Nanda Rea

We report on the detection of polarized X-ray emission from the new Galactic magnetar EP J223759.5+531421, discovered by the Einstein Probe Wide-field X-ray Telescope on 2026 June 28. The Imaging X-ray Polarimetry Explorer (IXPE) follow-up observation started on 2026 July 6 and detected the source at a (absorbed) flux of ≈4.5 × 10 ⁻¹¹ erg cm ⁻² s ⁻¹ (2–7.5 keV range), confirming the presence of two thermally emitting regions on the star surface, with temperatures ≈0.5 and ≈1 keV. A significant (at >4 σ confidence level) phase- and energy-integrated polarization degree of ≈7% was detected with the polarization angle ≈−2 ^∘ East of the celestial North. The two thermal components exhibit quite different polarization properties. The difference is larger in the phase-folded data, with the polarization degree reaching ∼63% in the range 4–7.5 keV, in correspondence with the secondary peak of the pulse profile, and never exceeding ∼30% at lower energies. The polarization angle continuously oscillates from −90 ^∘ to +90 ^∘ over one rotational cycle and is well fit by the rotating vector model, with an inclination of the line of sight and of the magnetic dipole axis relative to the star spin axis of ≈28 ^∘ and ≈109 ^∘ , respectively. The data point to an emission geometry in which the warm thermal component could originate from two antipodal caps, possibly in a magnetically condensed state, while the hot one could come from a smaller region covered by an atmosphere.

Understanding the Galactic population of isolated neutron stars within a unified framework provides key insights into their birth properties, evolutionary pathways, and the connections between different neutron star classes. In this work, we aim to reproduce both the observed radio and quiescent X-ray emission from isolated neutron stars, including radio pulsars, magnetars, and X-ray-dim isolated neutron stars (XDINSs). We develop a comprehensive population-synthesis framework that models neutron star birth properties, their dynamical, rotational, and magnetothermal evolution, as well as radio and X-ray emission, and the selection effects of corresponding surveys. To simulate realistic X-ray spectra, we account for magnetospheric resonant cyclotron scattering and interstellar absorption. Additionally, we model the observational bias introduced by magnetar outbursts by linking the outburst rate to magnetic stresses in the stellar crust. We then employ a simulation-based inference method, namely truncated sequential neural posterior estimation, to reconstruct the birth properties, such as the initial magnetic-field distribution. We find that the Galactic neutron star population can be described by a two-component log-normal distribution of birth magnetic fields with peaks at 5 × 10 ¹² G and 10 ¹⁴ G, respectively. We further find that the Galactic neutron star birth rate is around 3–4 per century. Our results help establish the contribution of neutron stars as central engines of powerful astrophysical transient phenomena, such as fast radio bursts, superluminous supernovae, and gamma-ray bursts.

Accreting millisecond pulsars (AMSPs) and transitional millisecond pulsars (tMSPs) are neutron star low-mass X-ray binaries that can evolve into recycled radio millisecond pulsars. In both types of systems, X-ray pulsations have been detected during phases of X-ray activity when matter accretion through a disc was turned on. On the other hand, when accretion stops, and these systems enter the quiescent low-luminosity X-ray state, and only tMSPs become visible as radio pulsars. Despite several attempts, radio pulsations have never been detected in quiescent AMSPs, except for IGR J18245−2452. We present the results of two observational campaigns performed on the AMSPs IGR J00291+5934 and MAXI J1957+032 with the Five-hundred-meter Aperture Spherical Telescope ( FAST ) in L band (1–1.5 GHz). The two sources have most likely been observed in quiescence, as suggested by the upper limits on their X-ray and optical flux obtained with Swift and the Las Cumbres Observatory, respectively. We performed a deep search for coherent periodicities in radio, but found no significant candidate signal at the known spin frequency of these sources or at other frequencies. Assuming a pulse duty cycle of 10%, we derive upper limits on the pulsed radio flux density of 3.3 μJy and 5.6 μJy for IGR J00291+5934 and MAXI J1957+032, respectively, which are the most stringent limits for any known persistent AMSP so far.

