arXiv subjects

N. A. Webb

Publications and source records attributed to N. A. Webb.

At least 19 recordsLinked to original sources

arXiv2026

A systematic assessment of the short-term X-ray behaviour of the $γ$ Cas analogues

Context. Overall, $γ$ Cas analogues represent a class of OBe stars displaying bright and hard X-ray emission. This emission was recently determined to be the result of accretion onto a companion white dwarf (WD), which might possibly be a magnetic WD. Aims. We present a systematic review of the properties of $γ$ Cas analogues on short-term (kilosecond, ks) scales to further investigate the nature of the X-ray source. In particular, the presence of periodicities could be associated with the rotation of a magnetic WD. Methods. We generated the power spectra and a generalised Fourier algorithm. We applied an epoch folding method, along with Z2n approaches, to 131 X-ray detections of 26 $γ$ Cas analogues and two candidate sources. Results. We identified five sources ($γ$ Cas, $ζ$ Tau, HD45314, V771 Sgr, and $π$ Aqr), with recurrent periodic variability (P $\sim$ 1.2-5.4 ks) across observations. Period values and pulsed fractions are consistent with those reported for intermediate polars. However, while similar periods are found in all exposures of an object, the exact period value or folded light curve shape can vary. We also identify another nine sources displaying periodicities that cannot be confirmed, as there was only one observation available for their analysis. In other cases, periodicities appear (at best) transient and/or marginally significant. Aperiodic variability could be fitted by a power law in 18 of the sources, while the power-law index always is $\sim$ 1, as observed in other accreting WDs. Conclusions. The global properties of the observed short-term X-ray variability of the $γ$ Cas analogues are consistent with those observed from other accreting WD systems.

astro-ph.SR↗

arXiv2026

Mass and radius measurements of the neutron star 47~Tuc X7 -- A new bias-free method

Neutron star (NS) radius measurements provide precious information to constrain the dense matter equation of state (EOS). Quiescent low-mass X-ray binaries (qLMXBs) have been used for this purpose, but a number of sources of systematic biases were uncovered, making other sources more favored for EOS studies. We aim to reintroduce qLMXBs as reliable sources of NS mass and radius measurements with a new method, free of systematic biases. We test our implementation on the qLMXB X7 in the globular cluster 47 Tucanae. We used X-PSI to perform the spectral analysis of the 47Tuc X7 observations. X-PSI accurately models the effects of the unknown NS rotation and possible surface anisotropies (two sources of biases in qLMXBs) on the NS spectra. The most significant source of bias on the radius is usually the chemical composition of the NS atmosphere, which, in the case of 47Tuc X7, is known to be hydrogen-rich. A broad range of masses and radii was explored. We obtain a NS radius at 1.4 $M_\odot$ of $R_{1.4} = 12.9\pm0.4$ km (68% credible interval). A shift of the radius by less than a % is measured compared to the model where these sources of systematic uncertainties are neglected. More importantly, including rotation and surface anisotropies in the modeling does not significantly broaden the radius posteriors. We also place strong constraints on the X-ray pulsed fraction (upper limit of 6.0% at a 99.97% credible level) caused by the possible presence of a hot spot. This suggests that, for 47Tuc X7, robust radius constraints can be obtained even without considering systematics, likely because of the deep exposures. We use the resulting M-R constraints from this NS to quantify the improvement on an EOS inference when combined with other measurements. We show that, using recently developed tools, qLMXBs can be exploited to infer reliable NS masses and radii, which can in turn constrain the EOS.

astro-ph.HE↗

arXiv2026

STONKS first results: Long-term transients in the XMM-Newton Galactic plane survey

