REDUCTION OF GALFACTS DATA AND VARIABLE RADIO SOURCES Scott Barenfeld with Tapasi Ghosh and Chris Salter

VARIABLE RADIO SOURCES First discovered in 1965 Radio galaxies and quasars that vary in brightness and sometimes polarization Time scales from months and years to hours and days

VARIABLE RADIO SOURCES Cause of variability is unclear Based on the timescale of the variability, the size of the active region can be constrained Much smaller than the angular resolution of even VLBI

VARIABLE RADIO SOURCES-THEORIES Extrinsic causes: Interstellar scintillation Gravitational microlensing Both probably contribute to some cases of variability Won’t work for most cases though

VARIABLE RADIO SOURCES-THEORIES Intrinsic causes: Brightness temperatures of up to 10^21 K, so non-thermal emission Probably syncotron Adiabatic expansion Shocks in jets More work needed Two Methods to Study Variable Sources

GALFACTS Survey of the Arecibo sky ALFA MHz Study of Milky Way and extragalactic sources

DATA ANALYSIS Average By Channel Pick Out Good Channels Average By Time Original Data Pick Out Sources

DATA ANALYSIS Original DataRemove NaNs Convolution Average by Channel Find and Remove RFI Average Good Channels by Time Filter Convolution Again Pick Out Individual Sources Fit Gaussians to Sources Generate Source List Repeat for Each Day’s Data

ORIGINAL DATA # RA DEC AST I Q U V

ORIGINAL DATA Test data first- known source 2048 Files RA, DEC, Time, I, Q, U, V

REMOVE NaNs NaNs create problems when averaging Program runs fastest if they are removed first Each channel’s file is searched Channel numbers with no NaNs are listed These channels are then averaged

AVERAGE BY CHANNEL Once the NaNs are removed, data can be averaged The I, Q, U, and V values are averaged for each channel The results are outputted to a new file, and plotted

AVERAGE BY CHANNEL

HOW TO FIND RFI? Next Step was to find channels with RFI Anything above a certain value is RFI?

HOW TO FIND RFI? Need to flatten spectrum while maintaining spikes Convolution Answer:

CONVOLUTION

One Up, Two Down

Now, anything above or below a certain value can be removed RMS of several intervals ± 6 sigma Channels with no RFI are written in a separate file FIND AND REMOVE RFI

AVERAGE BY TIME Go through original data and average I, Q, U, and V by time Use only channels with no RFI or NaNs Can anyone guess where the source is?

FILTER Pick out sources like we picked out spikes- convolution Noise created a problem Noise had to be removed first Convolution with sin(x)/x

FILTER BeforeAfter

CONVOLUTION AGAIN Still problems 1 up, 2 down didn’t work Tried several different functions Settled on 1 up, 4 down

Calculated theoretical sigma.017 K Only used the I data from this point on ±8 Sigma PICK OUT INDIVIDUAL SOURCES

FIT GAUSSIANS TO SOURCES Find what points to use 8 points taken on either side to form Gaussian 5 points taken on either side 28 points away for a baseline

FIT GAUSSIANS TO SOURCES Program created by GMRT scientists using Numerical Recipes Needed estimates of position, height, width of Gaussian, and slope and y-intercept of baseline Used width from slope of line fitted earlier Calculated the other parameters for each source Final fitted parameters are then returned

GENERATE SOURCE LIST Sources are sorted into two lists The scan direction, position, time, fit parameters, and chi squared are listed for each source JD Beam Source Direction RA DEC Time A W Chisq beam0 Source 4 Down beam0 Source 5 Down beam0 Source 6 Up beam0 Source 7 Down

REPEAT FOR EACH DAYS DATA All this was done for just one day and one beam of ALFA Next step to run the program on multiple days Run two levels up, with a list of file names JD and beam number are included in the final source list Sources sorted by RA

FUTURE WORK Still a lot of work to do The extended sources need to be fitted with multiple Gaussians Program needs to be run on the full data set Fluxes need to be calculated for each source Variability of sources needs to be analyzed All this needs to be done for Q, U, and V, as well as I

LIGHTNING

BREAKING NEWS…

QUESTIONS?

REFERENCES Altschluer 1990 Wagner and Witzel