[Paper Review] Observations of IntraDay Variable sources with the Effelsberg and Urumqi Radio Telescopes

N. Marchili, T. P. Krichbaum+8 more|ArXiv.org|Apr 17, 2008

Astrophysics and Cosmic Phenomena23 references3 citations

TL;DR

This study monitors IntraDay Variable (IDV) radio sources using the 25m Urumqi and 100m Effelsberg radio telescopes to investigate the origin of rapid flux variations. Despite its smaller aperture, Urumqi proves effective for IDV studies after correcting for a spurious 24-hour modulation. Key results show tentative evidence of annual modulation in S5 0716+714’s variability timescales—supporting extrinsic models like interstellar scintillation—while S4 0954+658 shows no such trend.

ABSTRACT

A sample of classical IntraDay Variable (IDV) and IDV candidate sources has been monitored with the Urumqi 25m telescope and the Effelsberg 100m telescope. Aim of the project is to investigate the origin of IntraDay Variability, a phenomenon which has been observed in about 30% of flat spectrum radio quasars. Simultaneous Effelsberg-Urumqi observations demonstrated that the Urumqi antenna, although relatively small in diameter, is well suitable for IDV experiments. A few Urumqi datasets, however, turned out to be affected by a spurious $\sim 24$ hours modulation, an effect which has been removed by means of a new procedure for data reduction. In about 14 months, 12 epochs of observation have been collected, for a total observing time of more than 45 days. The epochs are regularly distributed over the whole year, in order to check for the presence of systematic annual changes in the variability time scales - a crucial test for verifying the consistency of source-extrinsic models of the variability. Preliminary time-analysis of the monitored sources revealed some hint for a slowing down of the characteristic time scales of S5~0716+714, a result that, if confirmed, would be compatible with a source-extrinsic origin of the variability, in contrast to previous IDV studies. No significant modulation of the time scales has been detected for S4~0954+658.

Motivation & Objective

• To investigate the origin of IntraDay Variability (IDV) in flat-spectrum radio quasars, particularly distinguishing between intrinsic and extrinsic mechanisms.
• To test whether variability time scales exhibit systematic annual modulation—a key prediction of interstellar scintillation (ISS) models.
• To validate the Urumqi 25m telescope as a viable instrument for IDV monitoring despite its smaller aperture compared to the Effelsberg 100m telescope.
• To correct for a spurious 24-hour modulation in Urumqi data caused by pointing and day-night effects, improving data reliability.
• To compare variability characteristics of IDV sources across multiple epochs to detect long-term trends in time scales and modulation indices.

Proposed method

• Conducted 12 epochs of simultaneous radio observations at 4.8–4.85 GHz using the Urumqi 25m telescope and Effelsberg 100m telescope.
• Applied a new data reduction procedure to remove a spurious 24-hour periodic modulation caused by pointing instabilities and diurnal effects.
• Used standard time-series analysis techniques: Structure Function, Discrete AutoCorrelation Function (DIACF), and Wavelet analysis to estimate variability time scales.
• Compared variability curves from both telescopes to validate data consistency, confirming Urumqi’s reliability despite its smaller aperture.
• Estimated time scales and modulation indices (M.I.) for S5 0716+714 and S4 0954+658 across 14 months of observations.
• Tested for annual modulation by fitting time-scale data to isotropic and anisotropic models of interstellar scintillation.

Experimental results

Research questions

• RQ1 Does the variability time scale of S5 0716+714 exhibit systematic annual modulation, as predicted by interstellar scintillation models?
• RQ2 Is there evidence of annual modulation in the variability of S4 0954+658, another classical type II-IDV source?
• RQ3 Can the Urumqi 25m telescope reliably detect and measure IDV, given its smaller aperture compared to the Effelsberg 100m telescope?
• RQ4 To what extent does a spurious 24-hour modulation affect Urumqi data, and can it be effectively removed through a new data reduction procedure?
• RQ5 Do the observed variability patterns in IDV sources support an extrinsic (e.g., interstellar scintillation) or intrinsic origin of the flux variations?

Key findings

• The Urumqi 25m telescope, after correction for a spurious 24-hour modulation, produced variability curves that agree well with those from the larger Effelsberg 100m telescope, confirming its suitability for IDV experiments.
• A new data reduction procedure successfully removed a spurious 24-hour modulation in Urumqi data caused by pointing issues and diurnal effects.
• For S5 0716+714, a possible slowing down of the variability time scale was observed around day of year 300, suggesting a hint of annual modulation compatible with interstellar scintillation.
• The time scale of S5 0716+714 varied from 0.5 to 3.0 days across observations, with a peak modulation index of 4.93% and average flux of 0.74 Jy.
• No significant annual modulation was detected in S4 0954+658; its time scales remained relatively stable (0.3–2.3 days) with modulation indices between 0.76% and 2.28%.
• The lack of systematic time-scale variation in S4 0954+658, despite strong variability, challenges simple ISS models and suggests a more complex or intrinsic origin.