The study of astronomical transients at high energies provides insights into some of the most extreme physical events in the universe; however, carrying out their detection and fast follow-up studies are limited by instrumental constraints. Search for Transient Object in New observations using Known Sources (STONKS) is a near-real-time transient detection system for XMM-Newton offering the capability to detect transients in XMM-Newton observations at fainter fluxes than can be achieved with wide survey instruments. We present the transients detected with the STONKS pipeline found in an XMM-Newton multi-year heritage survey of the Galactic plane to identify and classify highly variable X-ray sources that have recently been reported in this region. We examined the alerts created by the STONKS pipeline from over 200 XMM observations of the Galactic plane, screening for instrumental effects. The 78 alerts associated with 70 real astrophysical sources were then subjected to further temporal and spectral analysis. From the 70 sources we identified, we were able to classify 32 with a high degree of confidence, including 7 X-ray binaries, 1 $γ$-Cas analogue, and 1 magnetar candidate. Of the 70 sources, 23 were detected for the first time in X-rays. This systematic analysis of publicly available data has shown the value and potential of STONKS in the application to XMM-Newton observations. It will enable the community to detect transient and highly variable sources at fainter fluxes than with any other X-ray transient detection systems.

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arXiv2025

Needle in a Poissonian haystack -- An X-ray astronomer's guide to QPE fishing

After six years of studies following the discovery of GSN069, a link is starting to appear between the elusive Quasi-Periodic Eruptions (QPEs) and other types of nuclear transients, among which are Tidal Disruption Events (TDEs). As such, observing strategies are adapting, with a current trend focusing on late-time X-ray follow-ups of (optical) TDEs. While these campaigns are so far proving quite successful, the inherent confirmation bias they introduce in our sample could lead the community to hasty, and perhaps erroneous, conclusions. It is thus important to still pursue the search for nuclear transients in other, more agnostic directions. In this work, we focus on the observational aspects of our field, and lay out two different methods that can be deployed in order to reveal new QPE sources. These complementary methods enable the detection of long-term ($\sim$years) and short term ($\sim$minutes) transient events, that would have otherwise been missed by the standard detection pipelines. Both of these methods can be used either for data mining in the 25 years worth of XMM-Newton archive, or to trigger real-time follow-ups upon a more recent discovery.

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arXiv2025

4XMM J175136.8-275858: A New Magnetar Candidate?

Magnetars are very rare astrophysical objects, with $\sim$31 known to date. They are best understood as highly magnetised neutron stars, but a greater number need to be found to constrain their role in stellar evolution pathways. We apply a novel approach for the detection of fast, transient X-ray sources, using a revised version of the EPIC XMM-Newton Outburst Detector (EXOD) with the aim of detecting and identifying new and rare variable compact objects. We detect a transient, variable source notable for its strong variability and hard spectrum. The emission from 4XMM J175136.8-275858 is well characterised by a blackbody, with temperatures between $\sim$1.8--5\,keV during its lower luminosity phase. Its temperature is poorly constrained during its brightest phase, and we observe an increase in luminosity by two orders of magnitude over timescales of a few ks. This is driven by increased emission of X-rays at energies above 2\,keV, with a luminosity decay potentially over weeks or months. Derived luminosities for 4XJ1751-2759 range up to $\sim10^{35} \text{\,erg s}^{-1}$ at 8\,kpc at the Galactic centre, but neutral hydrogen column densities are greater than predicted Galactic values possibly implying a greater distance to the source, still within our galaxy, further increasing its luminosity. A consideration of optical and IR information in combination with the X-ray observations allow us to exclude the possibility that 4XJ1751-2759 is a star, rotationally powered pulsar or supergiant fast X-ray transient. This rapid, hard, variability is closest to that of outbursts in magnetars than any other known class of X-ray transient.

astro-ph.HE↗

arXiv2025

Identification of high-redshift X-ray active galactic nuclei in the 4XMM-DR11 serendipitous catalogue using DES data: Comparative analysis with optically-selected QSOs

