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Evidences for a Pulsar Wind Nebula in SN1987A

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X-ray Plasma in Supernova Remnants

Physics in SNRs is deeply related to shock

Two broad classes of emission

Thermal (shock-heated plasma)

Bremsstrahlung
Radiative recombination
Line emission

Non-thermal (particle acceleration)

Synchrotron radiation

Other contributions may also come from sources lying within the SNR

(thermal emission from NS, non-thermal emission from PWN)

Before going into the details of the analysis, I provide you here with a quick overview of how SNRs work. After a massive star explodes as a SN, a shock front is generated. This shock front expands and sweeps and heats the interstellar/circumstellar medium. Another shock wave moves inwards, the reverse shock, heating the ejecta. The main emission processes occuring in SNRs can be divided in two broad classes: thermal emission, with BS, RRC and line emission and non-thermal emission, mostly due to synchrotron radiation and occasionally due to Non-thermal bremsstrahlung. We often also detect different type of contributions, coming from sources lyingh within the SNR, such as blackbody thermal radiation from the NS surface and non-thermal emission stemming from a Pulsar Wind Nebula, as already previously discussed by Barbara.

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SN 1987A…

Most recent naked-eye visible supernova at 51.4 kpc
Unique laboratory to study the link between progenitor-supernova-remnant
Embedded in an HII region and interacting with a dense ring-like structure
Reverse shock has not reached inner ejecta yet

Schematic representation of SN 1987A (McCray & Fransson 2016)

Red: ALMA (radio)

Green: Hubble (optical)

Blue: Chandra (X-ray)

SN 1987A is the most recent naked-eye visible SN. It exploded in the large magellanic cloud at a distance of 51.4 kpc. It is a unique source since it is the only known object for which we know the progenitor, we have seen the SN and now we can follow the evolution of the remnant. SN 1987A is characterized by a very inhomogenous environment: it is embedded in an HII region and interacts with a dense ring-like structure. The ring is clearly visibile in this multiwavelength images of SN 1987A in red as seen by ALMA in the radio band, in green by Hubble in the optical and in Blue by Chandra in the X-rays. Because of its young age, the reverse shock has not heated the inner ejecta yet and the inner area of 87A is threfore filled with cold and dense ejecta and dust, in a region where also a NS/PWN is expected to lie.

Riordinare il devloping remnant ecc

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…and its elusive compact object

BLOB

Cigan+19

Infact, during the explosion of SN 1987A neutrino were detected and according the SN explosion theory we should therefore expect the formation a Neutron star. However, no compact object has been observed so far at any wavelength. Only hint about its existence is provided by ALMA observations by Cigan19+, who found a blob, here shown, compatible with reprocessed emission stemming from a PWN/NS.The most likely explanation for the non-detection of the compact object is ascribable to the heavy absorption due to the photoelectric effect of the cold and dense inner ejecta. However, the cross section of photoelectric effect is much lower into the X-rays, and so we tackled this topic by looking into X-rays archival data. But how to estimate the ejecta absorption contribution? We can exploit the diagnostic provided by a state-of-the-art mHD simulation of 1987A, and include absorption effects estimated in the X-ray spectral analysis.

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State-of-the-art MHD modeling of SN1987A

Orlando+, 2020

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Estimating the absorption

Orlando+, 2020

300

500

700

Explosion site

Unshocked ejecta shelll

We define a photo-electric absorption component starting from the MHD model and included it in the spectral by folding them with the emission models for the pulsar wind nebula. We built these components taking into account the kick velocity of the NS, assuming a velocity of 500 km/s. Then, we could estimate the absorbing power year per year, taking into account the kick velocity of the NS which is delivered by the model. We found an ejecta absorption with an equivalent column density of 10^23, 100 times higher than the galactic absorption. To give an idea of the effect of the absorption, In this plot I show various ontribution from a model applied to Chandra+NuSTAR spectrum with the ejecta absorption component included. The absorption makes the Power-law component undetectable at energies lower than 10 keV.

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Hard X-ray emission in SN 1987A

Chandra spectrum: thermal emission from shocked material, mainly stemming from the circumstellar ring (interstellar absorption only)

Chandra 2014

NuSTAR spectrum: ???

Chandra best fit model

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Indication of a PWN in SN1987A (Greco+, 2021)

Simultaneous analysis of Chandra and NuSTAR data in 2012-2014 reveals non-thermal emission in the 10-20 keV band
Most likely explanation is emission from an heavily absorbed PWN
Diffusive schock acceleration is unlikely, but cannot be ruled out

Thermal only scenario

Thermal + PWN scenario

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From the cutoff energy of the synchrotron emission it is possible to derive the acceleration time in the DSA scenario

The DSA scenario appears less convincing (but alternative explanations may be viable, e.g. emission from secondary electrons in the dense circumstellar medium (TBI!)

What about DSA?

