On the Spectral Changes of OJ 287: The Lowest X-ray state Spectrum -- extended at Optical-UV and Hard at X-rays

Abstract

Optical-UV synchrotron spectrum has been argued to be the primary driver of the majority of X-ray spectral changes in the BL Lacertae object OJ 287 during its low and intermediate X-ray flux state. Here, we focus on the lowest recorded X-ray flux state of OJ 287 by the Swift facility and report the finding of a power-law optical-UV spectrum with a photon spectrum of $2.71\pm 0.03$ continuing into X-rays. Considering this at X-rays, we found a power-law X-ray spectrum of photon spectral index $1.22\pm 0.20$ that improves to $1.29\pm 0.06$ when considering other observations with similar X-ray spectra. This is the hardest reported X-ray spectrum (0.3-10 keV) and is consistent with the reported Swift-BAT hard X-ray spectrum. We further show that this X-ray spectrum can reproduce most of the flat X-ray spectra when combined with the corresponding optical-UV continuum during the low and intermediate flux states strengthening synchrotron as the primary driver of most of the X-ray spectral changes in the LBL state of the source. Combined with sharp-steepening/cutoff of the optical-UV spectrum during bright phases, the extended-spectrum indicates a comparatively larger emission region and could be the large-scale jet emission. The optical-UV spectrum implies a high-energy power-law particle spectrum of $\sim 4.4$ while X-ray implies a hard low-energy particle spectrum of $1.3-1.6$ which could be the real or can result from a higher low-energy cut-off in the particle spectrum.

5 pages; 3 figures; submitted

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Cited by in corpus (1)

• Simultaneous Optical to X-ray Spectra of OJ 287: Insights into X-ray spectral changes and particle spectra