Connecting neutron star diversity to that of supernovae explosions

Professor Samar Safi-Harb’s research addresses the physics of such objects as supernovae and neutron stars, and answers questions around their diversity, formation, and evolution, as well as their impact on their surroundings.

Supernovae (SN) are among the most fascinating, energetic events in the Universe that are relevant to a wide range of research areas, ranging from star formation and the Interstellar Medium, to high-energy cosmic rays and compact objects, to galaxy formation, evolution, and cosmology. These explosions make and scatter the heavy elements in the Universe, thus playing a major role in the chemical enrichment and evolution of galaxies. Furthermore, they accelerate cosmic rays to extremely high energies.

The supernova remnants (SNRs) resulting from core-collapse explosions make some of the most magnetic and exotic objects in the Universe: neutron stars (NSs), which have recently been in the spotlight thanks to LIGO’s discovery of gravitational waves from a neutron star merger – a discovery that has ushered in the new era of multi-wavelength, multi-messenger astronomy. Young NSs emit relativistic winds and jets that power beautiful nebulae referred to as Pulsar Wind Nebulae (PWNe), some of the most efficient particle accelerators known in the Universe.

Professor Samar Safi-Harb’s research addresses the physics of these objects and answers questions around their diversity, formation, evolution and impact on their surroundings. In an interview with Innovation News Network, she went into detail about her activities in these areas.

One of the driving questions of your research is connecting the neutron star diversity (or zoo) to the diversity of the supernovae explosions giving birth to these objects. Can you tell me more about this?

Neutron stars were first discovered as pulsars by Jocelyn Bell and Antony Hewish in 1967. Early observations showed that rotation-powered pulsars (RPPs) power PWNe like the famous Crab nebula. However, the past 20 or so years have shown accumulating evidence for a growing ‘zoo’ of neutron stars and PWNe, suggesting that the aftermath of the SN explosion of a massive star can be manifested in different ways. The zoo includes magnetars, high-magnetic field pulsars (HBPs), and Central Compact Objects (CCOs) amongst others.1 While the former class is commonly believed to be strongly magnetised neutron stars powered by their super-strong magnetic fields (B) exceeding the quantum critical field (BQED=4.4×1013 Gauss),2 the latter class (CCOs) point to low-B (~1010-11 Gauss) pulsars— thus referred to as ‘anti-magnetars’.3

Both magnetars and CCOs shine in X-rays and are commonly believed to lack PWNe. HBPs have magnetic fields that are intermediate between the magnetars and the rotation-powered pulsars. Their X-ray luminosities can be energetically powered by their rotational energy loss, but they also occasionally display some properties that are common to magnetars.

My research addresses this diversity and aims at studying their connection through studying their environment and hosting SNRs. The discovery of a magnetar-like behaviour and variability in two young HBPs4,5,6,7 otherwise known as rotation-powered for most of their lifetime powering PWNe, as well as the discovery of PWNe around a handful of HBPs and magnetars8,9,10,11 are blurring the distinction between the highly magnetised neutron stars, magnetars, and their classical cousins. Most recently, a Chandra X-ray study of a sample of PWNe revealed, through a spectral index mapping technique, that even the ‘classical’ PWNe, powered by rotation-powered pulsars, can display hidden variability.12

Spectroscopic and imaging studies of SNRs hosting this neutron stars zoo are shedding light on the nature of these objects. While earlier studies suggested that magnetars arise from high-mass (>20 solar-mass) progenitors13,14 a recent investigation of a handful of magnetar-hosting SNRs showed that these objects can arise from lower mass (<20 solar-mass) progenitor stars resulting from low-energy (<1051 ergs) explosions – at least an order of magnitude smaller than the canonical explosion energy.15,16 This challenges our current assumptions about supernova energetics and supports the fossil field origin for magnetars. Some explosions leading to these peculiar compact objects seem to be also associated with binary progenitors.

