[Paper Review] Interpreting a Dwarf Nova Eruption as Magnetic Flare Activity

Noam Soker, S. D. Vrtilek|ArXiv.org|Apr 4, 2009

Astrophysical Phenomena and Observations1 references3 citations

TL;DR

The paper proposes that the radio emission during the outburst of the dwarf nova SS Cyg originates from a magnetically heated corona formed above the accretion disk, not from jets. Using the radio-to-X-ray luminosity ratio $L_r/L_X \lesssim 10^{-5}$, consistent with the Güdel-Benz relation observed in magnetically active stars, the authors argue that coronal flaring—similar to solar magnetic activity—is the dominant source of radio emission, with jet-like outflows possibly launched via analogous coronal mass ejection processes.

ABSTRACT

We suggest that the radio emission from the dwarf nova SS Cyg during outburst comes from magnetic activity that formed a corona (similar to coronae found in magnetically active stars), rather than from jets. We base our claim on the recent results of Laor & Behar, who found that when the ratio between radio and X-ray flux of accretion disks in radio-quiet quasars is as in active stars, Lr/Lx=10^{-5}, then most of the radio emission comes from coronae. Using observations from the literature we find that for SS Cyg during outburst Lr/Lx<10^{-5}. This does not mean jets are not launched during outbursts. On the contrary, if the magnetic activity in erupting accreting disks is similar to that in stars, then mass ejection, e.g., as in coronal mass ejection, is expected. Hence magnetic flares similar to those in active stars might be the main mechanism for launching jets in a variety of systems, from young stellar objects to massive black holes.

Motivation & Objective

• To re-evaluate the origin of radio emission in the dwarf nova SS Cyg during outburst, challenging the prevailing jet-based interpretation.
• To test whether the radio-to-X-ray luminosity ratio $L_r/L_X$ in SS Cyg aligns with the Güdel-Benz relation observed in magnetically active stars.
• To investigate whether magnetic flare activity, analogous to solar coronal heating, can explain the observed radio emission in accreting white dwarfs.
• To explore the possibility that coronal magnetic activity, rather than jets, is the primary source of radio emission in accretion systems like SS Cyg.
• To propose that magnetic reconnection and coronal heating mechanisms may be the fundamental drivers of jet launching across diverse astrophysical systems.

Proposed method

• Calculated the radio luminosity $L_r$ from the average outburst radio flux of SS Cyg (0.3 mJy at 8.6 GHz), yielding $L_r \sim 3 \times 10^{-17}$ erg cm⁻² s⁻¹.
• Used X-ray luminosity $L_X$ from the 1996 outburst of SS Cyg (residual emission after obscuration), $L_X \sim 3.6 \times 10^{-10}$ erg cm⁻² s⁻¹, corrected for optical depth.
• Computed the radio-to-X-ray luminosity ratio $L_r/L_X \approx 10^{-6}$, which falls within the $\lesssim 10^{-5}$ range characteristic of stellar coronae.
• Applied the Güdel-Benz relation ($L_r \simeq 10^{-5}L_X$) to infer that radio emission is likely from a corona, not a jet, based on the observed ratio.
• Extended the analysis to argue that magnetic energy buildup via $α\Omega$ dynamo processes in high-accretion-rate disks can lead to coronal heating and magnetic flares.
• Proposed that magnetic reconnection and coronal heating processes in accretion disks are analogous to solar flares and CMEs, potentially launching jets via similar mechanisms.

Experimental results

Research questions

• RQ1 Does the radio-to-X-ray luminosity ratio in SS Cyg during outburst fall within the range expected for coronal emission in magnetically active stars?
• RQ2 Can the observed radio emission in SS Cyg be explained by a newly formed corona rather than by relativistic jets?
• RQ3 Is the magnetic activity in accreting white dwarfs during outbursts analogous to that in magnetically active stars, particularly in terms of energy dissipation and emission mechanisms?
• RQ4 What role do coronal magnetic flares play in launching collimated outflows or jets in accretion systems like cataclysmic variables?
• RQ5 How does the $L_r/L_X$ ratio help distinguish between jet-dominated and corona-dominated emission in accreting systems?

Key findings

• The radio-to-X-ray luminosity ratio $L_r/L_X \approx 10^{-6}$ for SS Cyg during outburst is consistent with the Güdel-Benz relation ($L_r \lesssim 10^{-5}L_X$) observed in magnetically active stars.
• The observed $L_r/L_X$ ratio strongly suggests that the radio emission originates from a corona formed above the accretion disk, not from a jet.
• Coronal heating via magnetic reconnection and energy buildup in high-accretion-rate disks can explain the observed radio emission, with the process analogous to solar magnetic flares.
• Magnetic flares in accretion disks may be the primary mechanism for launching jets in a wide range of systems, from young stellar objects to massive black holes.
• The study challenges the assumption that radio emission in accreting systems like SS Cyg is always jet-related, especially when $L_r/L_X \lesssim 10^{-5}$.
• The results support the idea that coronal magnetic activity—similar to that in active stars—can be the dominant source of radio emission in accretion disks, even in systems without clear jet signatures.