Investigation of the high-energy temporal characteristics of magnetar 4U 0142+61 covering a 16 year period

Abstract

In this bachelor project the temporal characteristics of magnetar 4U 0142+61 have been investigated using unexplored INTEGRAL ISGRI soft gamma-ray data in the 20-300 keV band, collected since Aug. 15, 2010 up to Jan. 21, 2019, and Fermi-LAT high-energy gamma-ray data (>30 MeV), collected since Aug. 4, 2008 up to Jan. 21, 2019.

Essential in this study is the availability of pulsar ephemerides i.e. timing models describing the rotation behaviour of the pulsar very accurately as a function of time for a certain time interval. Data from concurrent monitoring observations (commenced on July 29, 2011 and ceased at March 26, 2019) of 4U 0142+61 performed by the Swift-XRT instrument (0.3-10 keV) have been used to construct these required timing models.

In this work I prepared an accurate timing model for the period July 29, 2011 -- Feb. 26, 2015, covering more then 3.5 years, after downloading and pipeline reprocessing Swift-XRT data taken in Windowed-Timing (WT) mode. This model was subsequently applied and verified using hard X-ray data (3-79 keV) from observations taken by the NuSTAR telescope during March 27--29, 2014. Comparison with the contemporaneous Swift-XRT data yielded a consistent picture in the overlapping energy interval i.e. 3-10 keV, while the full picture clearly showed the well-known morphology changes of the pulse profile as function of energy, but now in (much) more detail.

Using the full set of Swift-XRT based ephemerides the unexplored soft gamma-ray data from the coded mask ISGRI instrument (20-300 keV) aboard INTEGRAL were processed in a timing analysis - after observation selection and data screening - to yield pulse-phase distributions across the 20-300 keV band. From these distributions (integrated across Aug. 15, 2010 up to Jan. 21, 2019) the significances of the pulsed emission have been estimated (and the morphology changes versus energy revealed) for various energy bands between 20 and 300 keV. I obtained signal strengths of 4.2 sigma, 4.3 sigma, 3.4 sigma and 1.3 sigma for the 20-50, 50-100, 100-150 and 150-300 keV bands, respectively, while the integral band, 20-150 keV, yielded a 6.9 sigma signal.

Combining these new ISGRI results with existing ISGRI products from the 2003-2010 period yielded the most accurate pulse profiles currently available, covering the full ~16 year INTEGRAL data period from 2003 to 2019. The overall pulsed emission significances are 6.7 sigma, 6.2 sigma, 5.4 sigma and 1.3 sigma for the 20-50, 50-100, 100-150 and 150-300 keV bands, respectively. The overall 20-150 keV signal reached even a 10.7 sigma strength.

Furthermore, the full set of Swift-XRT based ephemerides, along with some archival RXTE-PCA based ephemerides for the 2008--2011 period, has been used in a timing analysis of Fermi-LAT (>30 MeV) high-energy gamma-ray data in order to investigate the presence of pulsed emission at high-energy gamma-rays. I found no evidence for pulsed emission of 4U 0142+61 above >30 MeV in any energy band using more than 10 years of Fermi-LAT observations.

From the newly derived mission-wide INTEGRAL ISGRI (16 years) and Fermi-LAT (10 years) pulse-phase distributions spectral information of the pulsed emission can be reconstructed to yield the pulsed spectrum in great detail across the 20 keV - 10 GeV range, however, this is outside the scope of this bachelor project.