In addition, by studying the PSR-SNR associations with extreme magnetic fields (CCOs, magnetars, and HBPs) in an effort to reconcile the ages of PSRs and their hosting SNRs, we found that magnetic field evolution in neutron stars, particularly magnetic field growth in CCOs, plays a key role in connecting CCOs to other neutron star classes, supporting the idea of a magnetic field buried by SN fallback.17,18 In summary, this work is impacting our basic understanding of PSR evolution, dynamics and their SN progenitors.

What is SNRcat, and how are you involved?

At the University of Manitoba, we have developed the Supernova Remnants High-Energy Catalogue (SNRcat), a catalogue that is used extensively by the SNR community worldwide. This is the first public database19 of high-energy (X-ray and gamma-ray) observations of all (~400) Galactic SNRs, including Pulsar Wind Nebulae (PWNe). The catalogue can be accessed online at snrcat.physics.umanitoba.ca.

SNRcat has been heavily accessed and appreciated by the community worldwide, and it has become a unique reference for the field that is constantly developed and kept up to date. The catalogue has been used by both observers and theorists for the interpretation of existing SNR observations, as well as for the preparation of new ones. This catalogue has impacted a number of studies, including the HESS SNR population study and the HESS Galactic Plane Survey.20,21 It will also play a key role in future surveys and instruments, including the upcoming Cherenkov Telescope Array (CTA) in gamma-rays,22 ATHENA in X-rays, the Square Kilometre Array and ngVLA in the radio and the TMT/VLOT in the optical.

My team has recently released an updated version of the online catalogue which includes an imaging component (all SNR images in the radio and X-ray bands), galaxy maps (top-down and plane views), as well as a new interface for the website. All data are downloadable.

What are the future prospects for X-ray astrophysics missions?

Over the next decade, advancements in X-ray detectors will revolutionise our understanding of some of the most extreme events in the Universe. Upcoming X-ray missions will be equipped with new technology to provide high throughput, combined with high-resolution timing, imaging and/or spectroscopy. These include XRISM (to be launched by JAXA around 2022) and ATHENA (ESA, circa 2030 timescale).

Among the newly proposed upcoming X-ray mission concepts is the Colibrì satellite (see www.colibri-telescope.ca), Canada’s flagship X-ray telescope aimed at performing high throughput, high-resolution X-ray spectroscopy of compact objects.23,24 A recent FAST (Flights and Fieldwork for the Advancement of Science and Technology) Canadian Space Agency award to myself and Jeremy Heyl (University of British Columbia) will allow us and Colibrì collaborators to investigate the discovery space for this mission.

Colibrì will build on the strength of the JAXA-launched Hitomi satellite (high-resolution spectroscopy) and the NICER mission (collector optics and high timing resolution), and will be equipped with an unprecedented sensitivity that will achieve fine resolution in both timing and spectroscopic space, particularly around the Fe-K line energy. Colibrì is currently in its design phase and aims at using Transition Edge Sensors in space for the first time. This technology will be particularly crucial to probing the extreme physics of black holes and neutron stars, answering fundamental questions such as:

  • What is the structure of the spacetime surrounding black holes?
  • How do accretion disks transport material?
  • How are relativistic jets launched?
  • How does matter behave in extreme environments? and
  • What are the masses, radii, and atmospheric composition of neutron stars?

This is an exciting time for astrophysics as we look forward eagerly to the new and next generations of projects, missions, and instruments which will enable us to explore some of the highest energy astrophysical phenomena driving our understanding of the extreme, invisible Universe.

