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Broad-line type Ic SN 2020bvc: Signatures of an off-axis gamma-ray burst afterglow

Broad-line type Ic SN 2020bvc: Signatures of an off-axis gamma-ray burst afterglow
Broad-line type Ic SN 2020bvc: Signatures of an off-axis gamma-ray burst afterglow

Long-duration gamma-ray bursts (GRBs) are almost unequivocally associated with very energetic, broad-line supernovae of Type Ic-BL. While the gamma-ray emission is emitted in narrow jets, the SN emits radiation isotropically. Therefore, it has been hypothesized that some SN Ic-BL not associated with GRBs arise from events with inner engines such as off-axis GRBs or choked jets. Here we present observations of the nearby (d=120 Mpc) SN 2020bvc (ASAS-SN 20bs) that support this scenario. Swift-UVOT observations reveal an early decline (up to two days after explosion), while optical spectra classify it as a SN Ic-BL with very high expansion velocities (approximate to 70 000 km s(-1)), similar to that found for the jet-cocoon emission in SN 2017iuk associated with GRB 171205A. Moreover, the Swift X-Ray Telescope and CXO X-ray Observatory detected X-ray emission only three days after the SN and decaying onward, which can be ascribed to an afterglow component. Cocoon and X-ray emission are both signatures of jet-powered GRBs. In the case of SN 2020bvc, we find that the jet is off axis (by approximate to 23 degrees), as also indicated by the lack of early (approximate to 1 day) X-ray emission, which explains why no coincident GRB was detected promptly or in archival data. These observations suggest that SN 2020bvc is the first orphan GRB detected through its associated SN emission.

0004-6361
Izzo, Luca
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Auchettl, Katie
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Hjorth, Jens
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Colle, F. De
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Gall, Christa
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Angus, C. R.
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Raimundo, Sandra
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Ramirez-Ruiz, Enrico
fbcbe3f2-c6a2-45b1-a808-dd5373079f1c
Izzo, Luca
e6ffaf15-e044-4f4b-8cd4-75d723475821
Auchettl, Katie
24c92a3c-f229-4753-9bfe-8f1bcc0ff7cb
Hjorth, Jens
a37423df-8e2f-4dc5-ae78-8e70ed3cffb6
Colle, F. De
69083155-450c-4c3c-93a2-ee4beb907f0a
Gall, Christa
49c7031d-0457-499e-ba1e-8aba5083d10e
Angus, C. R.
7a190f2d-9816-4960-8695-e694c39f099c
Raimundo, Sandra
e409d9d3-17e8-4049-ad29-43ada60b24e2
Ramirez-Ruiz, Enrico
fbcbe3f2-c6a2-45b1-a808-dd5373079f1c

Izzo, Luca, Auchettl, Katie, Hjorth, Jens, Colle, F. De, Gall, Christa, Angus, C. R., Raimundo, Sandra and Ramirez-Ruiz, Enrico (2020) Broad-line type Ic SN 2020bvc: Signatures of an off-axis gamma-ray burst afterglow. Astronomy & Astrophysics, 639, [L11]. (doi:10.1051/0004-6361/202038152).

Record type: Article

Abstract

Long-duration gamma-ray bursts (GRBs) are almost unequivocally associated with very energetic, broad-line supernovae of Type Ic-BL. While the gamma-ray emission is emitted in narrow jets, the SN emits radiation isotropically. Therefore, it has been hypothesized that some SN Ic-BL not associated with GRBs arise from events with inner engines such as off-axis GRBs or choked jets. Here we present observations of the nearby (d=120 Mpc) SN 2020bvc (ASAS-SN 20bs) that support this scenario. Swift-UVOT observations reveal an early decline (up to two days after explosion), while optical spectra classify it as a SN Ic-BL with very high expansion velocities (approximate to 70 000 km s(-1)), similar to that found for the jet-cocoon emission in SN 2017iuk associated with GRB 171205A. Moreover, the Swift X-Ray Telescope and CXO X-ray Observatory detected X-ray emission only three days after the SN and decaying onward, which can be ascribed to an afterglow component. Cocoon and X-ray emission are both signatures of jet-powered GRBs. In the case of SN 2020bvc, we find that the jet is off axis (by approximate to 23 degrees), as also indicated by the lack of early (approximate to 1 day) X-ray emission, which explains why no coincident GRB was detected promptly or in archival data. These observations suggest that SN 2020bvc is the first orphan GRB detected through its associated SN emission.

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Accepted/In Press date: 30 June 2020
e-pub ahead of print date: 21 July 2020

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Local EPrints ID: 472592
URI: http://eprints.soton.ac.uk/id/eprint/472592
ISSN: 0004-6361
PURE UUID: e439c46c-bdef-4c71-8c31-a690d1df2c42
ORCID for Sandra Raimundo: ORCID iD orcid.org/0000-0002-6248-398X

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Date deposited: 09 Dec 2022 17:31
Last modified: 17 Mar 2024 04:03

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Contributors

Author: Luca Izzo
Author: Katie Auchettl
Author: Jens Hjorth
Author: F. De Colle
Author: Christa Gall
Author: C. R. Angus
Author: Sandra Raimundo ORCID iD
Author: Enrico Ramirez-Ruiz

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