KARON TAURARIN NEUTRON DA YA ƊAUKI SHEKARU BILIYAN 12 KAFIN YA AUKU

A 2017, na’urorin gano LIGO da Virgo sun kama raƙuman nauyi (gravitational waves) na taurarin neutron (neutron stars) biyu da ke kewaya juna har suka afka wa juna: GW170817. Na’urorin hangen nesa sai suka ga fashewar kanta, wani fitar hasken gamma da wani kilonova mai haske, a cikin damin taurari (galaxy) NGC 4993. An gano haɗewar taurarin neutron ta biyu, GW190425, tun daga lokacin, amma ba a tantance wurinta da kyau ba kuma ba a sami wani haske ba. Saboda haka GW170817 har yanzu ita ce kaɗai lamarin raƙuman nauyi da aka taɓa ɗaurawa da wani damin taurari.

Wannan damin taurari alama ce. Ryan Foley, na University of California, Santa Cruz, sun yi amfani da shi don tambayar tsawon lokacin da irin waɗannan ma’aurata ke jira kafin su haɗe — kuma sun sami amsar da ba ta dace da tsarin da aka saba ba.

Lokacin jira

Masana taurari suna kiransa lokacin jinkiri (delay time): lokacin da ke tsakanin haihuwar taurarin da haɗewarsu. Da zarar taurarin neutron biyu sun samu, a hankali suna rasa ƙarfin kewayawarsu (orbital energy) ga raƙuman nauyi. Lokacin da wannan ke ɗauka yana ƙaruwa da ƙarfi na huɗu (fourth power) na tazarar da ke tsakaninsu, don haka ma’auratan da aka haifa kusa da juna suna haɗewa da sauri, waɗanda aka haifa nesa da juna kuma suna haɗewa a makare.

Ga kewayon tazarar farko masu yiwuwa mai faɗi, wannan yana ba da wata ƙa’ida mai sauƙi: adadin haɗewa a kowace naúrar lokacin jinkiri yana raguwa kamar 1 raba da jinkirin. Idan aka ƙara mafi ƙanƙantar jinkiri na kusan shekaru miliyan 10, sai a sami rarrabuwa (distribution) ta gargajiya (canonical). Saƙonta: yawancin ma’aurata suna haɗewa jim kaɗan bayan haihuwar taurarinsu, kuma kaɗan ne ke jira shekaru biliyoyi.

Ƙiyasta shekarun NGC 4993

NGC 4993 damin taurari ne na “nau’in farko” (early-type), daga rukunin da masana taurari ke yi wa lakabi S0. Babu wani gungun taurari kuma babu wani yankin haifar da taurari kusa da wurin da kilonova ta haskaka, don haka wataƙila ma’auratan sun samu ne tun kafin haɗewar, a wani wuri dabam a cikin damin taurarin. Saboda haka Foley sun yi samfurin taurarin dukkan damin taurarin, suna haɗa ma’auni a launuka 14 daga ultraviolet zuwa infrared da wani bakan haske (spectrum) daga na’urar MUSE da ke kan Very Large Telescope.

Daidaitawar samfurin (fit) tana raba tarihin damin taurarin zuwa kwanduna 20 na lokaci kuma tana bar kowanne ya samar da taurari ba tare da ƙuntatawa ba. Sakamakon a fili yake. Matsakaicin shekarun taurarinsa shekaru biliyan 12.4 ne (tsakanin 11.7 da 12.9, da tabbacin 90%). Samar da taurari ya kai ƙololuwa tsakanin shekaru biliyan 12.5 da 13 da suka wuce, a sababbin taurari masu nauyin Rana 54 a kowace shekara, kuma ya kasance ƙasa da nauyin Rana ɗaya a shekara cikin shekaru biliyan 9 na ƙarshe. Kusan 96% na nauyin damin taurarin yana wurin tun shekaru biliyan 10.2 da suka wuce. Bawo-bawo (shells) da ke kewaye da damin taurarin suna nuna wata ƙaramar haɗewa kusan shekaru biliyan ɗaya da suka wuce, amma bakan hasken bai nuna wata alamar taurari matasa da wataƙila ta samar ba.

Don haka wataƙila GW170817 tana da jinkiri na kusan shekaru biliyan 12.

Dama ta 1.8%

Foley sai suka haɗa wannan shekaru da tarihin samar da taurari a sararin duniya, da yawan haɗewa da ƙungiyar LIGO-Virgo-KAGRA ta auna, da kuma haɗewa biyun da aka gano.

