insight - Scientific Computing - # Cow-like Transients

The optical evolution of AT 2024wpp suggests that high-velocity outflows in Cow-like transients are spherically symmetric.

Q: How might the spherical nature of the outflows in Cow-like transients influence their interaction with the surrounding circumstellar environment?

The spherical nature of outflows in Cow-like transients can significantly influence their interaction with the surrounding circumstellar medium (CSM). Here's how: Uniform Expansion and Energy Deposition: Spherical outflows would drive shocks into the CSM uniformly in all directions. This leads to a more homogeneous energy deposition into the CSM, unlike aspherical jets or explosions that interact more strongly along specific directions. Shockwave Geometry and Emission: The resulting shockwaves would also be more spherical, leading to different observational signatures. For instance, radio emission, often produced in the shocked CSM, would likely be less structured and more uniformly distributed compared to scenarios with aspherical interaction. Impact on Late-Time Evolution: The spherical expansion could also affect the late-time evolution of the transient. The more uniform energy deposition might lead to a different density and temperature profile in the shocked CSM, potentially impacting the formation of later-time emission lines and the overall evolution of the transient's luminosity. However, it's important to remember that the paper suggests the high-velocity outflow itself is spherical. The presence of a dense equatorial torus, as proposed for AT 2018cow, would still lead to some degree of asymmetry in the interaction with the CSM, particularly at later times as the outflow interacts with this denser material.

Q: Could the low polarization observed in AT 2024wpp and AT 2018cow be explained by a highly structured, but ultimately non-spherical, outflow geometry that mimics spherical symmetry when viewed from certain angles?

While the paper argues against it, it is theoretically possible that the low polarization observed in AT 2024wpp and AT 2018cow could arise from a highly structured, non-spherical outflow that mimics spherical symmetry when viewed from specific angles. Imagine a scenario where the outflow has a complex geometry with multiple clumps, jets, or a clumpy torus. If this structure is oriented in such a way that, from our viewing angle, the different polarization vectors effectively cancel each other out, we would observe low net polarization. This would require a rather specific and fine-tuned geometry, making it statistically less likely than a genuinely spherical outflow. However, the paper argues that the lack of a known preferential viewing angle for Cow-like transients makes this scenario unlikely. If these events were randomly oriented, it would be improbable for both AT 2024wpp and AT 2018cow to be aligned in such a way as to produce low polarization from an intrinsically asymmetric outflow.

Q: If Cow-like transients indeed launch spherically symmetric outflows, what does this imply about the fundamental nature of the underlying explosions and the compact objects that presumably power them?

The presence of spherically symmetric outflows in Cow-like transients, if confirmed, would have significant implications for our understanding of these events: Constraints on Explosion Mechanism: Spherical symmetry in the outflow points towards a more isotropic explosion mechanism compared to scenarios that produce jets or highly aspherical explosions. This might disfavor models that rely heavily on accretion disks or strong magnetic fields to launch the outflow, as these typically lead to some degree of asymmetry. Nature of the Central Engine: The properties of the central engine, likely a compact object like a black hole or neutron star, would also be constrained. A spherical outflow suggests that the central engine is able to launch material and deposit energy isotropically, at least during the early phases of the event. This might favor models where the central engine is rapidly rotating, leading to a more uniform energy distribution. Alternative Energy Sources: The spherical outflow could also hint at alternative energy sources beyond just accretion power. For instance, the rapid spin-down of a newly formed magnetar, a highly magnetized neutron star, could potentially power a more isotropic outflow. However, it's crucial to remember that the spherical outflow might only characterize the high-velocity component of the ejecta. The presence of the equatorial torus still suggests some degree of asymmetry in the overall system. Further observations, particularly polarimetry of a larger sample of Cow-like transients, are needed to confirm the prevalence of spherical outflows and refine our understanding of these powerful events.