References

1 Safi-Harb, S. 2017, Journal of Physics: Conference Series, Volume 932, Issue 1, article id. 012005 (2017). DOI: 10.1088/1742-6596/932/1/012005
2 Esposito, P., Rea, N., & Israel, G. L. 2018, in Timing Neutron Stars: Pulsations, Oscillations and Explosions, ed. T. Belloni, M. Mendez, & C. Zhang (Berlin: Springer) in press (arXiv:1803.05716)
3 Gotthelf, E.V., Halpern, J. P., Alford, J. The Astrophysical Journal, Volume 765, Issue 1, article id. 58, 16 pp. (2013)
4 Gavriil, F. et al. 2008, Science, Volume 319, Issue 5871, pp. 1802- (2008)
5 Kumar, H. & Safi-Harb, S. 2008, The Astrophysical Journal, Volume 678, Issue 1, pp. L43-L46
6 Göğüș E. et al. 2016, The Astrophysical Journal Letters, Volume 829, Issue 2, article id. L25, 7 pp. (2016)
7 Blumer, H., Safi-Harb, S. & McLaughlin, M. 2017, The Astrophysical Journal Letters, Volume 850, Issue 1, article id. L18, 6 pp. (2017)
8 Helfand, D., Collins, B. F. & Gotthelf, E.V. 2003, The Astrophysical Journal, Volume 582, Issue 2, pp. 783-792
9 Safi-Harb, S. & Kumar, H. 2008, The Astrophysical Journal, Volume 684, Issue 1, pp. 532-541 (2008)
10 Younes, G. et al. 2016, The Astrophysical Journal, Volume 824, Issue 2, article id. 138, 12 pp. (2016) (arXiv: 2008.01795)
11 Israel, G. L., Esposito, P., Rea, N., et al. 2016, Monthly Notices of the Royal Astronomical Society (MNRAS), 547, 3448
12 Guest, B. & Safi-Harb. S. 2020, MNRAS, https://doi.org/10.1093/mnras/staa2364
13 Gaensler, B.M. et al. 2005, The Astrophysical Journal, Volume 620, Issue 2, pp. L95-L98
14 Kumar, H., Safi-Harb, S., Slane, P.O & Gotthelf, E.V. 2014, The Astrophysical Journal, Volume 620, Issue 2, pp. L95-L98
15 Braun, C., Safi-Harb, S. & Fryer, C 2019, Monthly Notices of the Royal Astronomical Society, Volume 489, Issue 3, p.4444-4463
16 Zhou, P., Vink, J., Safi-Harb, S. & Miceli, M. 2019, Astronomy & Astrophysics, Volume 629, id.A51, 12
17 Rogers, A. & Safi-Harb, S. 2017, MNRAS, Volume 465, Issue 1, p.383-393
18 Rogers, A. & Safi-Harb, S. 2016, MNRAS, Volume 457, Issue 2, p.1180-1189
19 Ferrand, G. & Safi-Harb, S. 2012, Advances in Space Research, Volume 49, Issue 9, p. 1313-1319
20 HESS collaboration 2018, Astronomy & Astrophysics, Volume 612, id.A3, 18
21 HESS Collaboration 2018, Astronomy & Astrophysics, Volume 612, id.A1, 61
22 CTA collaboration, Science with the Cherenkov Telescope Array. Edited by CTA Consortium. Published by World Scientific Publishing Co. Pte. Ltd., ISBN #9789813270091
23 Heyl, J. et al. 2019, ‘Astro2020: Decadal Survey on Astronomy and Astrophysics, science white papers’, no. 491; Bulletin of the American Astronomical Society, Vol. 51, Issue 3, id. 491 (2019)’
24 Heyl, J. et al. 2019, ‘Astro2020: Decadal Survey on Astronomy and Astrophysics’, APC white papers, no. 175; Bulletin of the American Astronomical Society, Vol. 51, Issue 7, id. 175 (2019)

Professor Samar Safi-Harb
Professor, Physics & Astronomy
Lead for Equity, Diversity and Community, Faculty of Science
University of Manitoba (Canada)
+1 (204) 474 7104
Samar.Safi-Harb@umanitoba.ca
www2.physics.umanitoba.ca/u/samar



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