A ƙarƙashin rarrabuwar gargajiya, ga kowace haɗewa da ake gani a yau mai jinkiri na shekaru biliyan 10, ya kamata a sami kusan 2 masu jinkiri na shekaru biliyan 1 da 16 masu jinkiri na shekaru miliyan 100 (ga tazarar lokacin jinkiri mai tsawo iri ɗaya). Damar cewa haɗewar da aka gano ta tsufa kamar GW170817 1.8% ce kawai. Dalilin a fili yake: NGC 4993 ta samar da rabin taurarinta fiye da shekaru biliyan 12.6 da suka wuce, yayin da sararin duniya (Universe) gaba ɗaya ya samar da 1.1% kawai na taurarinsa a lokacin.

A matsakaici, masana taurari za su buƙaci ƙarin ganowa 55 kafin su yi tsammanin wata kamarta. Amma haɗewa ta farko da aka taɓa gano wurinta ta zama wannan batu da ba kasafai ake samu ba.

Abin da bayanan suka fi so

Idan aka bar mafi ƙanƙantar jinkirin a buɗe, bayanan sun fi son mai tsawo sosai. Rarrabuwar da babu ma’aurata da ke haɗewa a cikinta kafin shekaru biliyan 12.6 ta fi ta gargajiya samun goyon baya da ninki 40 zuwa 140, gwargwadon yadda aka haɗa yawan haɗewa a ciki, kuma ta fi rarrabuwar da aka samo daga gajerun fashe-fashen hasken gamma (short gamma-ray bursts) da kuma daga taurarin neutron biyu-biyu a Milky Way (Damin Taurarinmu) da ninki 80 zuwa 280. A wata hanyar nazari, mafi ƙanƙantar jinkirin ya wuce shekaru biliyan 7.5 da tabbacin 90%.

Hanyoyi uku na fita

Don tsarin gargajiya ya tsira, dole ne haɗewa masu gajeren jinkiri su kasance “ɓatattu”. Foley sun lissafa bayani uku.

• Wataƙila an haifi ma’auratan nesa da juna fiye da yadda aka ɗauka. Mafi ƙanƙantar jinkiri na shekaru biliyan 10 yana nufin babu ma’auratan taurarin neutron masu nauyin Rana 1.5 da suka fara kusa da juna fiye da naúrar sararin samaniya (astronomical units) 0.021. Foley na ganin hakan da wuya: zai ci karo da sanannun taurarin neutron biyu-biyu da kuma taurari biyu-biyu masu girma.
• Wataƙila na’urorin gano sun rasa haɗewa da yawa. Foley na kiran hakan abu mai matuƙar wuyar yiwuwa.
• Wata hanya dabam ta samar da ma’aurata, mai dogon jinkiri, za ta iya kishiyantar wadda aka saba ko ta rinjaye ta.

Foley sun fi son na ukun. Wanda ya fi kowa alƙawari shi ne gungun taurari masu cunkoso (dense star clusters), inda ma’aurata ke iya samuwa ta hanyar haɗuwar taurari da juna ta ƙarfin jan hankali, da jinkiri kusa da shekarun sararin duniya. Daga cikin kusan taurarin neutron biyu-biyu 24 da aka sani a Milky Way, ɗaya tabbatacce da uku marasa tabbaci suna cikin gungun taurari masu siffar ƙwallo (globular clusters), wani nau’in gungun taurari masu cunkoso. Amma ba a ga wani gungun taurari a wurin GW170817 ba, kuma ka’idodin baya-bayan nan suna hasashen haɗewa ƙalilan a cikin gungun taurari: dole ne yawansu ya fi yadda ake tsammani.

Tasirin da ya wuce lamari ɗaya

Idan haɗewa suna jira wannan dogon lokaci, suna zuwa a makare don wadatar da Damin Taurarinmu (Galaxy) a ƙuruciyarsa da sinadaran “r-process” da suke samarwa, kamar europium. Kuma binciken da ke neman hasken sababbin haɗewa ta hanyar fifita damin taurari masu samar da taurari na iya kasancewa ba ya nufar damin taurarin da suka fi alƙawari ba. Da mafi ƙanƙantar jinkiri na shekaru biliyan 10, babu irin wannan haɗewa da za ta iya faruwa kafin sararin duniya ya cika shekaru biliyan 10, don haka na’urorin gano na gaba za su ga ƙalilan daga cikinsu a nesa mai yawa.

Duk wannan ya dogara ne kan lamura biyu, ɗaya kawai aka gano wurinsa. Foley na sa ran ƙarin ganowa za su ƙara fayyace hoton, kuma na zargin cewa ainihin rarrabuwar ba ta da sauƙi kamar doka ɗaya ta ƙarfi (power law) mai mafi ƙanƙantar jinkiri.