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Irregular Variables

Irregular Variables (i)

Irregular variables are objects that show clear variability but lack a stable periodic signal. Their brightness changes are aperiodic, stochastic, or quasi-periodic, often driven by accretion, eruptions, or other non-deterministic physical processes.

Classification and numbers

  • Supertypes
  • variable
  • Subtypes
  • Flaring (fla): rapid brightening events (M dwarf flares, dwarf novae)
  • Dipping (dip): transient fading events (dust occultation, accretion structures)
  • Occurrence rate: very common; irregular variability is seen across many astrophysical source types

Description

Irregular variables are distinguished from periodic variables by the absence of a repeating, phase-coherent signal. Period-finding algorithms (Lomb-Scargle, conditional entropy, AOV) will generally not produce a convincing phase-folded light curve for these sources.

However, irregular and periodic variability are not mutually exclusive. Many sources show periodic modulation with superposed irregular behavior (e.g., RS CVn stars with flares, YSOs with rotational modulation and accretion bursts).

Light curve characteristics

  • No stable period, or period-finding yields incoherent phase folds
  • Variability amplitude ranges from millimagnitudes to several magnitudes
  • Timescales range from minutes (flares) to years (long-term drifts)
  • Common patterns include:
    • Sharp brightening episodes followed by decay (flaring)
    • Fading events of varying depth and duration (dipping)
    • Stochastic wandering resembling a damped random walk (AGN, accreting YSOs)

Astrophysical sources of irregular variability

  • M dwarf flares: magnetic reconnection events, minutes to hours
  • Dwarf novae: accretion disk instabilities in cataclysmic variables, days to weeks
  • YSO accretion variability: clumpy accretion, disk instabilities, hours to years
  • AGN: accretion disk variability, weeks to years
  • R Coronae Borealis stars: dust formation episodes causing deep fading

References and further reading:

  • Cody et al., 2014, AJ 147 82, CSI 2264: Simultaneous Optical and Infrared Light Curves of Young Disk-bearing Stars in NGC 2264 arxiv:1401.6582
  • Sesar et al., 2007, AJ 134 2236, Exploring the Variable Sky with the Sloan Digital Sky Survey arxiv:0704.1429