On the Mass and Magnetic Field of the Neutron Star in the Ultraluminous X-Ray Source NGC 300 ULX1

Abstract

The accreting compact objects in most of ultraluminous X-ray sources (ULXs) are likely to be neutron stars rather than black holes as suggested by the recent detection of periodic pulsations from some of these sources located in neighboring galaxies and one ULX that has hitherto been discovered in our own galaxy. As a member of the ULX family, NGC 300 ULX1 is a new pulsating ULX (PULX) spinning up at substantially high rates compared with other PULXs. In this paper, we infer the strength of the magnetic field on the surface of the neutron star from the energy of the cyclotron absorption line detected in the pulsed X-ray spectrum of NGC 300 ULX1 and estimate the plausible ranges for the neutron-star mass and beaming fraction using the observed spin period and period derivative of the pulsar and the measured X-ray flux of the source. Our analysis favors proton cyclotron resonance scattering as a viable mechanism to account for both the observed cyclotron energy and high spin-up rates provided that the absorption line is generated close to the surface of the neutron star.

Keywords

• Neutron stars
• Accretion
• X-ray binary stars

Aşırı Parlak X-Işın Kaynağı NGC 300 ULX1'deki Nötron Yıldızının Kütlesi ve Manyetik Alanı Üzerine

Öz

Şu ana kadar galaksimizde keşfedilen bir aşırı parlak X-ışın kaynağı (APX) ve komşu galaksilerde bulunan bazı APX’lerden geldiği kısa süre önce saptanan periyodik pulsasyonların düşündürdüğü üzere, APX’lerin çoğunda kütle yığıştıran yoğun cisimler karadeliklerden daha çok, büyük olasılıkla, nötron yıldızlarıdır. APX ailesinin bir üyesi olan NGC 300 ULX1, diğer pulsasyonlu APX’ler (PAPX) ile karşılaştırıldığında oldukça yüksek oranlarda dönüşü hızlanan yeni bir PAPX’tir. Bu makalede, NGC 300 ULX1’in pulsasyonlu X-ışın tayfında saptanan siklotron soğurma çizgisinin enerjisinden nötron yıldızının yüzeyindeki manyetik alan yeğinliğini çıkarsıyor ve kaynağın X-ışın akısı ile pulsarın gözlenen dönme periyodu ve periyot türevini kullanarak nötron yıldızının kütlesi ve hüzmeleme oranı için olası aralıkları kestiriyoruz. Analizimiz, soğurma çizgisinin nötron yıldızı yüzeyine yakın üretilmesi koşuluyla, gözlenen siklotron enerjisi ve dönme hızındaki yüksek artış oranlarının her ikisini de açıklayacak yeterli bir mekanizma olması bakımından proton siklotron rezonans saçılımını desteklemektedir.

Anahtar Kelimeler

• Nötron yıldızları
• Yığışma
• X-ışın çift yıldızları

Kaynakça

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