X-rays provide a robust method in identifying AGN. However, in the high-redshift Universe, their space density is relatively low, and, in combination with the small areas covered by X-ray surveys, the selected AGN are poorly sampled. Deep optical/infrared data are essential for locating counterparts and determining redshifts. In this work, we leverage the XMM-Newton 4XMM-DR11 serendipitous catalogue alongside the extensive optical Dark Energy Survey and the near-infrared VISTA Hemisphere Survey (VHS) to select one of the largest high-z X-ray AGN samples to date. Our analysis focuses on the overlapping area of these surveys, covering about 185 $deg^2$. In addition, we aspire to compare the properties of the X-ray AGN with the optically-selected QSOs. For sources without spectroscopic data (80%), we estimated the photometric redshifts using both SED fitting and machine-learning algorithms. Among the 65,000 X-ray sources in the 4XMM-DES-VHS area, we ended up with 833 z>3.5 AGN (11% having spec-z information) with high reliability and a fraction of outliers η<10%. The sample completeness is 90%, driven by the depth of DES data. Only 10% of the X-ray selected AGN are also optical QSOs and vice versa. Our findings indicate an observed absorbed fraction ($\log N_H~[cm^{-2}] \geq 23$) of 20-40% for the X-ray AGN, significantly higher than that of optical QSOs. X-ray AGN exhibit fainter observed optical magnitudes and brighter mid-IR magnitudes than optical QSOs. Their median rest-frame SED shapes differ notably with optical QSOs being dominated by AGN emission in the UV-optical wavelengths. While the median SEDs of X-ray AGN suggest extinction in the UV-optical range, individual sources exhibit a wide range of spectral shapes, indicating significant diversity within the population. This analysis supports that X-ray and optically-selected AGN represent distinct and complementary populations.

astro-ph.GA↗

arXiv2024

On the nature of the 2.4 hr-period eclipsing cataclysmic variable W2 in 47 Tuc

W2 (CXOGlb J002415.8-720436) is a cataclysmic variable (CV) in the Galactic globular cluster 47 Tucanae. Its modulation was discovered within the CATS@BAR project. The source shows all the properties of magnetic CVs, but whether it is a polar or an intermediate polar is still a matter of debate. This paper investigates the spectral and temporal properties of the source, using all archival X-ray data from Chandra and eROSITA Early Data Release, to establish whether the source falls within the category of polars or intermediate polars. We fitted Chandra archival spectra with three different models: a power law, a bremsstrahlung and an optically thin thermal plasma. We also explored the temporal properties of the source with searches for pulsations with a power spectral density analysis and a Rayleigh test ($Z_n^2$). W2 displays a mean luminosity of $\sim 10^{32}$ erg s$^{-1}$ over a 20-year span, despite lower values in a few epochs. The source is not detected in the latest observation, taken with Chandra in 2022, and we infer an X-ray luminosity $\leq 7 \times 10^{31}$ erg s$^{-1}$. The source spectral shape does not change over time and can be equally well fitted with each of the three models, with a best-fit photon index of 1.6 for the power law and best-fit temperatures of 10 keV for both the bremsstrahlung and the thermal plasma models. We confirm the previously detected period of 8649 s, ascribed to the binary orbital period, and found a cycle-to-cycle variability associated with this periodicity. No other significant pulsation is detected. Considering the source orbital period, luminosity, spectral characteristics, long-term evolution and strong cycle-to-cycle variability, we suggest that W2 is a magnetic CV of the polar type.

