Deciphering the Archeological Record Further Evidence for Ultra-High-Energy Cosmic Ray Acceleration in Starburst-Driven Superwinds Luis Alfredo Anchordoqui

2025-05-06 0 0 214.45KB 8 页 10玖币

侵权投诉

Deciphering the Archeological Record: Further Evidence for Ultra-High-Energy Cosmic Ray

Acceleration in Starburst-Driven Superwinds

Luis Alfredo Anchordoqui

Department of Physics and Astronomy, Lehman College, City University of New York, NY 10468, USA,

Department of Physics, Graduate Center, City University of New York, NY 10016, USA, and

Department of Astrophysics, American Museum of Natural History, NY 10024, USA

(Dated: October 2022)

Very recently, the Pierre Auger and Telescope Array collaborations reported strong evidence for a correlation

between the highest energy cosmic rays and nearby starburst galaxies, with a global signiﬁcance post-trial of

4.7σ. It is well known that the collective eﬀect of supernovae and winds from massive stars in the central

region of these galaxies drives a galactic-scale superwind that can shock heat and accelerate ambient interstellar

or circumgalactic gas. In previous work we showed that, for reasonable source parameters, starburst-driven

superwinds can be the carriers of ultra-high-energy cosmic ray acceleration. In this paper we assess the extent to

which one can approach the archaeological “inverse” problem of deciphering properties of superwind evolution

from present-day IR emission of their host galaxies. We show that the Outer Limits galaxy NGC 891 could

provide “smoking gun evidence” for the starburst-driven superwind model of ultra-high-energy cosmic rays.

I. INTRODUCTION

It is diﬃcult to overstate the scientiﬁc importance of under-

standing the origin of the highest energy cosmic rays. Ultra-

high-energy cosmic rays (UHECRs) have been observed since

the early 60’s [1] and a plethora of models have been pro-

posed to explain their origin, including acceleration (bottom-

up) processes in power astrophysical environments and (top-

down) decay of super-heavy (GUT scale) particles [2].

In the late 90’s we proposed a model predicting that

UHECR nuclei could be accelerated in the large-scale termi-

nal shock of the superwinds that ﬂow from the central region

of starburst galaxies [3]. A few years later we also predicted

that the highest energy cosmic ray nuclei will generate 20 to

30 degrees hot-spots around the starbursts, because of their

magnetic deﬂection in the Galactic magnetic ﬁeld [4].

Over the last decade, mounting evidence has been accumu-

lating suggesting that the UHECR composition becomes dom-

inated by nuclei at high-energy end of the spectrum [5–10].

Concurrently, the Pierre Auger Collaboration has provided a

compelling indication for a possible correlation between the

arrival directions of cosmic rays with energy E&1010.6GeV

and a model based on a catalog of bright starburst galax-

ies [11, 12]. The post-trial chance probability in an isotropic

cosmic ray sky gives a Gaussian signiﬁcance of 4.0σ. When

data from the Telescope Array are included in the statisti-

cal analysis the correlation with starburst galaxies is stronger

than the Auger-only result, with a post-trial signiﬁcance of

4.7σ[13]. In the best-ﬁt model, (12.1±4.5)% of the UHECR

ﬂux originates from the starbursts and undergoes angular dif-

fusion on a von Mises-Fisher scale ψ∼15.1+4.6

−3.0

◦.1

Together these observations lead to two critical questions:

1It is important to note that a likelihood analysis considering the biases in-

duced by the coherent deﬂection in the Galactic magnetic ﬁeld gives best-ﬁt

parameters which are consistent within 95%CL with those obtained adopt-

ing angular diﬀusion in the isotropic-scattering approximation [14].

•What imprints may the evolution of starburst-driven su-

perwinds leave in present-day observables?

•To what extent can we decipher this archaeological

record, exploiting information about the present-day

universe in order to learn about or constrain the pos-

sible acceleration of UHECR nuclei in starburst-driven

superwinds?

These are certainly very broad questions, and in this paper we

attempt to take a ﬁrst step towards answering these questions.

Before proceeding, we pause to note that median deﬂec-

tions of particles in the Galactic magnetic ﬁeld are estimated

to be

θG∼3◦ZE

1011 GeV ,(1)

where Zis the charge of the UHECR in units of the proton

charge [15]. Thus, the requirement θG.ψimplies that UHE-

CRs contributing to the starburst anisotropy signal should

have Z.10 and E/Z∼1010 GeV.