Long-period transients (LPTs) challenge our knowledge of the mechanism producing radio periodic pulsations in compact objects. Some LPTs have been associated with systems hosting a white dwarf and a low-mass star in a detached binary. Recently, a new LPT (ASKAP J174508.9-505149) has been classified as an accreting cataclysmic variable (CV). In the present Letter, we report on the detailed study of the X-ray variability of ASKAP J174508.9-505149 as observed by XMM-Newton and Einstein Probe between September 2025 and May 2026. Simultaneous optical and radio observations are also presented. We studied the timing variability of the source, and estimated an X-ray periodicity of P = 4868(22) s, consistent with radio and optical periods. We also observe the same periodicity in the hardness ratio extracted from the XMM-Newton observation, peaking at the minimum of the modulation. A long-term modulation is also present in the X-rays and in the B-band photometry, but it is poorly constrained by the current dataset. Spectral X-ray analysis shows the presence of a black-body component (∼0.1 keV), a collisionally ionised plasma (∼15 keV), and an absorption feature at 0.77 keV (possibly due to Oxygen-VII). This is the third LPT detected in the X-ray band, the second with a detected X-ray periodicity and variable X-ray emission, and the first conclusively recognised as an accreting magnetic CV.

Transient low-mass X-ray binaries (LMXBs) are usually first detected by all-sky X-ray monitors when they enter new outbursts. These detections typically occur at X-ray luminosities above ∼10 ³⁶ erg/s. Observations of these sources during the early rise of the outbursts have therefore been very limited so far. However, the launch of the Einstein Probe (EP) has greatly improved our ability to detect fainter X-ray activity, unlocking access to the early rise of the outburst. In September 2024, EP detected the early onset of a new outburst from the well-known neutron star LMXB Aql X-1, catching the source at a luminosity below 10 ³⁵ erg/s. We present results from a comprehensive multiwavelength campaign of this event, combining data from EP, NICER , NuSTAR , and Swift and Las Cumbres Observatory covering the full outburst from its early rise through its return to quiescence. By comparing X-ray and optical light curves obtained by the Las Cumbres Observatory during the initial rise, we show that the start of the X-ray emission lagged the optical rise by 13 days at most. We did not observe the exact moment at which the source changed from (optical or X-rays) quiescence to rise, so that other lags cannot be completely ruled out, however. Time-resolved X-ray spectroscopy revealed the evolution of the geometry and the physical properties of the accretion flow during this early stage of the outburst, as well as at higher luminosities as the source transitioned through the canonical X-ray spectral states, the hard, intermediate, and soft state. These data show that the source underwent a very rapid transition of about 12 h from the hard to the soft state about two weeks after the optical onset of the outburst. At the state transition, peculiar trends are observed for the temperature and physical sizes of the inner disk region and a blackbody near the surface of the neutron star, which might suggest that at this stage, a geometrically thicker inner disk emerges. We discuss these results in the context of timescales for outburst evolution and state transitions in accreting neutron stars and black holes.

We report multiwavelength observations of EP240305a, an uncatalogued X-ray transient detected by the Einstein Probe on March 5, 2024. The source exhibits distinct characteristics across the X-ray, optical, near-infrared, and radio bands. The soft X-ray observations show two significant flares lasting ~100 − 250 s, accompanied by rapid flux decay in a few days, and the optical and near-infrared data reveal a faint, candidate counterpart. In contrast, the radio observations expose a long-term spectral evolution from a self-absorbed to an optically thin state within two months, implying discrete jet ejection. We compare EP240305a with known classes of X-ray transients and find that it is unlikely to be associated with long-timescale transients such as jetted tidal disruption events or X-ray binaries. Its properties also disfavor a short-timescale stellar flare origin. Although the absence of optical spectroscopy prevents a redshift determination, the source exhibits properties similar to those of gamma-ray-dark gamma-ray burst-like transients, which may be associated with relativistic jets viewed off-axis or with choked jets. The discovery of EP240305a, along with other uncataloged transients detected by the Einstein Probe, underscores the scientific potential of highly sensitive X-ray survey telescopes and rapid-response multiwavelength follow-up observations in exploring the nature of atypical astronomical transients.

We report a serendipitous and unexpected discovery from a study of long-term X-ray variability in the nearby galaxy M83: a substantial fraction of X-ray sources classified as supernova remnants (SNRs) exhibit significant variability. This behavior is surprising because soft X-ray emission from remnants older than ∼100 yr is generally expected to evolve smoothly, with little variation on decade-long timescales. At least one source in our sample is associated with the historical supernova SN 1957D, for which the X-ray emission and variability are well explained by shock interaction between the expanding ejecta and the surrounding circumstellar medium. For most sources, however, the observed variability is difficult to reconcile with standard models of SNR evolution. Instead, the light curves and spectra are more naturally explained by the presence of an accretion-powered source coincident with the remnant. Plausible explanations include fallback accretion onto the compact object formed in the SN explosion or an X-ray binary physically associated with the remnant. The variable X-ray sources are statistically correlated with high-mass stars, indicating potential donor stars and supporting a high-mass X-ray binary interpretation. Their luminosities and spectral properties are also consistent with emission from cool accretion disks around stellar-mass black holes. Although the current data do not uniquely determine the nature of these systems, the results suggest that accretion-powered sources associated with SNRs may be more common than previously recognized.

Long-period radio transients (LPTs) are coherent bursts of polarized radio emission that repeat periodically on timescales of minutes to hours. Little is known about the physical origins of these systems. Astronomers have proposed magnetars that rotate slowly and white dwarfs that rapidly orbit with a companion star as potential explanations. While several recent examples appear to support the latter hypothesis, the mechanism generating these bright radio pulses remains poorly understood. Here we report our discovery and classification of the LPT ASKAP J174508.9-505149 as an accreting white dwarf binary. This object has an ~1.3 h spectroscopic orbital period and exhibits orbitally modulated X-ray emission and radio bursts. These elliptically polarized radio bursts drift in emission frequency, potentially due to a longer beat period, and turn off for several hours at a time. Some LPTs have been associated with non-interacting white dwarf binaries. We have spectroscopically confirmed this system as an accreting cataclysmic variable, identified through characteristic optical emission lines and an ongoing X-ray outburst. Our results strengthen the link between at least some LPTs and white dwarf binaries.

We report the discovery and multiwavelength characterization of the Galactic transient EPJ174942.2–384834, first detected by the Einstein Probe during a faint X-ray outburst in 2025 March. Coordinated follow-up observations revealed two major outbursts and a rebrightening over a 7 month period. Broadband X-ray spectral modeling shows that the outburst emission was dominated by thermal Comptonization of very soft seed photons. The absence of a detected thermal disk component, together with the low inferred seed-photon temperature, is consistent with a cool and possibly truncated accretion disk. The X-ray spectrum remained consistently hard throughout the outburst activity, with a power-law photon index of Γ ≈ 1–2, gradually softening as the flux declined. The optical/UV counterpart brightened in tandem with the X-ray emission and exhibited a blue continuum with broad Balmer absorption features. Together with the optical/UV–X-ray luminosity correlation, this supports a disk-dominated origin of the optical/UV outburst emission, with viscous heating likely playing a major role and irradiation possibly contributing, especially in the UV. No radio counterpart was detected, implying at most very faint jet activity. Taken together, the observed properties support the classification of EPJ174942.2–384834 as a very faint X-ray transient black hole candidate. This study demonstrates the ability of the Einstein Probe to uncover and characterize the faintest accreting compact objects in the Galaxy.