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A perfectly standard PWN

The absorbed PWN scenario provides reasonable (though poorly constrained) results

From the pulsar population of young pulsars the population of the PWNe can be constructed by scaling time and luminosity with the characteristic ones (Truelove & McKee 1999)

and from the measure of L_X, we explore different ambient densities and different η_X

Greco et al. 2021

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Analysis of multi-epochs Chandra, XMM and NuSTAR observations, including a new NuSTAR observation (2020)

The hard X-ray excess can be well fitted with an additional thermal component

kT₁

kT₂

kT₃

DSA

(not in the model)

Thermal emission in the hard X-rays?

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Alp et al. 2021

Time after the shock heating 100-1000 yr

Do the parameters make sense?

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Alp et al. 2021

Alp et al. 2021 show that also 2 thermal components + a power law (not including absorption from the cold ejecta) fits the spectra as well as the thermal scenario, though providing too large Fe abundances.

Absorbed power law not considered

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Enlarging the sample

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Chandra+RGS+PN+NuSTAR analysis: 3 kT

(Greco+, 2022)

Detector legend

Chandra

XMM-Newton/PN

XMM/RGS1O1

XMM/RGS1O2

XMM/RGS2O1

XMM/RGS2O2

NuSTAR

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Chandra+RGS+PN+NuSTAR analysis: 3 kT+PL

Detector legend

Chandra

XMM-Newton/PN

XMM/RGS1O1

XMM/RGS1O2

XMM/RGS2O1

XMM/RGS2O2

NuSTAR

(Greco+, 2022)

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Chandra+RGS+PN+NuSTAR analysis: 3 kT

Detector legend

Chandra

XMM-Newton/PN

XMM/RGS1O1

XMM/RGS1O2

XMM/RGS2O1

XMM/RGS2O2

NuSTAR

(Greco+, 2022)

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Chandra+RGS+PN+NuSTAR analysis: 3 kT+PL

Detector legend

Chandra

XMM-Newton/PN

XMM/RGS1O1

XMM/RGS1O2

XMM/RGS2O1

XMM/RGS2O2

NuSTAR

(Greco+, 2022)

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Chandra+RGS+PN+NuSTAR analysis: 3 kT

Detector legend

Chandra

XMM-Newton/PN

XMM/RGS1O1

XMM/RGS1O2

XMM/RGS2O1

XMM/RGS2O2

NuSTAR

(Greco+, 2022)

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Chandra+RGS+PN+NuSTAR analysis: 3 kT+PL

Detector legend

Chandra

XMM-Newton/PN

XMM/RGS1O1

XMM/RGS1O2

XMM/RGS2O1

XMM/RGS2O2

NuSTAR

(Greco+, 2022)

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A step beyond the standard spectral fit

The standard phenomological analysis of the enlarged data sample confirm the results from Greco+, 2021

Self-consistent synthesis from the MHD model by Orlando+

We also include doppler and thermal broadening effects
Physical interpretation of all available X-ray data thanks to the MHD model
Epochs: 2012-2014-2020

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MHD model over RGS data: 2012

Continuum and line emission are well reproduced

No fitting performed

(Greco+, 2022)

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MHD model over RGS data: 2014

No fitting performed

Continuum and line emission are well reproduced

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Origin of the hard X-ray emission: summary

Pulsar Wind Nebula

Thermal emission

Diff. Shock accel.

Spectral shape

Physically sound par.

temporal evolution

multi-λ consistency

Spatial location

Y

N

TBI

Y

TBI

N

Y

TBI

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Challenging the DSA scenario

Alp+, 2021

Dramatic flux �variation

Other hints on the nature of this emission can be inferred by looking at the variation of the flux in different wavelengths. Here, we can see how dramatically the flux changed all over the years in basically all the wavelength, except the 10-20 keV one where we only see a slight increase (10%) in 6 years. It is quite hard to explain why the thermal emission in every wavelength changes dramatically while it is basically constant in the 10-24 keV. Thanks to the MHD model we estimated that this 10% increase is due, with similar weights, to the expansion of the ejecta, which decrease their absorption power, and to higher contamination from the hottest thermal component in 2020 with respect to previous years. This also explain why in 2020 the improvement in the chi_squared with the addition of the absorbed power-law is less remarkable. Being the intrinsic flux of the PWN constant in 8 years we can rule out DSA as possible mechanism powering the synchrotron radiation because, otherwise we would expect a trend similar to that observed in the radio band.

Spiegare meglio discorso dsa radio

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Summary

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BACKUP SLIDES

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Image analysis and region selection

NuSTAR (3-20 keV)

Chandra (0.5-8 keV)

2014

Multi-epoch observations of SN 1987A: 2012-2013-2014

Greco+, 2021

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2012-2014 Chandra+NuSTAR analysis

2014

2013

2012

Greco, PhD thesis

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Chandra+NuSTAR analysis: power-law component

2014

2013

2012

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State-of-the-art MHD modeling of SN1987A

Orlando+, 2020

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Building the PWNe population

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Diffusive shock acceleration vs PWN

Greco+, 2021

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Considerations on DSA

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PWN SED

ALMA flux at 679 GHz taken as upper limit