Detection of kilosecond hard lags in the new pulsating ULX candidate NGC 7456 ULX-1
Abstract
Context. Ultraluminous X-ray sources (ULXs) are thought to be powered, in many cases, by super-Eddington accretion onto compact objects. While soft X-ray lags have been detected in several ULXs, hard lags remain rare and poorly understood. Aims. We investigate the temporal and energy dependence of X-ray lags in NGC 7456 ULX-1 to constrain their physical origin and probe the super-Eddington accretion flow. Methods. We analyzed the two deepest XMM-Newton observations, taken in 2018 and 2023. Hard (1-10 keV) and soft (0.3-1 keV) light curves were cross-correlated using adaptive-binning techniques optimized for Poissonian low-count data. We measured lags over consecutive 10 ks intervals and investigated their energy dependence. Spectra were modeled using thermal and Comptonization models. Results. We detect significant hard X-ray lags in both observations, with the hard emission delayed by ∼ 103 s during phases of rapid flux variability. The delays are primarily driven by the lowest-energy photons. Spectral modeling indicates a Comptonization-dominated flow comprising a cooler, extended outer region and a hotter, compact inner flow embedded in an optically thick wind. We interpret the delays as the combined effect of inward propagation of accretion-rate fluctuations and photon diffusion within the dense outflow. Fluctuations first enhance the soft-emitting outer regions and then propagate toward the hotter inner flow, where photons undergo stronger Comptonization before escaping with a kilosecond delay. The small inferred inner emitting radius disfavors an intermediate-mass black hole accretor. Conclusions. The sign, amplitude, and energy dependence of the delays disfavor standard reverberation. Propagation-driven variability coupled with radiative transfer in optically thick winds appears to play a major role in shaping the timing properties of super-Eddington accretion flows.
Related
- Detection of hard X-ray pulsations in non-ULX X-ray pulsar 3X J0042 in M31
- EK Cha b: Multi-tracer detection of a protoplanet candidate in a gas-rich environment
- Torque lags from magnetic unlocking in neutron stars and gravitational-wave smearing
- Beating and coupling in pulsating stars: A unified Fourier description and observational diagnostics
- Characterization of the Compact Multi-planet System TOI-789: Validation of Three Terrestrial Planets, Dynamical Characterization, and Detection of a Fourth Temperate Candidate
BibTeX
@misc{leone2026detection,
title = {{Detection of kilosecond hard lags in the new pulsating ULX candidate NGC 7456 ULX-1}},
author = {W. Leone and F. Pintore and C. Pinto and N. O. Pinciroli Vago and A. Sanna and A. Wolter and T. Di Salvo and R. Iaria and A. D'Aì and A. Anitra and R. Soria and P. Esposito and F. Barra and E. Ambrosi and S. Caserta and C. Salvaggio and R. Salvaterra and G. L. Israel and S. Banerjee and M. Marelli and G. Rodriguez-Castillo and L. Burderi},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.01444},
eprint = {2610.01444},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.01444},
note = {CC BY-SA 4.0},
}
RIS
TY - UNPB
TI - Detection of kilosecond hard lags in the new pulsating ULX candidate NGC 7456 ULX-1
AU - W. Leone
AU - F. Pintore
AU - C. Pinto
AU - N. O. Pinciroli Vago
AU - A. Sanna
AU - A. Wolter
AU - T. Di Salvo
AU - R. Iaria
AU - A. D'Aì
AU - A. Anitra
AU - R. Soria
AU - P. Esposito
AU - F. Barra
AU - E. Ambrosi
AU - S. Caserta
AU - C. Salvaggio
AU - R. Salvaterra
AU - G. L. Israel
AU - S. Banerjee
AU - M. Marelli
AU - G. Rodriguez-Castillo
AU - L. Burderi
PY - 2026
DA - 2026/10/01
PB - arXiv
DO - 10.48550/arxiv.2610.01444
UR - https://arxiv.org/abs/2610.01444
AB - Context. Ultraluminous X-ray sources (ULXs) are thought to be powered, in many cases, by super-Eddington accretion onto compact objects. While soft X-ray lags have been detected in several ULXs, hard lags remain rare and poorly understood. Aims. We investigate the temporal and energy dependence of X-ray lags in NGC 7456 ULX-1 to constrain their physical origin and probe the super-Eddington accretion flow. Methods. We analyzed the two deepest XMM-Newton observations, taken in 2018 and 2023. Hard (1-10 keV) and soft (0.3-1 keV) light curves were cross-correlated using adaptive-binning techniques optimized for Poissonian low-count data. We measured lags over consecutive 10 ks intervals and investigated their energy dependence. Spectra were modeled using thermal and Comptonization models. Results. We detect significant hard X-ray lags in both observations, with the hard emission delayed by ∼ 10^3 s during phases of rapid flux variability. The delays are primarily driven by the lowest-energy photons. Spectral modeling indicates a Comptonization-dominated flow comprising a cooler, extended outer region and a hotter, compact inner flow embedded in an optically thick wind. We interpret the delays as the combined effect of inward propagation of accretion-rate fluctuations and photon diffusion within the dense outflow. Fluctuations first enhance the soft-emitting outer regions and then propagate toward the hotter inner flow, where photons undergo stronger Comptonization before escaping with a kilosecond delay. The small inferred inner emitting radius disfavors an intermediate-mass black hole accretor. Conclusions. The sign, amplitude, and energy dependence of the delays disfavor standard reverberation. Propagation-driven variability coupled with radiative transfer in optically thick winds appears to play a major role in shaping the timing properties of super-Eddington accretion flows.
ER -