About the Project
Scientific Problem | Planned Observations | Data Release | Instruments
Scientific Problem
One of the most important classes of Fermi (formerly GLAST) sources is expected to be blazars--active galactic nuclei in which a relativistic jet is aligned with the line of sight (see reviews by Urry and Padovani 1995 and Urry 1999). Relativistic boosting results in very luminous, highly variable emission. These sources are thus ideal for studying the physics of the ubiquitous, but poorly understood astrophysical jets.
We now have a basic understanding of blazars, including rough scenarios for jet geometry, bulk speeds, and at least some of the relevant physical processes and radiation mechanisms. The spectral energy distributions (SEDs) are dominated by two components, one at lower energies (radio-optical-UV), and one at higher energies (X-ray/ gamma-ray) (See Fossati, et al. 1998). The lower energy component is generally agreed to be due to synchrotron radiation from relativistic electrons accelerated at shocks in the jet (See Marscher and Gear 1985 and Dermer and Schlickeiser 2003, 1993), but the source of the high energy component of blazar SEDs is still disputed, with both leptonic and hadronic models able to fit many current constraints.
One key approach to this problem is to search for temporal correlations between the flux and spectra of the two major emission components. In hadronic models the high-energy and low energy components are produced by different particle populations that are not necessarily simply related. In contrast, in the simplest leptonic models (e.g., synchrotron self-Compton or inverse Compton) the two components are produced by the same particles, which leads to very specific predictions for correlations, lags and leads between the components, and spectral responses within each component. Observations across a sample of sources should be able to strongly restrict the available parameter space, and thus test the viability of the models generally. Such observations also serve as a starting point for constructing detailed physical models of relevant effects. We have considerable experience in the interpretation of correlations in such multiwavelength curves (e.g. Hartman et al. 2001).
Multi-wavelength studies have been attempted to address these issues in the past (for recent examples see Dolcini et al. 2007 and Bach et al. 2007) but they are difficult to organize and hard to sustain, since they require large amounts of simultaneous time on ground-based observatories and space-based gamma-ray missions. Particular difficulties arise for IR imaging, which is important for extending broad-band coverage beyond the narrow optical window, and for optical IR/emission-line spectroscopy, which is important for distinguishing between emission from the jet and from the accretion disk or clouds.
On the high energy side, Fermi is a huge improvement in temporal coverage at GeV energies. At short timescales (hours to days), the unprecedented sensitivity will allow appropriate temporal resolution, while at longer time scales (months to years), the continuous sky coverage will provide continuous long-term lightcurves. Recognizing the importance of correlated multi-wavelength observations, the Fermi-LAT team will provide immediate data release of almost two dozen bright blazars, in Cycle 1. This project will generate and analyze a homogeneous publically-accessible optical/IR database for the southern subset of the LAT-monitored blazars using Yale's share of access to the SMARTS 1m-class telescopes in Chile.
Back to top of pagePlanned Observations
The observations will be carried out with three telescopes located at Cerro Tololo Interamerican Observatory (CTIO) operated by the Small and Moderate Aperture Research Telescope System (SMARTS). Altogether, 70 nights of Yale University telescope time on SMARTS will be dedicated to this task. A list of the targets to be observed can be found here.
- Photometric Monitoring
- Spectroscopic Monitoring
- Intensive Monitoring Programs
Our daily monitoring program will be carried out with the SMARTS 1.3m+ANDICAM at CTIO. ANDICAM is a dual-channel imager with a dichroic that feeds an optical CCD and an IR imager. Thus this instrument can obtain simultaneous data from 0.4 to 2.2 microns. We will calibrate comparison stars in the field, and will be able to determine magnitudes of our targets even in mildly adverse weather conditions. We have been carrying out such monitoring campaigns for years on optical counterparts of galactic X-ray binaries, as shown below. We expect to obtain data of similar quality for our target blazars.
Many blazars have featureless optical spectra, but some show emission lines (e.g. 3C 273, 1510-089), and it is beneficial to see how the line strength varies. It is also possible that some of the featureless blazars may develop lines in certain states. Thus we will monitor the brighter blazars in our sample spectroscopically as well. The SMARTS 1.5m+RC Spec allows for only one spectral setup on any given night, so we will not be able to carry out daily monitoring. Initially (Aug-Dec 2008) we will obtain spectra of our targets once every two weeks. We hope to increase the frequency of observations in 2009.
The 1.3m+ANDICAM cannot be used all night on a single program, thus variability on timescales short compared to one day are difficult to study with the 1.3m. However, the 1.0m and 0.9m SMARTS telescopes are equipped with CCD imagers, and are scheduled classically. We expect to periodically schedule one or two week long intensive monitoring programs that will observe the sources with a cadence of 1-5 minutes. During these programs, the 1.3m and 1.5m will observe the source several times during the night.
Data Release
We intend to release much of our data and reduced data products to the community quickly, on the same 1-2 day timescale as the LAT monitoring program. We will also continuously search for correlations between our data and LAT data projects, and we will be able to change our observing strategy in response to interesting general results and specific time constrained events.
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