Evolution of timing and spectral characteristics of 4U 1901+03 during its 2019 outburst using the Swift and NuSTAR observatories

We report the results from a detailed timing and spectral study of transient X-ray pulsars, 4U 1901+03 during its 2019 outburst. We performed broadband spectroscopy in the 1-70 keV energy band using four observations made with Swift and NuSTAR at different intensity levels. Our timing results reveal the presence of highly variable pulse profiles dependent on both luminosity and energy. Our spectroscopy results showed the presence of a cyclotron resonance scattering feature (CRSF) at ~30 keV. This feature at 30 keV is highly luminosity and pulse-phase dependent. Phase-averaged spectra during the last two observations, made close to the declining phase of the outburst showed the presence of this feature at around 30 keV. The existence of CRSF at 30 keV during these observations is well supported by an abrupt change in the shape of pulse profiles found close to this energy. We also found that 30 keV feature was significantly detected in the pulse-phase resolved spectra of observations made at relatively high luminosities. Moreover, all spectral fit parameters showed a strong pulse phase dependence. In line with the previous findings, an absorption feature at around 10 keV is significantly observed in the phase-averaged X-ray spectra of all observations and also showed a strong pulse phase dependence.

[1]  T. Zhang,et al.  Insight-HXMT insight into switch of the accretion mode: The case of the X-ray pulsar 4U 1901+03 , 2020, 2004.13307.

[2]  T. Zhang,et al.  Switches between accretion structures during flares in 4U 1901+03 , 2020, 2002.08919.

[3]  K. Pottschmidt,et al.  The giant outburst of 4U 0115+634 in 2011 with Suzaku and RXTE , 2019, Astronomy & Astrophysics.

[4]  S. Tsygankov,et al.  Discovery of a Pulse-phase-transient Cyclotron Line in the X-Ray pulsar GRO J2058+42 , 2019, The Astrophysical Journal.

[5]  D. Klochkov,et al.  Cyclotron lines in highly magnetized neutron stars , 2018, Astronomy & Astrophysics.

[6]  C. Bailer-Jones,et al.  Estimating Distance from Parallaxes. IV. Distances to 1.33 Billion Stars in Gaia Data Release 2 , 2018, The Astronomical Journal.

[7]  C. Maitra.  Cyclotron Lines: From Magnetic Field Strength Estimators to Geometry Tracers in Neutron Stars , 2017, Journal of Astrophysics and Astronomy.

[8]  B. Paul,et al.  Understanding the nature of the intriguing source X Persei: a deep look with a Suzaku observation , 2017, 1705.07568.

[9]  P. Reig,et al.  Accretion regimes in the X-ray pulsar 4U 1901+03 , 2016, 1608.06118.

[10]  B. Paul,et al.  Post-flare formation of the accretion stream and a dip in pulse profiles of LMC X–4 , 2016, 1705.05205.

[11]  Moscow,et al.  Transient X-ray pulsar V 0332+53: pulse-phase-resolved spectroscopy and the reflection model , 2015, 1502.03783.

[12]  S. Tsygankov,et al.  The critical accretion luminosity for magnetized neutron stars , 2014, 1409.6457.

[13]  T. Dauser,et al.  Formation of phase lags at the cyclotron energies in the pulse profiles of magnetized, accreting neutron stars , 2014, 1405.6855.

[14]  B. Paul,et al.  Torque reversals and pulse profile of the pulsar 4U 1626−67 , 2014, 1401.2936.

[15]  O. Nishimura.  VARIATIONS OF CYCLOTRON LINE ENERGY WITH LUMINOSITY IN ACCRETING X-RAY PULSARS , 2014 .

[16]  S. Tsygankov,et al.  A REFLECTION MODEL FOR THE CYCLOTRON LINES IN THE SPECTRA OF X-RAY PULSARS , 2013, 1304.2633.

[17]  P. Reig,et al.  Patterns of variability in Be/X-ray pulsars during giant outbursts , 2012, 1212.5944.

[18]  B. Paul,et al.  PULSE-PHASE-RESOLVED SPECTROSCOPY OF VELA X-1 WITH SUZAKU , 2012, 1212.1538.

[19]  D. Klochkov,et al.  Spectral formation in accreting X-ray pulsars: bimodal variation of the cyclotron energy with luminosity , 2012, 1205.5316.

[20]  S. Naik,et al.  Timing and broad-band spectroscopy of 1A 1118−61 with Suzaku , 2011, 1111.3299.

[21]  C. Ferrigno,et al.  4U 0115 + 63: phase lags and cyclotron resonant scattering , 2011, 1106.4930.

[22]  Bangalore,et al.  RXTE-PCA observations of 1A 1118−61: timing and spectral studies during an outburst , 2011, 1101.5020.

[23]  P. Reig.  Be/X-ray binaries , 2011, 1101.5036.

