Non-Uniformity and Possible Structure in GRB Sky Distribution

Krista L. Smith with advisors

Dr. Kirsten Tollefson & Dr. Jim Linnemann

Gamma-Ray Bursts (GRBs)

Illustration from “GRB Lights Up Ancient Hidden Galaxy”, Universe Today.

Discovered accidentally by Los Alamos National Lab in 1967

Over 5,000 papers were published on GRBs (1973 – 2000)

Two classes, both lead to the formation of a black hole

GRBs are not well understood

Most powerful events ever observed

Project GoalsConfirm or disprove results from a paper published earlier this year that observed a possible structure at redshift 1.6 to 2.1 http://arxiv.org/abs/1401.0533

Perform analysis of the GRB redshift distribution using The GRB Catalog and look for activity greater than 3

Timeline, Big Bang Central

GRB Catalog

This database was created by former postdoc at MSU and Swift team member Tilan Ukwatta.

The database currently includes 984 GRBs, with the majority having been detected by Swift.

Our study includes 284 total GRBs (those with known redshifts confirmed using the spectroscopic technique)

NASA Swift satellite, launched November 20, 2004 and actively

detecting GRBs

284 GRB Pool

Distribution as seen in the zx plane

Distribution as seen in the xy plane

Celestial coordinate system (NASA Swift

Sonoma State)

98% of GRBs in pool detected by Swift over 9 yrs.

Celestial coordinates for the x-axis are RA (Right Ascension), y-axis are Dec (Declination) and z-axis are Redshift.

The graph below at left shows distribution when Dec equals zero, and the graph below at right shows distribution when Redshift equals zero.

MethodWe partitioned the sky into grids of size 4 x 4, 6 x 6, and 8 x 8 to vary the binning of BAT RA and BAT Dec

We also partitioned the redshift.

These were first done by hand, then with code.

We are looking for activity that will be significant in all grid sizes.All graphs generated by the GRB

Catalog

AnalysisWe have used the population standard deviation as well as the sample standard deviation.

The sample standard deviation is most likely a better estimate since the GRB pool is rather large.

The GRB count from each bin was treated as a data point.

The standard deviation is taken from the mean GRB count for the redshift interval.

Sample Standard Deviation

Results

4 x 4 grid

Standard Deviation Results

This is the 4 x 4 grid partitioning of the sky for Redshift 1.6 – 2.7.

We observe 3 activity in this range using the population standard deviation.

Results

The grid sizes of 6 x 6 and 8 x 8 both show 3 activity using the sample standard deviation.

Results

What does it mean?Results from NASA’s BATSE satellite concluded that GRB distributions were uniform and not pertaining to any form of structure in the sky.

These results show a non-uniform distribution where activity is 3 standard deviations from the mean.

GRB sky distribution of all GRBs detected by NASA’s BATSE satellite (1991-2000)

These areas are subsets of the same region of the sky:

C3 = (180 – 270, 0 – 45)

D4 = (180 – 240, 0 – 30)

F5 = (180 – 225, 22.5 – 45)

Conclusive RemarksBased on the 3 GRB activity in redshift 1.6 – 2.7, these findings appear to support a recent publication which suggests the redshift region 1.6 – 2.1 possibly contains a structure even larger than the Sloan Great Wall, the second largest known galactic structure in the Universe.

Left: Example of Hakkila paper results which identify a redshift binning only. Right: Our results, which identify a larger (yet compatible) redshift

region as well as RA and Dec regions

Future WorkCompleting more sophisticated analysis on results presented here.

Comparing different redshift regions to look for evidence of non-uniform expansion.

Percent difference for RA values in redshift region 0.0 – 3.0 compared with 3.0 – 10.0

AcknowledgmentsProf. Kirsten Tollefson, REU advisor

Prof. Jim Linnemann, REU advisor

Tilan Ukwatta, former postdoc at MSU

Prof. Dan Stump, REU coordinator

The National Science Foundation

Michigan State University

Kim Crosslan