II. STARBURST ARCHEOLOGY

Starburst-driven superwinds are complex, multi-phase phe-

nomena primarily powered by the momentum and energy in-

jected by massive stars in the form of supernova (SN) ex-

plosions, stellar winds, and radiation [16]. According to the

book, these superwinds are ubiquitous in galaxies where the

star-formation rate per unit area exceeds 10−1Myr−1kpc−2.

This type of starbursting object, nicknamed far-IR galaxy

(FIRG), can be characterized by: (i) an IR luminosity, LIR &

1044 erg s−1, which is large relative to its optical luminosity

LIR LOPT, and (ii) a “warm” IR spectrum (ﬂux density at

60 µm>50% the ﬂux density at 100 µm).

The deposition of mechanical energy by supernovae and

stellar winds results in a bubble ﬁlled with hot (T.108K)

gas that is unbound by the gravitational potential because its

temperature is greater than the local escape temperature. The

arXiv:2210.15569v3 [astro-ph.HE] 4 Apr 2023

over-pressured bubble expands adiabatically, becomes super-

sonic at the edge of the starburst region, and eventually blows

out of the disk into the halo forming a strong shock front on

the contact surface with the cold gas in the halo.

Two distinct mechanisms have been proposed to explain the

starburst anisotropy signal:

•UHECRs can be accelerated by bouncing back and

forth across the superwind’s terminal shock (hereafter

ARC model) [3].

•UHECR acceleration can occur in the disproportion-

ally frequent extreme explosions that take place in the

starburst nucleus due to the high star-formation ac-

tivity [17]; e.g., low-luminosity gamma-ray bursts (ll-

GRBs) [18].

A point worth noting at this juncture is that one would ex-

pect llGRB explosions to stochastically sample the locations

of cosmic star-formation throughout the volume of the Uni-

verse in which they can be observed. Then the probability

for a given type of galaxy to host a llGRB during some pe-

riod of time would be proportional to its star-formation rate.

Starburst galaxies represent about 1% of the fraction of galax-

ies containing star forming galaxies [19], and the probabil-

ity of SN explosions is about one to two orders of magnitude

larger in starbursts than in normal galaxies, e.g., the SN rate

for M82 is about 0.2−0.3 yr−1[20] whereas for the Milky

Way is ∼3.5±1.5 century−1[21]. Note that these two eﬀects

tend to compensate each other, and so a straightforward cal-

culation shows that UHECRs accelerated in llGRBs will have

a stronger correlation with the nearby matter distribution than

with starbursts [22].

Indeed, given the ubiquity of llGRB explosions we can

ask ourselves why the correlation of UHECRs with starburst

galaxies would be explained by the presence of this common

phenomenon. Rather there must be some other inherently

unique feature of starburst galaxies to account for this cor-

relation. With this in mind, herein we focus on the ARC

model [3]. In previous work [23, 24], we investigated the

constraints imposed by the starburst anisotropy signal on the

ARC model and we readjusted free parameters to remain con-

sistent with the most recent astrophysical observations. We

now investigate the minimum power requirement for UHECR

acceleration at shocks.

The cosmic ray maximum energy for any multiplicative

acceleration process is given by the Hillas criterion, which

yields Emax ∼ZeuBR, where Ris the size and Bthe mag-

netic ﬁeld strength of the acceleration region, and uis the

speed of the scattering centers (i.e., the shock velocity) [25].

Now, the magnetic ﬁeld Bcarries with it an energy density

B2/(2µ0), and the out-ﬂowing plasma carries with it an en-

ergy ﬂux ∼uR2B2/(2µ0), where µ0is the permeability of free

space. This sets a constraint on the maximum magnetic power

delivered through the shock [26]. Following [27], we combine

the Hillas criterion with the constraint of the magnetic energy

ﬂux to arrive at the minimum power needed to accelerate a

nucleus to a given rigidity R,

Pmin =R2

2µ0u∼1044 erg s−1u

0.01 c−1 R

1010 GV !2

.(2)

TABLE I: Infrared luminosities [30] and kinetic energy output.

