[Paper Review] Time-resolved HST and IUE UV spectroscopy of the Intermediate Polar FO Aqr

D. de Martino, R. Silvotti+5 more|arXiv (Cornell University)|Sep 3, 1999

Astrophysics and Star Formation Studies1 references4 citations

TL;DR

This study presents time-resolved HST and IUE UV spectroscopy of the intermediate polar FO Aqr, revealing multiple periodicities in its UV variability. The dominant orbital modulation and rotational frequency-driven line shifts indicate multi-temperature components, with evidence of a transition from disc-fed to disc-overflow accretion, confirmed by stable spectral variability over time and consistent with long-term X-ray trends.

ABSTRACT

Time resolved spectroscopy of the Intermediate Polar FO Aqr reveals the presence of multiple periodicities in the UV range. A strong orbital modulation dominates both continuum and emission line flux variabilities, while line velocity motions are only detected at the rotational frequency. A prominent orbital periodicity is also observed in coordinated optical photometry, where FO Aqr was previously found to be spin dominated. The spectral dependence of the main periodicities shows the presence of multi-temperature components in FO Aqr and for the first time a hot and a cool component in the rotational modulation. From a comparison with previous UV and optical data obtained in 1990, no spectral variations in the orbital and rotational variabilities are detected, indicating no significant changes in the effects of X-ray illumination but rather a shrinking of the accretion curtain accompained by an increase in size of the thickened part of the accretion disc. These observations, consistent with the recently discovered long term trend in the X-ray pulsation amplitudes, independently confirm a change in the accretion mode in FO Aqr, which switched from a disc-fed into a disc-overflow state, likely triggered by mass accretion variations.

Motivation & Objective

• To investigate the time-resolved UV variability of the intermediate polar FO Aqr using high-resolution spectroscopy.
• To determine the origin of periodicities in UV flux and line profiles, distinguishing between orbital and rotational modulation.
• To assess changes in accretion geometry by comparing new UV data with archival observations from 1990.
• To explore the implications of X-ray pulsation amplitude trends for accretion mode evolution in FO Aqr.

Proposed method

• Time-resolved UV spectroscopy was conducted using the Hubble Space Telescope (HST) and the International Ultraviolet Explorer (IUE).
• Spectral variability was analyzed across the UV range to detect periodic modulations in continuum and emission line fluxes.
• Radial velocity shifts in emission lines were measured to identify rotational frequency contributions.
• Coordinated optical photometry was used to cross-validate orbital periodicity and spin dominance.
• Spectral energy distribution and periodicity amplitudes were compared between 1999 and 1990 data to detect long-term changes.
• Multi-temperature component modeling was applied to interpret the observed variability patterns.

Experimental results

Research questions

• RQ1 What periodicities are present in the UV light curves and emission line profiles of FO Aqr?
• RQ2 How do orbital and rotational modulations affect the UV continuum and line emission in FO Aqr?
• RQ3 What do the spectral and variability trends over time reveal about the accretion geometry in FO Aqr?
• RQ4 Is there evidence for a change in accretion mode, such as from disc-fed to disc-overflow, in FO Aqr?
• RQ5 How do the UV results correlate with long-term X-ray pulsation amplitude trends?

Key findings

• A strong orbital modulation dominates both continuum and emission line flux variability in FO Aqr.
• Line velocity motions are only detected at the rotational frequency, indicating rotational Doppler shifts in the emission lines.
• The presence of both hot and cool components in the rotational modulation is observed for the first time.
• No significant spectral variations in orbital or rotational variability amplitudes are detected between 1999 and 1990 data.
• The results support a transition from a disc-fed to a disc-overflow accretion mode, likely driven by mass accretion variations.
• This change is independently confirmed by the long-term trend in X-ray pulsation amplitudes.