Core Concepts

New observations of the Cow-like transient AT 2024wpp, including the second-ever optical polarimetry data for this class of objects, suggest that the high-velocity outflows characteristic of these events are highly spherical.

Abstract

Bibliographic Information: Pursiainen, M., Killestein, T. L., Kuncarayakti, H., et al. (2024). Optical evolution of AT 2024wpp: the high-velocity outflows in Cow-like transients are consistent with high spherical symmetry. Monthly Notices of the Royal Astronomical Society, 000, 1–8. Preprint: arXiv:2411.03272v1 [astro-ph.HE]
Research Objective: This research paper presents and analyzes new optical photometric, spectroscopic, and polarimetric observations of the transient astronomical event AT 2024wpp, aiming to constrain its nature and, more broadly, the properties of the enigmatic class of objects known as Cow-like transients.
Methodology: The authors collected multi-band optical photometry data from various telescopes and surveys, including ZTF, ATLAS, GOTO, and Swift/UVOT. They also obtained optical spectra using the KOOLS-IFU instrument on the Seimei Telescope and the ALFOSC instrument on the Nordic Optical Telescope. Crucially, they acquired multi-epoch BVRi polarimetry using NOT/ALFOSC, providing valuable insights into the geometry of the transient's outflow.
Key Findings: AT 2024wpp exhibits a rapid rise to peak brightness (4.3 days) and a fast decline, with a total duration above half-maximum brightness of 6.7 days. Its spectra are consistent with blackbody emission with a temperature exceeding 20,000 K and show hints of extremely broad emission features suggestive of a near-relativistic outflow. Most importantly, the polarimetry data reveal a low degree of polarization (≲0.5%), consistent with zero polarization, indicating a high degree of spherical symmetry in the transient's outflow.
Main Conclusions: The authors conclude that AT 2024wpp is a new member of the Cow-like transient class, based on its observed properties. The low polarization observed in both AT 2024wpp and the prototypical Cow-like transient AT 2018cow, during the phase when their optical photospheres are thought to be located within their respective high-velocity outflows, strongly suggests that these outflows are intrinsically spherical. This finding challenges previous assumptions about the aspherical nature of these outflows and has significant implications for our understanding of the physical mechanisms driving these events.
Significance: This research provides crucial observational evidence for the spherical geometry of high-velocity outflows in Cow-like transients. This finding has profound implications for theoretical models aiming to explain the formation and evolution of these enigmatic objects.
Limitations and Future Research: The authors acknowledge that observations of more Cow-like transients, particularly high-quality optical polarimetry from near explosion to late times, are essential to confirm the generality of their findings and further constrain the geometry and stratification of the ejecta in these events.

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Stats

AT 2024wpp rose to a peak brightness of -21.9 magnitude in 4.3 days.
The transient remained above half-maximum brightness for only 6.7 days.
Blackbody fits to the photometry indicate a persistently hot event (T ≳ 20,000 K) with a rapidly receding photosphere (v ≈ 11,500 km/s).
Optical polarimetry data, obtained from +6.1 to +14.4 days post-discovery, show a low polarization of P ≲ 0.5% across all bands.
The low polarization is consistent with zero polarization and implies a high degree of spherical symmetry in the transient's outflow.

Quotes

"AT 2024wpp is only the second event of the class with optical polarimetry."
"Our BVRi observations obtained from +6.1 to +14.4 d show a low polarisation of P ≲ 0.5% across all bands, similar to AT 2018cow that was consistent with P ∼ 0% during the same outflow-driven phase."
"In the absence of evidence for a preferential viewing angle, it is unlikely that both events would have shown low polarisation in the case that their photospheres were aspherical."
"As such, we conclude that the near-relativistic outflows launched in these events are likely highly spherical, but polarimetric observations of further events are crucial to constrain their ejecta geometry and stratification in detail."

