[Paper Review] Compact Intraday Variable Radio Cores: New Observational Approaches

L. Fuhrmann, Giuseppe Cimò+5 more|arXiv (Cornell University)|Jul 22, 2002

Astrophysics and Cosmic Phenomena3 citations

TL;DR

This paper presents two new observational approaches to distinguish intrinsic from refractive interstellar scintillation (RISS) origins of intraday variability (IDV) in quasars and BL Lacs. First, it reports tentative detection of sub-mm IDV at 345 GHz in 0716+714, suggesting intrinsic variability since RISS should be suppressed at such high frequencies. Second, it presents the first detection of a high-latitude molecular CO cloud in front of the IDV source 0954+658, implying such clouds may host the scattering screens responsible for RISS.

ABSTRACT

The evidence for refractive interstellar scintillation (RISS) being the main cause for rapid intraday variations (Intraday Variability, IDV) in Quasars and BL Lacs has recently become stronger. If IDV is still a complex composition of extrinsic and source intrinsic effects, the intrinsic part of the IDV pattern should show up in the millimeter and sub-millimeter regime due to the frequency dependence of RISS. Hence, observations at higher frequencies are essential in order to exclude RISS as the sole cause of IDV. Here we report on our new attempt to search for rapid variations at much higher frequencies. In addition, the possibility of a direct detection of the postulated scattering screen in front of IDV sources will be discussed. Our recent line observations towards a few IDV sources lead to the first detection of a high latitude molecular cloud in front of an intraday variable radio core.

Motivation & Objective

• To test whether intraday variability (IDV) in quasars and BL Lacs is intrinsic or caused by refractive interstellar scintillation (RISS), particularly by probing higher frequencies where RISS is expected to vanish.
• To directly detect the interstellar scattering screen responsible for RISS by searching for associated ionized or molecular material in the line of sight to IDV sources.
• To investigate the spatial and physical association between IDV sources and high-latitude molecular clouds (HLCs), which may harbor the scattering plasma.
• To determine whether the observed IDV patterns can be disentangled into intrinsic and extrinsic components using multi-frequency and spectral line observations.

Proposed method

• Conducted 40-hour 32 GHz observations of IDV sources using the Effelsberg telescope with dense time sampling and secondary calibrators to detect high-frequency IDV.
• Performed a 3-week multifrequency campaign at 345 GHz using the HHT (Herschel Telescope) to search for sub-mm IDV in flat-spectrum radio sources.
• Carried out spectral line observations at 230 GHz using an SIS receiver to detect CO(2–1) emission toward IDV sources, particularly 0954+658.
• Produced a 7′×7′ raster map of CO emission toward 0954+658 with 15 hours of total integration time to locate molecular material in the line of sight.
• Analyzed the spatial relationship between IDV sources and known high-latitude molecular clouds (HLCs), such as the Ursa Major complex.
• Planned follow-up observations of ionized species (e.g., HCO+, H30α) and radio recombination lines (RRLs) to confirm the presence of ionized scattering material.

Experimental results

Research questions

• RQ1 Can sub-mm IDV (at 345 GHz) be detected in IDV sources, and if so, does this rule out RISS as the sole cause of IDV?
• RQ2 Is there direct observational evidence linking IDV sources to molecular clouds in the local interstellar medium (LISM)?
• RQ3 Can the scattering screen responsible for RISS be directly detected via spectral line emission from molecular or ionized gas?
• RQ4 What is the spatial and kinematic relationship between IDV sources and high-latitude molecular clouds, and does this support their role as scattering screens?
• RQ5 Do extreme scattering events (ESEs), such as those observed in 0954+658, correlate with the presence of dense, clumpy molecular cloud boundaries?

Key findings

• Preliminary results from the 345 GHz campaign suggest that the BL Lac object 0716+714 exhibits significant variability, indicating potential intrinsic IDV at sub-mm frequencies, which would be inconsistent with RISS dominance.
• A CO(2–1) emission line was detected toward the IDV source 0954+658 with a main beam brightness temperature of 1.005 K at a LSR velocity of -1.54 km/s, confirming the presence of a molecular cloud in the line of sight.
• The detected CO cloud extends over 440″ and lies east of 0954+658, with the source located in the outer western shell of the cloud, suggesting a possible association with a scattering screen.
• The distance to the CO cloud is estimated to be ~100 pc, consistent with the expected distance of a scattering screen producing 10-hour timescale RISS.
• The source 0954+658 is located behind the outer shell of the CO cloud, which may host an ionized boundary layer capable of producing scattering effects.
• Additional observations of HCO+ and H30α lines show weak, inconclusive signals, indicating the need for higher-sensitivity follow-up, particularly with the IRAM 30m telescope.