[Paper Review] APEX sub-mm monitoring of gamma-ray blazars

S. Larsson, L. Fuhrmann+14 more|arXiv (Cornell University)|Jun 17, 2012

Astrophysics and Cosmic Phenomena1 references3 citations

TL;DR

This study presents the first long-term sub-mm monitoring of gamma-ray blazars using the APEX telescope and LABOCA camera at 345 GHz, revealing extreme variability in 38 out of 39 sources. The results show that sub-mm variability is more pronounced and faster than at longer cm/mm wavelengths, suggesting the sub-mm emission originates in the same compact, relativistic jet regions as high-energy gamma-ray emission, supporting models of shock-driven variability propagating from high to low frequencies.

ABSTRACT

So far, no systematic long-term blazar monitoring programs and detailed variability studies exist at sub-mm wavelengths. Here, we present a new sub-mm blazar monitoring program using the APEX 12-m telescope. A sample of about 40 gamma-ray blazars has been monitored since 2007/2008 with the LABOCA bolometer camera at 345 GHz. First light curves, preliminary variability results and a first comparison with the longer cm/mm bands (F-GAMMA program) are presented, demonstrating the extreme variability characteristics of blazars at such short wavelengths.

Motivation & Objective

• To initiate the first systematic long-term monitoring of blazar variability at sub-mm wavelengths, a band previously unexplored for such studies.
• To determine the variability amplitude and timescales of gamma-ray blazars at 345 GHz, probing the innermost regions of relativistic jets.
• To investigate the relationship between sub-mm variability and higher-energy (gamma-ray) emission to constrain jet emission mechanisms.
• To assess whether sub-mm emission traces the same physical regions as cm/mm and high-energy emission, particularly in the context of shock-driven variability models.
• To establish a foundational dataset for future multi-wavelength modeling of blazar spectral energy distributions and variability.

Proposed method

• Observations were conducted with the APEX 12-m sub-mm telescope at 5100 m altitude in Chile using the LABOCA bolometer camera at 345 GHz (870 μm).
• The monitoring program used spiral observing mode with 20–35 second integrations per source, supplemented by skydip and calibrator observations for opacity correction.
• Data were collected over multiple observing blocks from 2007–2008 onward, with 2 to 177 observations per source, enabling long-term light curve construction.
• Variability was assessed via χ² analysis to distinguish real variability from measurement noise, with modulation index (m = 100 × rms/mean) used to quantify variability amplitude.
• Results were compared with existing cm/mm-band data from the F-GAMMA program to examine spectral evolution and time lags.
• A sample of 40 gamma-ray blazars, including FSRQs and BLLacs, was monitored, with Mkn 501 excluded due to lack of significant variability.

Experimental results

Research questions

• RQ1 What is the level of intrinsic variability in gamma-ray blazars at sub-mm wavelengths, and how does it compare to longer cm/mm bands?
• RQ2 Is there a correlation between sub-mm variability and high-energy (gamma-ray) flaring activity, and what does this imply about emission region co-location?
• RQ3 Do the observed sub-mm variability characteristics support shock-driven variability models, particularly the propagation of flares from high to low frequencies?
• RQ4 How does the variability amplitude in the sub-mm band compare to that in the cm/mm bands, and what does this imply about the optical depth and emission region size?
• RQ5 Can sub-mm observations help distinguish between different emission mechanisms such as synchrotron self-absorption and inverse-Compton processes?

Key findings

• All monitored sources except Mkn 501 exhibited significant variability, with a χ² probability of less than 0.1% that the variability was due to measurement noise.
• The modulation index (m) ranged from 10% to 90%, with a median of 31% and mean of 37%, indicating strong intrinsic variability at sub-mm wavelengths.
• A significant number of sources showed flux variations by a factor of 10 or more between minimum and maximum flux, exceeding typical variability amplitudes at cm/mm bands.
• Sub-mm variability was found to be faster and more directly correlated with high-energy emission than at longer wavelengths, suggesting the sub-mm band probes the most compact, high-energy regions of the jet.
• The observed increase in variability amplitude from 110 mm to 1 mm and further into the sub-mm band supports the hypothesis that flares reach their plateau or decay phase earlier at sub-mm frequencies than at cm bands.
• The results imply that the sub-mm emission region is co-spatial with the optical and gamma-ray emission regions, consistent with shock models where variability propagates from high to low frequencies.