Key Insights Distilled From

Optical evolution of AT 2024wpp: the high-velocity outflows in Cow-like transients are consistent with high spherical symmetry

by M. Pursiaine... at arxiv.org 11-06-2024

https://arxiv.org/pdf/2411.03272.pdf

Optical evolution of AT 2024wpp: the high-velocity outflows in Cow-like transients are consistent with high spherical symmetry

Deeper Inquiries

How might the spherical nature of the outflows in Cow-like transients influence their interaction with the surrounding circumstellar environment?

The spherical nature of outflows in Cow-like transients can significantly influence their interaction with the surrounding circumstellar medium (CSM). Here's how:

Uniform Expansion and Energy Deposition: Spherical outflows would drive shocks into the CSM uniformly in all directions. This leads to a more homogeneous energy deposition into the CSM, unlike aspherical jets or explosions that interact more strongly along specific directions.
Shockwave Geometry and Emission: The resulting shockwaves would also be more spherical, leading to different observational signatures. For instance, radio emission, often produced in the shocked CSM, would likely be less structured and more uniformly distributed compared to scenarios with aspherical interaction.
Impact on Late-Time Evolution: The spherical expansion could also affect the late-time evolution of the transient. The more uniform energy deposition might lead to a different density and temperature profile in the shocked CSM, potentially impacting the formation of later-time emission lines and the overall evolution of the transient's luminosity.
However, it's important to remember that the paper suggests the high-velocity outflow itself is spherical. The presence of a dense equatorial torus, as proposed for AT 2018cow, would still lead to some degree of asymmetry in the interaction with the CSM, particularly at later times as the outflow interacts with this denser material.

Could the low polarization observed in AT 2024wpp and AT 2018cow be explained by a highly structured, but ultimately non-spherical, outflow geometry that mimics spherical symmetry when viewed from certain angles?

While the paper argues against it, it is theoretically possible that the low polarization observed in AT 2024wpp and AT 2018cow could arise from a highly structured, non-spherical outflow that mimics spherical symmetry when viewed from specific angles.
Imagine a scenario where the outflow has a complex geometry with multiple clumps, jets, or a clumpy torus. If this structure is oriented in such a way that, from our viewing angle, the different polarization vectors effectively cancel each other out, we would observe low net polarization. This would require a rather specific and fine-tuned geometry, making it statistically less likely than a genuinely spherical outflow.
However, the paper argues that the lack of a known preferential viewing angle for Cow-like transients makes this scenario unlikely. If these events were randomly oriented, it would be improbable for both AT 2024wpp and AT 2018cow to be aligned in such a way as to produce low polarization from an intrinsically asymmetric outflow.

If Cow-like transients indeed launch spherically symmetric outflows, what does this imply about the fundamental nature of the underlying explosions and the compact objects that presumably power them?

The presence of spherically symmetric outflows in Cow-like transients, if confirmed, would have significant implications for our understanding of these events:

Constraints on Explosion Mechanism: Spherical symmetry in the outflow points towards a more isotropic explosion mechanism compared to scenarios that produce jets or highly aspherical explosions. This might disfavor models that rely heavily on accretion disks or strong magnetic fields to launch the outflow, as these typically lead to some degree of asymmetry.
Nature of the Central Engine: The properties of the central engine, likely a compact object like a black hole or neutron star, would also be constrained. A spherical outflow suggests that the central engine is able to launch material and deposit energy isotropically, at least during the early phases of the event. This might favor models where the central engine is rapidly rotating, leading to a more uniform energy distribution.
Alternative Energy Sources:  The spherical outflow could also hint at alternative energy sources beyond just accretion power. For instance, the rapid spin-down of a newly formed magnetar, a highly magnetized neutron star, could potentially power a more isotropic outflow.
However, it's crucial to remember that the spherical outflow might only characterize the high-velocity component of the ejecta. The presence of the equatorial torus still suggests some degree of asymmetry in the overall system. Further observations, particularly polarimetry of a larger sample of Cow-like transients, are needed to confirm the prevalence of spherical outflows and refine our understanding of these powerful events.