Pulsars, transients and relativistic astrophysics

Abstract:

Context. X-ray binaries are excellent laboratories to study the coupling between accretion and ejection phenomena around compact objects through multiwavelength observations. Swift J1753.5–0127 is a black hole low-mass X-ray binary discovered in outburst in 2005 that remained active for 11 years, followed by two mini-outbursts. This X-ray transient is known for exhibiting hard-only outbursts and was one of the first outliers of the radio/X-ray correlation. Aims. We investigate the behavior of the compact jets of Swift J1753.5–0127 during its 2023–2024 outburst. Methods. We report a dense multiwavelength monitoring in the radio (MeerKAT, AMI–LA), NIR/optical/UV (REM, LCO, ATLAS, Swift /UVOT), and X-ray ( Swift /XRT, NICER/XTI, INTEGRAL/IBIS) bands. Using new and archival luminosity measurements, we construct a refined, smoother radio/X-ray plane that covers the outlier (steep) and transition (shallow) branches of the radio/X-ray correlation. We also present spectral energy distributions (SEDs) constructed using quasi-simultaneous observations from four epochs. Results. Swift J1753.5–0127 underwent a faint, hard-only, double-peaked outburst, followed by a rebrightening in the optical. The updated radio/X-ray plane offers a clear view of the region occupied by this source, similar to MAXI J1348–630, but at radio luminosities lower than H 1743–322, and we confirm its hybrid behavior. The broadband SED fitting reveals that a cool, truncated accretion disk dominates the NIR/optical/UV band. We find that the compact jets have a variable but weak contribution in the NIR, with a break frequency ν break  ≲ 5 × 10 12 Hz (≲2 × 10 13 Hz if they do not contribute to the hard X-rays), in line with previous studies, providing additional evidence that radio-quiet sources are also infrared-quiet. We also find evidence that the radio spectral index evolves with X-ray luminosity, transitioning from inverted values near quiescence to flatter or negative values at higher luminosities. In the context of adiabatic jet models, this trend may indicate that jet geometry becomes increasingly confined as the accretion rate rises.

Abstract:

Context. The detection of multi-TeV radiation from several black-hole X-ray binaries has positioned the source class as a promising candidate to explain the observed cosmic-ray flux in the PeV energy range. Aims. We aim to determine what distinguishes the systems detected in the gamma-ray range from the rest of the population. Methods. We built on existing catalogues of black-hole X-ray binaries by including information on the radio and gamma-ray emission from these systems. We then compared the > 100 TeV gamma-ray luminosity to different properties of the systems and evaluated the strength of the (rank) correlation ( τ K ) between them. Additionally, we compared the distribution of sources in our sample with the positions of unassociated gamma-ray sources in the 1LHAASO and 1HGPS catalogues to determine whether a correlation exists between them. Results. We find no correlation with the mass of the compact object, the companion mass, or the inclination angle. Instead, we find evidence for a strong correlation ( τ K  > 0.5) between the gamma-ray luminosity and the radio and hard-state X-ray luminosities in our sample. The limited number of gamma-ray detections means that this correlation cannot yet be claimed as statistically significant. We find indications that a high (≳20%) duty cycle (defined as the fraction of time spent in outburst) might be a requirement for > 100 TeV gamma-ray loudness. We find significant evidence (> 3 σ ) that black-hole X-ray binaries are located near unassociated gamma-ray sources in the region of the sky not covered by LHAASO. This evidence is only significant when comparing to X-ray binaries detected in the radio band, not for the sample without a reported radio detection. Conclusions. We conclude that radio-bright black-hole X-ray binaries with a high duty cycle are more likely to be detected as gamma-ray sources than sources that are not detected in the radio band or those with low duty cycles. This result suggests that the particles responsible for the gamma-ray emission are associated with the jets, either accelerated within them or via their interaction with their surroundings.

Abstract:

After sixteen years in quiescence, EXO 0748–676 unexpectedly returned to outburst in March 2024. However, in stark contrast to its previous outburst (1985 - 2008), EXO 0748–676 returned to quiescence in January 2025, after ∼10 months in outburst. Here, we collate data from nine observing facilities, covering wavelengths from radio to X-ray, to provide the first multi-wavelength study of the 2024/25 outburst. Unlike the 1985 outburst, both the rise and decay are fully captured at X-ray and optical wavelengths. The light curves do not exhibit the hallmarks of a canonical transient low-mass X-ray binary outburst. Instead, the X-ray and optical fluxes remain reasonably stable during the 2024/25 outburst. The wealth of optical data of EXO 0748-676 reveals a skewed optical phase curve and an irradiation-dominated optical light curve which lasts until the onset of hydrogen recombination in the outer disc at ∼11000K, triggering a rapid fade at the end of the outburst. X-ray observations by XMM-Newton and NICER revealed that EXO 0748–676 was heavily obscured early on, with spectra exhibiting a dampened soft X-ray contribution. We find that a spectral model including a clumpy, ionised absorber describes the obscuration, although the exact cause of the early-time obscuration remains uncertain. X-ray colour analysis indicates that EXO 0748–676 spent most of the ∼10 month outburst in a faint, hard state. EXO 0748-676 increased marginally in brightness towards the end of the outburst, but never entered a soft spectral state. This behaviour is characteristic of a failed transition outburst.

Abstract:

ABSTRACT Determining the relationship between star formation rate (SFR) and the radio luminosity ($L_{1.4}$) is critical if we are to trace the star formation history of the Universe dust-agnostically using current and future radio facilities. However, until now, such work has relied on potentially biased binary classifications of sources to remove contaminating active galactic nuclei (AGNs). We present a new, statistically driven methodology for deriving the SFR–$L_{1.4}$ relation, removing the need for problematic cuts. We use a Bayesian hierarchical mixture model fit to the radio-detected sources in the deep MIGHTEE COSMOS DR1 catalogue, incorporating the full SFR posterior probability distributions generated by state-of-the-art spectral energy distribution fitting code GRAHSP. This allows us to probabilistically determine a mean SFR–$L_{1.4}$ relation for the SF-dominated galaxies, whilst accounting for changing fractions of SF dominated sources across redshift, radio luminosity, and stellar mass ranges. We find that the SFR–radio luminosity correlation exhibits a significant dependence on redshift, but a stellar mass dependence that is weaker than previous studies. Our resultant SFR–radio correlation is $\log _{10}(\text{SFR}/{\rm M}_{\odot }\, \text{yr}^{-1}) = 0.790\times (\log _{10}(L_{1.4}/\text{W {}Hz}^{-1})-23) + 1.244 \times (1+z)^{0.122} -0.033 \times (\log _{10}(M_*/{\rm M}_{\odot })-10)$, with an intrinsic scatter of 0.178 dex. We show that this redshift evolution could be explained by a moderate evolution in the radio spectral index of SF galaxies. We attribute the lack of observed strong dependence on stellar mass, compared to recent studies, to the novel statistical approach that does not rely on cuts to remove AGNs.