Case Studies
Imaging Short-timescale Low-amplitude variability in Blazars with iXon+ DU897-BV
Quasars are the most luminous, continuously emitting, objects in the Universe. The
standard quasar model invokes a super massive black hole at the centre of a galaxy,
onto which matter flows from an accretion disk. In about 10% of cases, jet-like
structures are seen to emanate from the core in radio observations. If the jet is
beamed directly at the earth, the output may be dominated by emission from the jet,
rather than the underlying accretion disk. Quasars that fit into this category are
called blazars. Variability in blazars is known to occur on timescales ranging from
hours to tens of years. The most rapid variations are likely to originate in shock
fronts in the jet, where particles are accelerated via Fermi acceleration.
Rapid variability measurements, with time resolutions of the order of a minute,
are important because they probe structures with angular diameters that cannot be
imaged directly. Even planned space-borne mm-interferometry will fall short of this
angular resolution by some three orders of magnitude. Blazars have been monitored
intensively by many observers using conventional CCD technology, with typical integration
times of 1 – 5 minutes. Whenever the sampling has been dense and temporally fast,
there has been evidence for very fast variations. However, the observations have
been hampered by the need to integrate for long enough to ensure the signal is well
above the read-noise floor. The lack of sufficiently precise photometry at high
time resolution has made it difficult to draw conclusions about the temporal shapes
of fast flares and possible substructures contained within.
Searching for fast variations almost inevitably results in low integrated fluxes
per frame, hence optimum signal-to-noise (S/N) ratios must be achieved at very low
photon fluxes.
Image plot of Blazar 0954+51. 0.2 sec exposure with EMCCD gain.
This prompted the group to make a series of observations of a small sample of blazars
with the advanced iXonEM+ DU-897 EMCCD camera from Andor, featuring the
CCD 97 back-illuminated L3 sensor from E2V, that offers unsurpassed sensitivity
performance at high time resolution. Light curves of a number of blazars were recorded
and clear evidence of variability was detected on timescales of 30 minutes and longer.
No convincing evidence was found for variations on timescales of minutes. Significantly,
the fast readout rates employed (by blazar monitoring standards) generated large
numbers of datapoints. By binning these, they we were able to estimate the empirical
errors for each datapoint and improve the reliability of the photometry. They have
chosen the new Andor iXonEM+ with –100°C TE cooling and Linear Gain to
advance future efforts in this field.