Cyclotron Resonance Scattering Features in Binary X-ray Pulsars Sachindra Naik Physical Research Laboratory, Ahmedabad

Binary X-ray Pulsars Most of the neutron stars are rapidly rotating and highly magnetic. Misalignment of spin axis and magnetic axis causes pulsation in the emitted radiation Electrons and protons are accelerated off its surface through the intense magnetic field giving off radiation.

Binary X-ray Pulsars Becker & Wolff 2007

Cyclotron Resonance Scattering Features Cyclotron Resonance Scattering Features (CRFs) in X-ray pulsars are due to resonant scattering of line of sight X-ray photons against electrons embedded in the magnetic field. Detection of CRSF in the spectrum provides DIRECT and most accurate estimation of magnetic field of the pulsar. Magnetic field in the range 1012-13 G → Ecyc in X-ray range

Cyclotron Resonance Scattering Features Truemper et al. (1978) 4U 0115+63: Santangelo et al. (1999)

Detection of of a Cyclotron Resonance Scattering Feature in X-Ray Pulsar 4U 1909+07

CRSF in Be/X-ray Binary Pulsars during outbursts (with Suzaku and NuSTAR) Cyclotron features with Suzaku and NuSTAR during outbursts 1A 1118-61 : CRSF ~55, 110 keV Suchy et (2011) GX 304-1: CRSF ~54 keV Yamamoto et al. (2011)

CRSF in Be/X-ray Binary Pulsars during outbursts (with Suzaku and NuSTAR) Fuerst et al. (2014) KS 1947+300 Cyclotron line at ~12 keV Cyclotron line at ~80 keV GRO J1008-57 Yamamoto et al. (2011)

CRSF in 4U 1909+07 Cyclotron Abs. Line Poorly studied HMXB Pulsation Period: 604 s Orbital Period: 4.4 days Model independent cyclotron absorption like feature at 44 keV. Magnetic field of the pulsar is estimated to be 3.8x1012 Gauss. (Jaisawal , Naik, and Paul, 2013)

CRSF in SMC X-2 (with Swift/XRT and NuSTAR) MAXI light curve of SMC X-2 during 2015 October outburst. (Jaisawal and Naik, 2016) CRSF detected at ~28 keV in all three observations (New Detection). Magnetic field ~2.3x1012 Gauss. CRSF energy during observations is found to be different, showing a marginal negative dependence on luminosity.

Swift/BAT light curve during 2014 June- July outburst. CRSF in Cep X-4 Swift/BAT light curve during 2014 June- July outburst. Pulsation period : 66 s Orbital period – Unknown Cyclotron absorption feature reported at ~30 keV (Mihara et al. 1991). We have studied two Suzaku and NuSTAR observations of Cep X-4 during 2014 June-July outburst.

CRSF in Cep X-4 Apart from CRSF at 28 keV, an additional absorption like feature was detected at 45 keV. Close to the twice of the fundamental cyclotron energy but less i.e. 1.7 Identified as first cyclotron harmonic of 28 keV line. (Jaisawal and Naik, 2015) Suzaku Obs-II

Luminosity dependent line energy ratio CRSF in Cep X-4 Luminosity dependent line energy ratio Fundamental line energy obtained from NuSTAR is nearly constant. A significant variation in the first harmonic line energy. Luminosity dependent line energy ratio. Both the cyclotron lines are forming at different heights in accretion column or viewed at higher angles.

Asymetric CRSF in Cep X-4 Asymmetric cyclotron line in Cep X-4 Fuerst et al. (2015) Best fit FDCut model without and with 19 keV feature (b and c panels) in addition to fundamental energy at ~30 keV Fuerst et al. (2015)

Asymetric CRSF in V0332+53 (b) No line; (c) one line at 25.5 keV; (d) two lines, one at 26.6 keV and one at 50 keV; (e) two lines, a fundamental line that is best modeled with two Gaussian components at 25.2 keV and 26.6 keV and its harmonic at 49.9 keV; (f) three lines, the fundamental that modeled by two components at ∼25 and ∼27 keV, and two lines at ∼51 and ∼74 keV. Pottschmidt et al. (2005) Pottschmidt et al. (2005)

Asymetric CRSF in Cep X-4 (Phase Resolved Spectroscopy) CRSF parameters are variable over pulse phase. Can be interpreted as due to the changes in magnetic field. Fuerst et al. (2015) Jaisawal & Naik (2016)

Anharmonicity in CRSF 4U 0115+63: 14, 24, 36, 48, 62 keV (Heindl et al. 1999) 4U 1538-52: 22, 47 keV (Rodes-Roca et al. 2009) V 0332+53 : 27, 51, 74 keV (Nakajima et al. 2010) Cep X-4 : 28, 45 kev (Jaisawal and Naik, 2015) Her X-1 : 39, 73 keV (Enoto et al. 2008) MAXI J1409-619: 44, 73, 128 keV (Orlandini et al. 2012) A 0535+262 : 45, 100 keV (Terada et al. 2006) Effect of viewing angle (Nishimura 2013) or CRSF taking place at different height??? Does it affect the matter distribution in the accretion stream and hence pulse profiles (dips??)

Pulsars with detected CRSF in X-ray Spectrum List of Cyclotron Line Sources

Pulsars with detected CRSF in X-ray Spectra List of Cyclotron Line Sources Tentative detection LAXPC of Astrosat can play a major role in new detections

Luminosity Dependent of CRSF with Astrosat 4U0115+63 : Nakajima et al. 2006 A0535+262 : Caballero et al. 2007

Investigation of CRSF with Astrosat Detection of Cyclotron Resonance Scattering Features in new sources Study of Luminosity Dependence of CRSFs Detailed investigation of anharmonicity in cyclotron line profile Aysmmetry nature of cyclotron absorption line in several sources

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Summary and Conclusions We detected cyclotron lines in 4U 1909+07 (fundamental), Cep X-4 (first harmonics), and SMC X-2 (fundamental) for the first time. Anharmonicity in cyclotron line energies of Cep X-4, indicating line forming regions are at different height? We have also performed the phase-resolved spectroscopy of cyclotron lines in order to map the magnetic field around the pulsars such as Cep X- 4 and SMC X-2 . The cyclotron parameters were found to be variable within <20 %, indication of local distortion in the magnetic field or effect of viewing angle.