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arXiv2024

STONKS: Quasi-real time XMM-Newton transient detection system

Over recent decades, astronomy has entered the era of massive data and real-time surveys. This is improving the study of transient objects - although they still contain some of the most poorly understood phenomena in astrophysics, as it is inherently more difficult to obtain data on them. In order to help detect these objects in their brightest state, we have built a quasi-real time transient detection system for the XMM-Newton pipeline: the Search for Transient Objects in New detections using Known Sources (STONKS) pipeline. STONKS detects long-term X-ray transients by automatically comparing new XMM-Newton detections to any available archival X-ray data at this position, sending out an alert if the amplitude of variability between observations is over 5. This required an initial careful cross-correlation and flux calibration of various X-ray catalogs from different observatories (XMM-Newton, Chandra, Swift, ROSAT, and eROSITA). We also systematically computed the XMM-Newton upper limits at the position of any X-ray source covered by the XMM-Newton observational footprint, even without any XMM-Newton counterpart. The behavior of STONKS was then tested on all 483 observations performed with imaging mode in 2021. Over the 2021 testing run, STONKS provided $0.7^{+0.7}_{-0.5}$ alerts per day, about 80% of them being serendipitous. STONKS also detected targeted tidal disruption events, ensuring its ability to detect other serendipitous events. As a byproduct of our method, the archival multi-instrument catalog contains about one million X-ray sources, with 15% of them involving several catalogs and 60% of them having XMM-Newton upper limits. STONKS demonstrates a great potential for revealing future serendipitous transient X-ray sources, providing the community with the ability to follow-up on these objects a few days after their detection.

astro-ph.HE↗

arXiv2023

Tormund's return: Hints of quasi-periodic eruption features from a recent optical tidal disruption event

Quasi-periodic eruptions (QPEs) are repeating thermal X-ray bursts associated with accreting massive black holes, the precise underlying physical mechanisms of which are still unclear. We present a new candidate QPE source, AT 2019vcb (nicknamed Tormund by the ZTF collaboration), which was found during an archival search for QPEs in the XMM-Newton archive. It was first discovered in 2019 as an optical tidal disruption event (TDE) at $z=0.088$, and its X-ray follow-up exhibited QPE-like properties. Our goals are to verify its robustness as QPE candidate and to investigate its properties to improve our understanding of QPEs. We performed a detailed study of the X-ray spectral behaviour of this source over the course of the XMM-Newton archival observation. We also report on recent Swift and NICER follow-up observations to constrain the source's current activity and overall lifetime, as well as an optical spectral follow-up. The first two Swift detections and the first half of the 30 ks XMM-Newton exposure of Tormund displayed a decaying thermal emission typical of an X-ray TDE. However, the second half of the exposure showed a dramatic rise in temperature (from 53 to 114 eV) and 0.2-2 keV luminosity (from $3.2\times10^{42}$ to $1.2\times10^{44}$ erg s$^{-1}$). The late-time NICER follow-up indicates that the source is still X-ray bright more than three years after the initial optical TDE. Although only a rise phase was observed, Tormund's strong similarities with a known QPE source (eRO-QPE1) and the impossibility to simultaneously account for all observational features with alternative interpretations allow us to classify Tormund as a candidate QPE. If confirmed as a QPE, it would further strengthen the observational link between TDEs and QPEs. It is also the first QPE candidate for which an associated optical TDE was directly observed, constraining the formation time of QPEs.

astro-ph.HE↗

arXiv2023

Gender and Precarity in Astronomy

Following the survey Well-being in astrophysics that was sent out in March 2021, to establish how astrophysics researchers, primarily in France, experience their career, some of the results were published in Webb et al. (2021). Here we further analyse the data to determine if gender can cause different experiences in astrophysics. We also study the impact on the well-being of temporary staff (primarily PhD students and postdocs), compared to permanent staff. Whilst more temporary staff stated that they felt permanently overwhelmed than permanent staff, the experiences in astrophysics for the different genders were in general very similar, except in one area. More than three times more females than males experienced harassment or discrimination, rising sharply for gender discrimination and sexual harassment, where all of those having experienced sexual harassment and who had provided their gender in the survey, were female. Further, as previously reported (Webb et al. 2021), 20% of the respondents had suffered mental health issues before starting their career in astrophysics. We found that whilst this group was split approximately equally with regards to males and females, the number rose sharply to almost 45% of astronomers experiencing mental health issues since starting in astrophysics. Of this population, there were 50% more females than males. This excess of females was almost entirely made up of the population of women that had been harassed or discriminated against.

astro-ph.IM↗

arXiv2022

Discovery of a recurrent spectral evolutionary cycle in the ultra-luminous X-ray sources Holmberg II X-1 and NGC 5204 X-1

Most ultra-luminous X-ray sources (ULXs) are now thought to be powered by stellar-mass compact objects accreting at super-Eddington rates. While the discovery of evolutionary cycles have marked a breakthrough in our understanding of the accretion flow changes in the sub-Eddington regime in Galactic Black Hole Binaries, their evidence in the super-Eddington regime remained elusive. However, recent circumstantial evidence had hinted the presence of a recurrent evolutionary cycle in two archetypal ULXs: Holmberg II X-1 and NGC 5204 X-1. Here we build on our previous work and exploit the long-term high-cadence monitoring of Swift-XRT in order to provide evidence of the evolutionary cycle in these two sources and investigate the main physical parameters inducing their spectral transitions. We study the long-term evolution of both sources using hardness-intensity diagrams (HID) and by means of Lomb-Scargle periodograms and Gaussian processes modelling to look for periodic variability. We show that both sources follow a recurrent evolutionary pattern in the HID that can be characterized by the hard ultraluminous (HUL) and soft ultraluminous (SUL) spectral regimes, and a third state with characteristics similar to the supersoft ultraluminous (SSUL) state. The transitions between the soft states seem aperiodic, as revealed by timing analysis of the light curve of Holmberg II X-1, albeit further investigation is warranted. The light curve of NGC 5204 X-1 shows a periodicity of $\sim$ 200 days, possibly associated with the duration of the evolutionary cycle. We support a scenario in which the spectral changes from HUL to SUL are due to a periodic increase of the mass-transfer rate and subsequent narrowing of the opening angle of the supercritical funnel. The narrower funnel, combined with stochastic variability imprinted by the wind, might explain the SUL--SSUL spectral changes.

astro-ph.HE↗

arXiv2022

Well-being in French Astrophysics

It has become clear that early career astrophysics researchers (doctoral researchers, post-docs, etc) have a very diverse appreciation of their career, with some declaring it the best job that you can have and others suffering from overwork, harrassment and stress from the precarity of their job, and associated difficulties. In order to establish how astrophysics researchers, primarily in France, experience their career, we sent out a survey to understand the impact that their job has on their well-being. 276 people responded to the survey. Whilst around half of the respondents expressed pleasure derived from their career, it is clear that many (early career) researchers are suffering due to overwork, with more than a quarter saying that they work in excess of 50 hours per week and 2\% in excess of 90 h per week. Almost 30\% professed to having suffered harrassment or discrimination in the course of their work. Further, whilst only 20\% had suffered mental health issues before starting their career in astrophysics, $\sim$45\% said that they suffered with mental health problems since starting in astrophysics. Here we provide results from the survey as well as possible avenues to explore and a list of recommendations to improve (early) careers in astrophysics.

astro-ph.IM↗

arXiv2020

The XMM-Newton serendipitous survey IX. The fourth XMM-Newton serendipitous source catalogue

Sky surveys produce enormous quantities of data on extensive regions of the sky. The easiest way to access this information is through catalogues of standardised data products. {\em XMM-Newton} has been surveying the sky in the X-ray, ultra-violet, and optical bands for 20 years. The {\em XMM-Newton} Survey Science Centre has been producing standardised data products and catalogues to facilitate access to the serendipitous X-ray sky. Using improved calibration and enhanced software, we re-reduced all of the 14041 {\em XMM-Newton} X-ray observations, of which 11204 observations contained data with at least one detection and with these we created a new, high quality version of the {\em XMM-Newton} serendipitous source catalogue, 4XMM-DR9. 4XMM-DR9 contains 810795 detections down to a detection significance of 3 $σ$, of which 550124 are unique sources, which cover 1152 degrees$^{2}$ (2.85\%) of the sky. Filtering 4XMM-DR9 to retain only the cleanest sources with at least a 5 $σ$ detection significance leaves 433612 detections. Of these detections, 99.6\% have no pileup. Furthermore, 336 columns of information on each detection are provided, along with images. The quality of the source detection is shown to have improved significantly with respect to previous versions of the catalogues. Spectra and lightcurves are also made available for more than 288000 of the brightest sources (36\% of all detections).

