9$ to $\sim 2.4$ with photon energy between 3 and 30 keV. PWN G54.1+0.3's X-ray spectrum is consistent with a broken power law, with break energy $E_{\rm break} \approx 5$ keV, consistent with synchrotron cooling of a single power-law particle spectrum. The best-fit broadband spectral energy distribution model after the inclusion of this new spectral data indicates a maximum particle $E_{\rm max} \sim 400$ TeV. We discuss PSR J1930+1852 and PWN G54.1+0.3 in the context of other PWNe powered by young energetic pulsars.

astro-ph.HE↗

arXiv2023

The High Energy X-ray Probe (HEX-P): Magnetars and Other Isolated Neutron Stars

The hard X-ray emission from magnetars and other isolated neutron stars remains under-explored. An instrument with higher sensitivity to hard X-rays is critical to understanding the physics of neutron star magnetospheres and also the relationship between magnetars and Fast Radio Bursts (FRBs). High sensitivity to hard X-rays is required to determine the number of magnetars with hard X-ray tails, and to track transient non-thermal emission from these sources for years post-outburst. This sensitivity would also enable previously impossible studies of the faint non-thermal emission from middle-aged rotation-powered pulsars (RPPs), and detailed phase-resolved spectroscopic studies of younger, bright RPPs. The High Energy X-ray Probe (HEX-P) is a probe-class mission concept that will combine high spatial resolution X-ray imaging ($<5$ arcsec half-power diameter (HPD) at 0.2--25 keV) and broad spectral coverage (0.2--80 keV) with a sensitivity superior to current facilities (including XMM-Newton and NuSTAR). HEX-P has the required timing resolution to perform follow-up observations of sources identified by other facilities and positively identify candidate pulsating neutron stars. Here we discuss how HEX-P is ideally suited to address important questions about the physics of magnetars and other isolated neutron stars.

astro-ph.HE↗

arXiv2023

Do Central Compact Objects have Carbon Atmospheres?

Only three of the dozen central compact objects (CCOs) in supernova remnants (SNRs) show thermal X-ray pulsations due to non-uniform surface temperature (hot-spots). The absence of X-ray pulsations from several unpulsed CCOs has motivated suggestions that they have uniform-temperature carbon atmospheres (UTCAs), which adequately fit their spectra with appropriate neutron star (NS) surface areas. This is in contrast to the two-temperature blackbody or hydrogen atmospheres that also fit well. Here we investigate the applicability of UTCAs to CCOs. We show the following: (i) The phase-averaged spectra of the three pulsed CCOs can also be fitted with a UTCA of the appropriate NS area, despite pulsed CCOs manifestly having non-uniform surface temperature. A good spectral fit is therefore not strong support for the UTCA model of unpulsed CCOs. (ii) An improved spectrum of one unpulsed CCO, previously analyzed with a UTCA, does not allow an acceptable fit. (iii) For two unpulsed CCOs, the UTCA does not allow a distance compatible with the SNR distance. These results imply that, in general, CCOs must have hot, localized regions on the NS surface. We derive new X-ray pulse modulation upper limits on the unpulsed CCOs, and constrain their hot spot sizes and locations. We develop an alternative model that accounts for both the pulsed and unpulsed CCOs: a range of angles between hot spot and rotation axes consistent with an exponential distribution with scale factor $λ\sim 20^{\circ}$. We discuss physical mechanisms that could produce such small angles and small hot-spots.

astro-ph.HE↗

arXiv2021

Measuring the Non-Axially-Symmetric Surface Temperature Distribution of the Central Compact Object in Puppis A

The surface temperature distributions of central compact objects (CCOs) are powerful probes of their crustal magnetic field strengths and geometries. Here we model the surface temperature distribution of RX J0822$-$4300, the CCO in the Puppis A supernova remnant (SNR), using $471$ ks of XMM-Newton data. We compute the energy-dependent pulse profiles in sixteen energy bands, fully including the general relativistic effects of gravitational redshift and light bending, to accurately model the two heated surface regions of different temperatures and areas, in addition to constraining the viewing geometry. This results in precise measurements of the two temperatures: $kT_{\rm warm} = (1+z) \times 0.222_{-0.019}^{+0.018}$ keV and $kT_{\rm hot} = (1+z) \times 0.411\pm0.011$ keV. For the first time, we are able to measure a deviation from a pure antipodal hot-spot geometry, with a minimum value of $1.\!^{\circ}1 \pm 0.\!^{\circ}2$, and an expectation value of $9.\!^{\circ}35 \pm 0.\!^{\circ}17$ among the most probable geometries. The discovery of this asymmetry, along with the factor of $\approx2$ temperature difference between the two emitting regions, may indicate that RX J0822$-$4300 was born with a strong, tangled crustal magnetic field.