[24]  N. Masetti,et al.  BeppoSAX OBSERVATIONS OF THE X-RAY PULSAR MAXI J1409−619 IN LOW STATE: DISCOVERY OF CYCLOTRON RESONANCE FEATURES , 2010, 1012.1218.

[25]  B. Paul,et al.  Flares, broadening of the pulse-frequency peak and quasi-periodic oscillations in the transient X-ray pulsar 4U 1901+03 , 2010, 1008.2815.

[26]  Y. Lei,et al.  PHASE-RESOLVED SPECTRAL ANALYSIS OF 4U 1901+03 DURING ITS OUTBURST , 2009, 0912.4772.

[27]  Wei Chen,et al.  A Study on the Pulse Profiles of the HMXB 4U 1901+03 , 2008 .

[28]  A. Harding,et al.  Emission processes in x‐ray pulsars , 2008 .

[29]  I. Kreykenbohm,et al.  A model for cyclotron resonance scattering features , 2007, 0707.2105.

[30]  Moscow,et al.  4U 0115+63 from RXTE and INTEGRAL data: Pulse profile and cyclotron line energy , 2007, 0704.2874.

[31]  S. Tsygankov,et al.  V0332+53 in the outburst of 2004–2005: luminosity dependence of the cyclotron line and pulse profile , 2005, astro-ph/0511237.

[32]  A. Lutovinov,et al.  First results from TOO observations of the Aql X-1 field with INTEGRAL , 2003, astro-ph/0309630.

[33]  I. Kreykenbohm,et al.  Magnetic Fields of Accreting X-Ray Pulsars with the Rossi X-Ray Timing Explorer , 2001, astro-ph/0207325.

[34]  A. Harding,et al.  Cyclotron-Line Features from Near-critical Fields. II. On the Effect of Anisotropic Radiation Fields , 2000, astro-ph/0007191.

[35]  L. Burderi,et al.  The 0.1-100 keV Spectrum of Centaurus X-3: Pulse Phase Spectroscopy of the Cyclotron Line and Magnetic Field Structure , 2000 .

[36]  A. Harding,et al.  Cyclotron Line Features from Near-critical Magnetic Fields: The Effect of Optical Depth and Plasma Geometry , 1999 .

[37]  R. W. Nelson,et al.  Observations of Accreting Pulsars , 1997, astro-ph/9707125.

[38]  S. Kitamoto,et al.  X-Ray Spectroscopy of Centaurus X-3 with ASCA over an Eclipse , 1996 .

[39]  G. Ferland,et al.  Atomic data for astrophysics. II. New analytic fits for photoionization cross sections of atoms and ions , 1996, astro-ph/9601009.

[40]  H. Ruder,et al.  Analyzing X-Ray Pulsar Profiles: Asymmetry as a Key to Geometry and Beam Pattern , 1995 .

[41]  Lev Titarchuk,et al.  GENERALIZED COMPTONIZATION MODELS AND APPLICATION TO THE RECENT HIGH-ENERGY OBSERVATIONS , 1994 .

[42]  L. Stella,et al.  The transient 42 second X-ray pulsar EXO 2030+375. II: The luminosity dependence of the pulse profile , 1989 .

[43]  R. Corbet,et al.  The three types of high-mass X-ray pulsator , 1986 .

[44]  P. Mészáros,et al.  X-ray pulsar models. I. Angle-dependent cyclotron line formation and comptonization , 1985 .

[45]  W. Priedhorsky,et al.  Long-term observations of x-ray sources: The aquila-serpens-scutum region , 1984 .

[46]  W. Nagel.  Radiative Transfer in a Strongly Magnetized Plasma - Part Two - Effects of Comptonization , 1981 .

[47]  M. Basko.  K-fluorescence lines in spectra of X-ray binaries , 1978 .

[48]  Christine Jones,et al.  Uhuru observations of the galactic plane in 1970, 1971, and 1972 , 1976 .

[49]  Sophie Papst,et al.  Statistics A Guide To The Use Of Statistical Methods In The Physical Sciences , 2016 .

[50]  Submitted to ApJ Preprint typeset using L ATEX style emulateapj v. 6/22/04 DISCOVERY OF PULSATIONS IN THE X-RAY TRANSIENT 4U 1901+03 , 2005 .

[51]  R. McCray,et al.  Astrophysical Journal, in press Preprint typeset using L ATEX style emulateapj v. 26/01/00 ON THE ABSORPTION OF X-RAYS IN THE INTERSTELLAR MEDIUM , 2000 .

[52]  K. Arnaud.  XSPEC: The First Ten Years , 1996 .

[53]  C. Day,et al.  The Evolution of X-ray Binaries , 1994 .

[54]  Y. Tanaka,et al.  Observations of compact x-ray sources , 1986 .