Stochastic Optical Variability and an rms-flux Relation in the Intermediate Polar EP240309a
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2026
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| author | Wu, S. -Y. Hu, Y. -D. Perez-Garcia, I. Castro-Tirado, A. J. Gritsevich, M. Fernandez-Garcia, E. J. Caballero-Garcia, M. D. Guziy, S. Garcia-Segura, G. Sanchez-Ramirez, R. Kilpatrick, C. D. Bom, C. R. Santana, L. Santos, A. Meintjes, P. J. van Heerden, H. J. Martin-Carrillo, A. Hanlon, L. Maury, A. Xiong, D. -R. Zhang, B. -B. |
| author_facet | Wu, S. -Y. Hu, Y. -D. Perez-Garcia, I. Castro-Tirado, A. J. Gritsevich, M. Fernandez-Garcia, E. J. Caballero-Garcia, M. D. Guziy, S. Garcia-Segura, G. Sanchez-Ramirez, R. Kilpatrick, C. D. Bom, C. R. Santana, L. Santos, A. Meintjes, P. J. van Heerden, H. J. Martin-Carrillo, A. Hanlon, L. Maury, A. Xiong, D. -R. Zhang, B. -B. |
| contents | Magnetic cataclysmic variables provide a natural laboratory for studying how accretion interacts with compact-object magnetospheres and generates stochastic variability. We present an optical variability study of the intermediate-polar candidate EP240309a, an Einstein Probe X-ray transient, using BOOTES photometry, high-cadence TESS light curves, and a SOAR/Goodman optical spectrum. Previous studies found a white-dwarf spin period of 3.97 min (Pspin ~ 238 s) and an orbital period of Porb = 3.7614(4) h. Power spectral densities from the BOOTES data are consistent with single power laws with slopes alpha ~ 1.2-1.8, with no statistically significant evidence for a bend across the sampled frequency range. Using red-noise simulations and injection-recovery tests, we place one-sided constraints on any putative break frequency, which translate, under standard dynamical identifications, into an upper limit on the magnetospheric radius of Rm <= few x 10^10 cm for MWD = 0.8 Msun. In the TESS data, we detect a linear rms-flux relation on hour timescales in three high-cadence sectors, while two other sectors do not show a robust detection, indicating epoch-dependent rms-flux behavior. The SOAR spectrum shows Balmer and He II emission lines with FWHM about 1000-1600 km s^-1; under a Keplerian interpretation, these imply characteristic radii of r about (0.9-3.4) x 10^10 cm, broadly comparable to the timing-based constraints. Overall, the data provide conservative, order-of-magnitude radius constraints consistent with accretion onto a magnetic white dwarf, but they do not establish the detailed accretion geometry or exclude stream-fed or mixed accretion scenarios. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2604_07540 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Stochastic Optical Variability and an rms-flux Relation in the Intermediate Polar EP240309a Wu, S. -Y. Hu, Y. -D. Perez-Garcia, I. Castro-Tirado, A. J. Gritsevich, M. Fernandez-Garcia, E. J. Caballero-Garcia, M. D. Guziy, S. Garcia-Segura, G. Sanchez-Ramirez, R. Kilpatrick, C. D. Bom, C. R. Santana, L. Santos, A. Meintjes, P. J. van Heerden, H. J. Martin-Carrillo, A. Hanlon, L. Maury, A. Xiong, D. -R. Zhang, B. -B. Solar and Stellar Astrophysics High Energy Astrophysical Phenomena Magnetic cataclysmic variables provide a natural laboratory for studying how accretion interacts with compact-object magnetospheres and generates stochastic variability. We present an optical variability study of the intermediate-polar candidate EP240309a, an Einstein Probe X-ray transient, using BOOTES photometry, high-cadence TESS light curves, and a SOAR/Goodman optical spectrum. Previous studies found a white-dwarf spin period of 3.97 min (Pspin ~ 238 s) and an orbital period of Porb = 3.7614(4) h. Power spectral densities from the BOOTES data are consistent with single power laws with slopes alpha ~ 1.2-1.8, with no statistically significant evidence for a bend across the sampled frequency range. Using red-noise simulations and injection-recovery tests, we place one-sided constraints on any putative break frequency, which translate, under standard dynamical identifications, into an upper limit on the magnetospheric radius of Rm <= few x 10^10 cm for MWD = 0.8 Msun. In the TESS data, we detect a linear rms-flux relation on hour timescales in three high-cadence sectors, while two other sectors do not show a robust detection, indicating epoch-dependent rms-flux behavior. The SOAR spectrum shows Balmer and He II emission lines with FWHM about 1000-1600 km s^-1; under a Keplerian interpretation, these imply characteristic radii of r about (0.9-3.4) x 10^10 cm, broadly comparable to the timing-based constraints. Overall, the data provide conservative, order-of-magnitude radius constraints consistent with accretion onto a magnetic white dwarf, but they do not establish the detailed accretion geometry or exclude stream-fed or mixed accretion scenarios. |
| title | Stochastic Optical Variability and an rms-flux Relation in the Intermediate Polar EP240309a |
| topic | Solar and Stellar Astrophysics High Energy Astrophysical Phenomena |
| url | https://arxiv.org/abs/2604.07540 |