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Variability of extragalactic X-ray jets on kiloparsec scales

Nature Astronomy volume 7, pages 967–975 (2023)Cite this article

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Abstract

Unexpectedly strong X-ray emission from extragalactic radio jets on kiloparsec scales has been one of the major discoveries of Chandra, the only X-ray observatory capable of sub-arcsecond-scale imaging. The origin of this X-ray emission, which appears as a second spectral component from that of the radio emission, has been debated for over two decades. The most commonly assumed mechanism is inverse-Compton upscattering of the cosmic microwave background by very low-energy electrons in a still highly relativistic jet. Under this mechanism, no variability in the X-ray emission is expected. Here we report the detection of X-ray variability in the large-scale jet population, using a novel statistical analysis of 53 jets with multiple Chandra observations. Taken as a population, we find that the distribution of P values from a Poisson model is strongly inconsistent with steady emission, with a global P value of 1.96 × 10−4 under a Kolmogorov–Smirnov test against the expected uniform (0, 1) distribution. These results strongly imply that the dominant mechanism of X-ray production in kiloparsec-scale jets is synchrotron emission by a second population of electrons reaching multi-teraelectronvolt energies. X-ray variability on the timescale of months to a few years implies extremely small emitting volumes much smaller than the cross-section of the jet.

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Fig. 1: A Chandra X-ray image (0.3–7 keV) of PKS 1136-135, one of the 53 X-ray emitting jets in our sample.
Fig. 2: Level plots showing the P value for a one-sided KS test comparing simulated data sets with our observed single-region P-value distribution.
Fig. 3: Histogram of the single-region P values from the directional test, not adjusted for multiple comparison.

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Data availability

All observations used in this study are publicly available. In particular, the Chandra X-Ray Observatory archive can be accessed on the web at https://cda.harvard.edu/chaser/. Radio observations with NRAO facilities are available at https://data.nrao.edu, ACTA observations from https://atoa.atnf.csiro.au/. Extensive tables of reduced data necessary to repeat these analyses are available as supplementary Excel files, further described in Supplementary Section 3.g

Code availability

The analysis software used in this study for reducing astronomical data is described in Methods and publicly available from the respective observatories. The statistical analysis is fully described in Methods and can be repeated using any software suitable for data analysis, such as R or Python.

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Acknowledgements

The following grant funding is acknowledged: Chandra Archival Grant 16700615 (E.T.M.), ADAP grant NNX15AE55G (E.T.M.) and NSF grant 1714380 (E.T.M.), as well as support from the Natural Sciences and Engineering Research Council of Canada, grant ID RGPIN-2020-05897 (Y.T. and N.R.). This research was made possible through use of data obtained from the Chandra Data Archive and the Chandra Source Catalog, and software provided by the Chandra X-Ray Center (CXC) in the application packages CIAO and sherpa. The Australia Telescope Compact Array is part of the Australia Telescope National Facility, which is funded by the Australian Government for operation as a national facility managed by CSIRO. We acknowledge the Gomeroi people as the traditional owners of the observatory site. This paper makes use of the following ALMA data sets: 2012.1.00688.S, 2016.1.01481.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. The NRAO is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

Author information

Authors and Affiliations

  1. Department of Physics, University of Maryland, Baltimore County, Baltimore, MD, USA

    Eileen T. Meyer, Aamil Shaik, Karthik Reddy, Markos Georganopoulos, Nat DeNigris & Max Trevor

  2. Department of Mathematics, Imperial College London, London, UK

    Yanbo Tang

  3. Department of Statistical Sciences, University of Toronto, Toronto, Ontario, Canada

    Nancy Reid

  4. School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA

    Karthik Reddy

  5. The William H. Miller III Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD, USA

    Peter Breiding & Marco Chiaberge

  6. Space Telescope Science Institute for the European Space Agency, Baltimore, MD, USA

    Marco Chiaberge

  7. Department of Aerospace, Physics and Space Sciences, Florida Institute of Technology, Melbourne, FL, USA

    Eric Perlman & Devon Clautice

  8. SETI Institute, Mountain View, CA, USA

    William Sparks

  9. Space Telescope Science Institute, Baltimore, MD, USA

    William Sparks

  10. Department of Astronomy, University of Massachusetts Amherst, Amherst, MA, USA

    Nat DeNigris

  11. Department of Physics, University of Maryland, College Park, College Park, MD, USA

    Max Trevor

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Contributions

E.T.M. conceived the project, developed the data analysis methods, contributed to the multiwavelength data analysis and wrote the paper. A.S. carried out the X-ray data analysis with contributions from E.T.M., K.R., P.B., N.D., D.C. and M.T. Y.T. and N.R. contributed the statistical analysis methods and expertise. K.R. and P.B. contributed to the data analysis and interpretation. M.G. contributed to the interpretation and theoretical implications. All authors contributed to the editing of the paper and interpretation of results.

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Correspondence to Eileen T. Meyer.

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Extended data

Extended Data Fig. 1

Histogram of the single-region p-values for the full sample of 155 regions. The distribution shows an excess at low values relative to the expected uniform (0, 1) distribution, indicating variability in the population.

Extended Data Fig. 2 Histogram of the percent difference of each epoch source rate from \(\bar{\mu }\) for all regions.

Including all epochs on all regions, there are 545 distinct observations. The distribution has a mean of 1.02% and a standard deviation of 28.5%. The mean of the absolute value of the percent difference is 18%.

Supplementary information

Supplementary Information

Supplementary Figs. 1–5, Table 1, Methods, Discussion and description of Supplementary Tables 2–5.

Supplementary Data 1

Supplementary Tables 2–5.

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Meyer, E.T., Shaik, A., Tang, Y. et al. Variability of extragalactic X-ray jets on kiloparsec scales. Nat Astron 7, 967–975 (2023). https://doi.org/10.1038/s41550-023-01983-1

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