Flux monitoring of Sagittarius A* at mm-wavelengths

The launching mechanism of the jets of active galactic nuclei is poorly constrained observationally, owing to the large distances to these objects and the very small scales (sub-parsec) involved. To better constrain theoretical models, it is especially important to get information from the region close to the physical base of the jet, where the plasma acceleration takes place. In this paper, we report multi-epoch and multifrequency continuum observations of the z = 2.5 blazar PKS 1830−211 with ALMA, serendipitously coincident with a strong γ-ray flare reported by Fermi-LAT. The blazar is lensed by a foreground z = 0.89 galaxy, with two bright images of the compact core separated by 1 . Our ALMA observations individually resolve these two images (although not any of their substructures), and we study the change in their relative flux ratio with time (four epochs spread over nearly three times the time delay between the two lensed images) and frequency (between 350 and 1050 GHz, rest frame of the blazar), during the γ-ray flare. In particular, we detect a remarkable frequency-dependent behavior of the flux ratio, which implies the presence of a chromatic structure in the blazar (i.e., a core-shift effect). We rule out the possibility of micro-and milli-lensing effects and propose instead a simple model of plasmon ejection in the blazar's jet to explain the time and frequency variability of the flux ratio. We suggest that PKS 1830−211 is likely to be one of the best sources to probe the activity at the base of a blazar's jet at submillimeter wavelengths, thanks to the peculiar geometry of the system. The implications of the core shift in absorption studies of the foreground z = 0.89 galaxy (e.g., constraints on the cosmological variations of fundamental constants) are discussed.

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“…wavelengths, lasting several weeks, is not rare in sources as instrinsically weak as Sgr A* (Miyazaki et al 2006).…”

Section: Comparison With Mm/submm Flaring Activity In Other Agnmentioning

confidence: 99%

Probing the jet base of the blazar PKS 1830−211 from the chromatic variability of its lensed images

Martí-Vidal

Müller

Combes

et al. 2013

A&A

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“…Variability at radio through sub-millimeter wavelengths has been studied extensively, showing that variations occur on timescales from hours to years (Wright & Backer 1993, Bower et al 2002, 2003, 2004, 2005a, Zhao et al 2003, 2004, Mauerhan et al 2005, Yusef-Zadeh et al 2007, Miyazaki et al 2006, Marrone et al 2008. Several flares have provided evidence for decaying millimeter and sub-millimeter emission following NIR/Xray flares.…”

Section: Introductionmentioning

confidence: 99%

Modeling mm- to X-ray flare emission from Sagittarius A*

Eckart

Baganoff

Morris

et al. 2009

A&A

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Context. We report on new modeling results based on the mm-to X-ray emission of the SgrA* counterpart associated with the massive ∼4 × 10 6 M black hole at the Galactic Center. Aims. We investigate the physical processes responsible for the variable emission from SgrA*. Methods. Our modeling is based on simultaneous observations carried out on 07 July, 2004, using the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope and the ACIS-I instrument aboard the Chandra X-ray Observatory as well as the Submillimeter Array SMA on Mauna Kea, Hawaii, and the Very Large Array in New Mexico. Results. The observations revealed several flare events in all wavelength domains. Here we show that the flare emission can be described with a combination of a synchrotron self-Compton (SSC) model followed by an adiabatic expansion of the source components. The SSC emission at NIR and X-ray wavelengths involves up-scattered sub-millimeter photons from a compact source component. At the start of the flare, spectra of these components peak at frequencies between several 100 GHz and 2 THz. The adiabatic expansion then accounts for the variable emission observed at sub-mm/mm wavelengths. The derived physical quantities that describe the flare emission give a blob expansion speed of v exp ∼ 0.005 c, magnetic field of B around 60 G or less and spectral indices of α = 0.8 to 1.4, corresponding to a particle spectral index p ∼ 2.6 to 3.8. Conclusions. A combined SSC and adiabatic expansion model can fully account for the observed flare flux densities and delay times covering the spectral range from the X-ray to the mm-radio domain. The derived model parameters suggest that the adiabatic expansion takes place in source components that have a bulk motion larger than v exp or the expanding material contributes to a corona or disk, confined to the immediate surroundings of SgrA*.

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“…The flare emissions in radio, submillimeter, and millimeter wavelengths are known to vary on hourly timescales (Marrone et al 2007;Miyazaki et al 2006;Mauerhan et al 2005;Yusef-Zadeh et al 2006a). The near-IR measurements show a high degree of linearly polarized emission during the 10-20 minute timescale of typical flare emission.…”

Section: Introductionmentioning

confidence: 99%

The Variability of Polarized Radiation from Sagittarius A*

Yusef‐Zadeh

Wardle

Cotton

et al. 2007

ApJ

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Sgr A* is variable at radio and submillimeter wavelengths on hourly timescales, showing time delays between the peaks of flare emission as well as linearly polarized emission at millimeter and submillimeter wavelengths. To determine the polarization characteristics of this variable source at radio frequencies, we present VLA observations of Sgr A* and report the detection of polarized emission at a level of 0.77% ‫ע‬ 0.01% and 0.2% ‫ע‬ 0.01% at 43 and 22 GHz, respectively. The change in the time-averaged polarization angle between 22 and 43 GHz corresponds to a RM of (Ϫ2.5 ‫ע‬ 0.6) # 10 3 rad m with no phase wrapping (or ∼5 # 10 4 rad m 2 with 2p phase wrap). We also note a Ϫ2 rise and fall timescale of 1.5-2 hr in the total polarized intensity. The light curves of the degree of linearly polarized emission suggest a correlation with the variability of the total intensity at 43 GHz. The available polarization data at radio and submillimeter wavelengths suggest that the rotation measure decreases with decreasing frequency. This frequency dependence, and observed changes in polarization angle during flare events, may be caused by the reduction in rotation measure associated with the expansion of synchrotron-emitting blobs.

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Flux monitoring of Sagittarius A* at mm-wavelengths

Cited by 6 publications

References 18 publications

Probing the jet base of the blazar PKS 1830−211 from the chromatic variability of its lensed images

Probing the jet base of the blazar PKS 1830−211 from the chromatic variability of its lensed images

Modeling mm- to X-ray flare emission from Sagittarius A*

The Variability of Polarized Radiation from Sagittarius A*

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