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arXiv2020

The XMM-Newton serendipitous survey. X: The second source catalogue from overlapping XMM-Newton observations and its long-term variable content

The XMM-Newton Survey Science Centre Consortium (SSC) develops software in close collaboration with the Science Operations Centre to perform a pipeline analysis of all XMM-Newton observations. In celebration of the 20th launch anniversary, the SSC has compiled the 4th generation of serendipitous source catalogues, 4XMM. The catalogue described here, 4XMM-DR9s, explores sky areas that were observed more than once by XMM-Newton. It was constructed from simultaneous source detection on the overlapping observations, which were bundled in groups ("stacks"). Stacking leads to a higher sensitivity, resulting in newly discovered sources and better constrained source parameters, and unveils long-term brightness variations. As a novel feature, positional rectification was applied beforehand. Observations with all filters and suitable camera settings were included. Exposures with a high background were discarded, which was determined through a statistical analysis of all exposures in each instrument configuration. The X-ray background maps used in source detection were modelled via adaptive smoothing with newly determined parameters. Source fluxes were derived for all contributing observations, irrespective of whether the source would be detectable in an individual observation. From 1,329 stacks with 6,604 contributing observations over repeatedly covered 300 square degrees in the sky, 4XMM-DR9s lists 288,191 sources. 218,283 of them were observed several times. Most stacks are composed of two observations, the largest one comprises 352. The number of observations of a source ranges from 1 to 40. Auxiliary products like X-ray images, long-term light curves, and optical finding charts are published as well. 4XMM-DR9s is considered a prime resource to explore long-term variability of X-ray sources discovered by XMM-Newton. Regular incremental releases including new public observations are planned.

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arXiv2020

Identification of 3XMM J000511.8+634018 as a new polar at P(orb)=133.5 min -- is it inside or outside the period gap?

We aimed to identify the variable X-ray source 3XMM J000511.8+634018, which was serendipitously discovered through routine inspections while the 3XMM catalogue was compiled. We analysed the archival XMM-Newton observation of the source, obtained BUSCA photometry in three colours, and performed optical spectroscopy with the LBT. These data were supplemented by archival observations from the Zwicky Transient Facility. Based on its optical and X-ray properties, 3XMM J000511.8+634018 is classified as a magnetic cataclysmic variable, or polar. The flux is modulated with a period of 2.22 hr (8009.1 +- 0.2 s), which we identify with the orbital period. The bright phases are highly variable in X-ray luminosity from one cycle to the next. The source shows a thermal plasma spectrum typical of polars without evidence of a luminous soft blackbody-like component. It is non-eclipsing and displays one-pole accretion. The X-ray and BUSCA light curves show a stream absorption dip, which suggests an inclination 50degr < i < 75degr. The phasing of this feature, which occurs at the end of the bright phase, requires a somewhat special accretion geometry with a stream running far around the white dwarf before it is magnetically channelled. The period of this polar falls within the period gap of the cataclysmic variables (2.15-3.18 h), but appears to fall just below the minimum period when only polars are considered.

