Spectral and temporal properties of the ultra-luminous X-ray pulsar in M82 from 15 years of Chandra observations and analysis of the pulsed emission using NuSTAR
- Cahill Center for Astrophysics, California Institute of Technology, 1216 East California Boulevard, Pasadena, CA 91125 (United States)
- NASA Goddard Space Flight Center, Code 662, Greenbelt, MD 20771 (United States)
- Physics Department, University of Crete, Heraklion (Greece)
- INAF/Osservatorio Astronomico di Cagliari, via della Scienza 5, I-09047 Selargius (Italy)
The recent discovery by Bachetti et al. of a pulsar in M82 that can reach luminosities of up to 10{sup 40} erg s{sup −1}, a factor of ∼100 times the Eddington luminosity for a 1.4 M{sub ⊙} compact object, poses a challenge for accretion physics. In order to better understand the nature of this source and its duty cycle, and in light of several physical models that have been subsequently published, we conduct a spectral and temporal analysis of the 0.5–8 keV X-ray emission from this source from 15 years of Chandra observations. We analyze 19 ACIS observations where the point-spread function (PSF) of the pulsar is not contaminated by nearby sources. We fit the Chandra spectra of the pulsar with a power-law model and a disk blackbody model, subjected to interstellar absorption in M82. We carefully assess for the effect of pile-up in our observations, where four observations have a pile-up fraction of >10%, which we account for during spectral modeling with a convolution model. When fitted with a power-law model, the average photon index when the source is at high luminosity (L{sub X} > 10{sup 39} erg s{sup −1}) is Γ = 1.33 ± 0.15. For the disk blackbody model, the average temperature is T{sub in} = 3.24 ± 0.65 keV, the spectral shape being consistent with other luminous X-ray pulsars. We also investigated the inclusion of a soft excess component and spectral break, finding that the spectra are also consistent with these features common to luminous X-ray pulsars. In addition, we present spectral analysis from NuSTAR over the 3–50 keV range where we have isolated the pulsed component. We find that the pulsed emission in this band is best fit by a power-law with a high-energy cutoff, where Γ = 0.6 ± 0.3 and E{sub C}=14{sub −3}{sup +5} keV. While the pulsar has previously been identified as a transient, we find from our longer-baseline study that it has been remarkably active over the 15-year period, where for 9/19 (47%) observations that we analyzed, the pulsar appears to be emitting at a luminosity in excess of 10{sup 39} erg s{sup −1}, greater than 10 times its Eddington limit.
- OSTI ID:
- 22882314
- Journal Information:
- Astrophysical Journal, Vol. 816, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); Since 2009, the country of publication for this journal is the UK.; ISSN 0004-637X
- Country of Publication:
- United Kingdom
- Language:
- English
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