astro-ph.HE↗

arXiv2021

The X-ray evolution and geometry of the 2018 outburst of XTE J1810-197

After 15 years, in late 2018, the magnetar XTE J1810-197 underwent a second recorded X-ray outburst event and reactivated as a radio pulsar. We initiated an X-ray monitoring campaign to follow the timing and spectral evolution of the magnetar as its flux decays using Swift, XMM-Newton, NuSTAR, and NICER observations. During the year-long campaign, the magnetar reproduced similar behaviour to that found for the first outburst, with a factor of two change in its spin-down rate from $\sim7.2\times10^{-12}$ s s$^{-1}$ to $\sim1.5\times10^{-11}$ s s$^{-1}$ after two months. Unique to this outburst, we confirm the peculiar energy-dependent phase shift of the pulse profile. Following the initial outburst, the spectrum of XTE J1810-197 is well-modelled by multiple blackbody components corresponding to a pair of non-concentric, hot thermal caps surrounded by a cooler one, superposed to the colder star surface. We model the energy-dependent pulse profile to constrain the viewing and surface emission geometry and find that the overall geometry of XTE J1810-197 has likely evolved relative to that found for the 2003 event.

astro-ph.HE↗

arXiv2019

The 2018 X-ray and Radio Outburst of Magnetar XTE J1810-197

We present the earliest X-ray observations of the 2018 outburst of XTE J1810-197, the first outburst since its 2003 discovery as the prototypical transient and radio-emitting anomalous X-ray pulsar (AXP). The Monitor of All-sky X-ray Image (MAXI) detected XTE J1810-197 immediately after a November 20-26 visibility gap, contemporaneous with its reactivation as a radio pulsar, first observed on December 8. On December 13 the Nuclear Spectroscopic Telescope Array (NUSTAR) detected X-ray emission up to at least 30 keV, with a spectrum well-characterized by a blackbody plus power-law model with temperature kT = 0.74+/-0.02 keV and photon index Gamma = 4.4+/-0.2 or by a two-blackbody model with kT = 0.59+/-0.04 keV and kT = 1.0+/-0.1 keV, both including an additional power-law component to account for emission above 10 keV, with Gamma_h = -0.2+/-1.5 and Gamma_h = 1.5+/-0.5, respectively. The latter index is consistent with hard X-ray flux reported for the non-transient magnetars. In the 2-10 keV bandpass, the absorbed flux is 2E-10 erg/s/cm^2, a factor of 2 greater than the maximum flux extrapolated for the 2003 outburst. The peak of the sinusoidal X-ray pulse lags the radio pulse by approx. 0.13 cycles, consistent with their phase relationship during the 2003 outburst. This suggests a stable geometry in which radio emission originates on magnetic field lines containing currents that heat a spot on the neutron star surface. However, a measured energy-dependent phase shift of the pulsed X-rays suggests that all X-ray emitting regions are not precisely co-aligned.

astro-ph.HE↗

arXiv2016

Radio disappearance of the magnetar XTE J1810-197 and continued X-ray timing

We report on timing, flux density, and polarimetric observations of the transient magnetar and 5.54 s radio pulsar XTE J1810-197 using the GBT, Nancay, and Parkes radio telescopes beginning in early 2006, until its sudden disappearance as a radio source in late 2008. Repeated observations through 2016 have not detected radio pulsations again. The torque on the neutron star, as inferred from its rotation frequency derivative f-dot, decreased in an unsteady manner by a factor of 3 in the first year of radio monitoring. In contrast, during its final year as a detectable radio source, the torque decreased steadily by only 9%. The period-averaged flux density, after decreasing by a factor of 20 during the first 10 months of radio monitoring, remained steady in the next 22 months, at an average of 0.7+/-0.3 mJy at 1.4 GHz, while still showing day-to-day fluctuations by factors of a few. There is evidence that during this last phase of radio activity the magnetar had a steep radio spectrum, in contrast to earlier behavior. There was no secular decrease that presaged its radio demise. During this time the pulse profile continued to display large variations, and polarimetry indicates that the magnetic geometry remained consistent with that of earlier times. We supplement these results with X-ray timing of the pulsar from its outburst in 2003 up to 2014. For the first 4 years, XTE J1810-197 experienced non-monotonic excursions in f-dot by at least a factor of 8. But since 2007, its f-dot has remained relatively stable near its minimum observed value. The only apparent event in the X-ray record that is possibly contemporaneous with the radio shut-down is a decrease of ~20% in the hot-spot flux in 2008-2009, to a stable, minimum value. However, the permanence of the high-amplitude, thermal X-ray pulse, even after the radio demise, implies continuing magnetar activity.

astro-ph.SR↗