[Paper Review] Extreme intranight variability in the BL Lacertae object AO 0235+164

Gustavo E. Romero, S. A. Cellone+1 more|arXiv (Cornell University)|Jul 26, 2000

Astrophysics and Cosmic Phenomena3 citations

TL;DR

This study presents extreme intranight optical variability in the BL Lac object AO 0235+164, detecting amplitude variations of up to 100% within 24 hours, with 0.5 mag changes in a single night and 1.2 mag from night to night. The lack of spectral index variation during a 30% outburst supports a geometric origin via a precessing helical jet modulating the viewing angle of a relativistic shock, as modeled by Kraus et al. (1999) and Qian et al. (2000).

ABSTRACT

We present results of two-colour photometry with high time resolution of the violently variable BL Lac object AO 0235+164. We have found extreme intranight variability with amplitudes of ~ 100 % over time scales of 24 hours. Changes of 0.5 magnitudes in both R and V bands were measured within a single night, and variations up to 1.2 magnitudes occurred from night to night. A complete outburst with an amplitude ~ 30 % was observed during one of the nights, while the spectrum remained unchanged. This seems to support an origin based on a thin relativistic shock propagating in such a way that it changes the viewing angle, as recently suggested by Kraus et al. (1999) and Qian et al. (2000).

Motivation & Objective

• To investigate the nature of extreme intranight optical variability in the BL Lac object AO 0235+164.
• To determine whether the observed rapid flux changes are due to intrinsic emission mechanisms or geometric effects.
• To test the hypothesis that variability arises from a relativistic shock whose viewing angle changes due to jet precession.
• To assess the viability of the precessing helical jet model in explaining achromatic, time-symmetric outbursts without spectral changes.
• To constrain the physical parameters of the emission region using high-time-resolution two-colour photometry.

Proposed method

• Conducted high-time-resolution two-colour CCD photometry using the 2.15-m CASLEO telescope with 100 s integration times.
• Employed differential photometry with an 8-pixel aperture (6.5 arcsec) to minimize seeing-induced variability effects.
• Used standard stars from Landolt (1992) for magnitude calibration and applied bias subtraction and flat-field correction.
• Calculated flux densities in V and R bands using Bessell (1979) calibrations and corrected for Galactic extinction (EB-V = 0.08).
• Computed spectral indices via power-law fitting (F ∝ ν^α) and monitored their scatter to assess spectral stability.
• Applied IRAF software package with APPHOT for aperture photometry and used comparison stars to control for systematic errors.

Experimental results

Research questions

• RQ1 What causes the extreme intranight variability (up to 100% amplitude) observed in AO 0235+164 on timescales of hours?
• RQ2 Is the observed flux variation accompanied by changes in the spectral index, indicating intrinsic spectral evolution or geometric modulation?
• RQ3 Can the precessing helical jet model of Kraus et al. (1999) and Qian et al. (2000) explain the achromatic, time-symmetric outbursts without spectral changes?
• RQ4 What is the role of relativistic beaming and Doppler factor modulation in producing rapid flux variations?
• RQ5 Could gravitational microlensing also explain the observed variability, and what constraints does the data place on this mechanism?

Key findings

• A 30% flux outburst was observed in the V band on November 5, 1999, within a single night, with no detectable change in the spectral index.
• Variations of 0.5 magnitudes in both R and V bands occurred within a single night, and changes of up to 1.2 magnitudes occurred between consecutive nights.
• The variability confidence level exceeded 26σ in the V band and 30.9σ in the R band, indicating high statistical significance.
• The spectral index scatter for field stars was |σα| ≈ 0.074, confirming that the source’s spectral index remained stable during the outburst.
• The lack of spectral variability during the 30% outburst supports a geometric origin via changing viewing angle, consistent with a precessing helical jet model.
• For a Lorentz factor Γ = 25, a 1° change in the shock’s direction relative to the line of sight can produce a flux variation of ~70%, consistent with the observed 30% change.