astro-ph.SR↗

arXiv2019

NICER X-ray Observations of Seven Nearby Rotation-Powered Millisecond Pulsars

NICER observed several rotation-powered millisecond pulsars to search for or confirm the presence of X-ray pulsations. When broad and sine-like, these pulsations may indicate thermal emission from hot polar caps at the magnetic poles on the neutron star surface. We report confident detections ($\ge4.7σ$ after background filtering) of X-ray pulsations for five of the seven pulsars in our target sample: PSR J0614-3329, PSR J0636+5129, PSR J0751+1807, PSR J1012+5307, and PSR J2241-5236, while PSR J1552+5437 and PSR J1744-1134 remain undetected. Of those, only PSR J0751+1807 and PSR J1012+5307 had pulsations previously detected at the 1.7$σ$ and almost 3$σ$ confidence levels, respectively, in XMM-Newton data. All detected sources exhibit broad sine-like pulses, which are indicative of surface thermal radiation. As such, these MSPs are promising targets for future X-ray observations aimed at constraining the neutron star mass-radius relation and the dense matter equation of state using detailed pulse profile modeling. Furthermore, we find that three of the detected millisecond pulsars exhibit a significant phase offset between their X-ray and radio pulses.

astro-ph.HE↗

arXiv2019

Discovery of a soft X-ray lag in the Ultraluminous X-ray Source NGC 1313 X-1

Ultraluminous X-ray Sources (ULXs) provide a unique opportunities to probe the geometry and energetics of super-Eddington accretion. The radiative processes involved in super-Eddington accretion are not well understood, and so studying correlated variability between different energy bands can provide insights into the causal connection between different emitting regions. We present a spectral-timing analysis of NGC 1313 X-1 from a recent XMM-Newton campaign. The spectra can be decomposed into two thermal-like components, the hotter of which may originate from the inner accretion disc, and the cooler from an optically thick outflow. We find correlated variability between hard (2-10 keV) and soft (0.3-2 keV) bands on kilosecond timescales, and find a soft lag of ~150 seconds. The covariance spectrum suggests that emission contributing to the lags is largely associated with the hotter of the two thermal-like components, likely originating from the inner accretion flow. This is only the third ULX to exhibit soft lags. The lags range over three orders of magnitude in amplitude, but all three are ~5 to ~20 percent of the corresponding characteristic variability timescales. If these soft lags can be understood in the context of a unified picture of ULXs, then lag timescales may provide constraints on the density and extent of radiatively-driven outflows.

astro-ph.HE↗

arXiv2019

New constraints on the nuclear equation of state from the thermal emission of neutron stars in quiescent low-mass X-ray binaries

This paper presents a new analysis of the thermal emission from the neutron star surface to constrain the dense matter equation of state. It is based on the use of a Markov-Chain Monte Carlo algorithm combined with an empirical parametrization of the equation of state, as well as the consistent treatment of seven neutron star quiescent low-mass X-ray binaries in globular clusters with well-measured distances. Previous analyses have indicated that the thermal emission of these neutron stars tends to prefer low neutron star radii, questioning basic knowledge from nuclear physics. We show that it is possible to reconcile the thermal emission analyses with nuclear physics knowledge, with or without including a prior on the slope of the symmetry energy $L_{\rm sym}$. We obtain radii of the order of about 12~km without worsening the fit statistic. With an empirical parametrization of the equation of state, we obtain the following values for the slope of the symmetry energy, its curvature $K_{\rm sym}$, and the isoscalar skewness parameter $Q_{\rm sat}$: $L_{\rm sym}=37.2^{+9.2}_{-8.9}$ MeV, $K_{\rm sym}=-85^{+82}_{-70}$ MeV, and $Q_{\rm sat}=318^{+673}_{-366}$ MeV. For the first time, we measure the values of the empirical parameters $K_{\rm sym}$ and $Q_{\rm sat}$. These values are only weakly impacted by our assumptions, such as the distances or the number of free empirical parameters, provided they are taken within a reasonable range. We also study the weak sensitivity of our results to the set of sources analyzed, and we identify a group of sources that dominates the constraints. The resulting masses and radii obtained are also discussed in the context of the independent constraints from GW 170817 and its electromagnetic counterpart, AT 2017gfo.

astro-ph.HE↗