Starburst Galaxy log10(LIR/L)Ptoday/(1043erg s−1)

NGC 253 10.44 1

NGC 891 10.27 0.7

NGC 1068 11.27 7

NGC 3034 (a.k.a. M82) 10.77 2

NGC 4945 10.48 1

NGC 5236 (a.k.a. M83) 10.10 0.5

NGC 6946 10.16 0.6

IC 342 10.17 0.6

This steady state argument provides a conservative upper limit

for the required minimum power in the superwind. Note that

the minimum power requirement can be relaxed if, e.g., the

energy carried by the out-ﬂowing plasma needed to maintain

a 100 µG magnetic ﬁeld strength on a scale of 15 kpc [24]

is supplied during periodic ﬂaring intervals. Throughout we

remain cautious and adopt (2) as our point of reference.

Next, in line with our stated plan, we adopt the functional

form of the energy injection rate from stellar winds and super-

novae estimated in [28] to determine the kinetic energy output

of the starburst from measurements of the IR luminosity,

Ptoday ∼4×1043 LIR,11 erg s−1,(3)

where LIR,11 is the total IR luminosity (in units of 1011L), and

where we have rescaled the normalization factor to accommo-

date a supernova rate in M82 of 0.3 yr−1[29], rather than the

0.07 yr−1used in the original calculation of [28]. The associ-

ated mass-loss rate to match the normalization u∼0.01 cis

found to be

M∼15 LIR,11 Myr−1.(4)

In Table I we list the present-day kinetic energy output of the

nearby starbursts contributing to the anisotropy signal.

By comparing (2) with the results on Table I we see that for

most of the starbursts the present-day power output falls short

by about an order of magnitude to accommodate the required

maximum rigidity to explain the anisotropy signal. However,

we note that the estimate in (2) is subject to large systematic

uncertainties; see Appendix I. Furthermore, the characteristic

time-scale for Fermi-acceleration in non-relativistic shocks is

O(107yr) [31], and the superwind power given in Table I does

not take into account any source evolution, but rather charac-

terizes the current state of the outgoing plasma assuming that

the star formation proceeded continuously at a constant rate.

The question is then: Could the superwind of the FIRGs

listed in Table I be more powerful in an earlier stage? The

answer to this question is, in principle, yes: the rationale be-

ing that very powerful FIRGs have been observed in our cos-

mic backyard. For example, Arp 220 and NGC 6240 are the

nearest and best-studied examples of very powerful FIRGs

(LIR ∼1012L), while IRAS 00182 - 7112 is the most FIR-

powerful galaxy yet discovered (LIR nearly 1013 L) [28]. We

note, however, that there is no solid evidence indicating that

文档加载中……请稍候！
如果长时间未打开，您也可以点击刷新试试。

下载文档到电脑，查找使用更方便

10 玖币 0人已下载

立即下载

摘要：

DecipheringtheArcheologicalRecord:FurtherEvidenceforUltra-High-EnergyCosmicRayAccelerationinStarburst-DrivenSuperwindsLuisAlfredoAnchordoquiDepartmentofPhysicsandAstronomy,LehmanCollege,CityUniversityofNewYork,NY10468,USA,DepartmentofPhysics,GraduateCenter,CityUniversityofNewYork,NY10016,USA,andDepa...

展开>> 收起<<

Deciphering the Archeological Record Further Evidence for Ultra-High-Energy Cosmic Ray Acceleration in Starburst-Driven Superwinds Luis Alfredo Anchordoqui.pdf

共8页,预览2页

还剩页未读，继续阅读

声明：本站为文档C2C交易模式，即用户上传的文档直接被用户下载，本站只是中间服务平台，本站所有文档下载所得的收益归上传人(含作者)所有。玖贝云文库仅提供信息存储空间，仅对用户上传内容的表现方式做保护处理，对上载内容本身不做任何修改或编辑。若文档所含内容侵犯了您的版权或隐私，请立即通知玖贝云文库，我们立即给予删除！

Deciphering the Archeological Record Further Evidence for Ultra-High-Energy Cosmic Ray Acceleration in Starburst-Driven Superwinds Luis Alfredo Anchordoqui

相关推荐

开通VIP享超值会员特权

作者详情

相关内容

热门标签

举报选择: