MULTIWAVELENGTH STUDY OF QUIESCENT STATES OF Mrk 421 WITH UNPRECEDENTED HARD X-RAY COVERAGE PROVIDED BY NuSTAR IN 2013

Abstract

We present coordinated multiwavelength observations of the bright, nearby BL Lacertae object Mrk 421 taken in 2013 January-March, involving GASP-WEBT, Swift, NuSTAR, Fermi-LAT, MAGIC, VERITAS, and other collaborations and instruments, providing data from radio to very high energy(VHE) γ-ray bands. NuSTAR yielded previously unattainable sensitivity in the 3-79keV range, revealing that the spectrum softens when the source is dimmer until the X-ray spectral shape saturates into a steep 3 G » power law, with no evidence for an exponential cutoff or additional hard components up to ∼80keV. For the first time, we observed both the synchrotron and the inverse-Compton peaks of the spectral energy distribution (SED) simultaneously shifted to frequencies below the typical quiescent state by an order of magnitude. The fractional variability as a function of photon energy shows a double-bump structure that relates to the two bumps of the broadband SED. In each bump, the variability increases with energy, which, in the framework of the synchrotron self-Compton model, implies that the electrons with higher energies are more variable. The measured multi band variability, the significant X-ray-to-VHE correlation down to some of the lowest fluxes ever observed in both bands, the lack of correlation between optical/UV and X-ray flux, the low degree of polarization and its significant (random) variations, the short estimated electron cooling time, and the significantly longer variability timescale observed in the NuSTAR light curves point toward in situ electron acceleration and suggest that there are multiple compact regions contributing to the broadband emission of Mrk 421 during low-activity states.

On the Origin of the Hard X-Ray Excess of High-Synchrotron-Peaked BL Lac Object Mrk 421

“…Gorham et al (2000) estimated the central black hole mass of order ∼ 10 9 M ⊙ based on the imagery and broadline velocity dispersion. Its synchrotron component peaks at the hard X-ray band during the bright state and moves to the soft X-ray band when the source gets fainter (Baloković et al 2016). During the faintest state in MJD 56302, its synchrotron component peaks at too low a frequency to make it similar to ISP/LSP.…”

“…They found that this excess follows the extension of the γ-ray spectra well; therefore, they argued that this hard X-ray excess must be the low-energy part of the SSC emissions of the same electron population that is producing the Fermi/LAT γ-ray. Actually, the broadband SED at this epoch had been well fitted by a one-zone SSC model (up to the TeV band but except for the hard X-ray spectra; Baloković et al 2016). However, it can be seen that the observed hard X-ray excess cannot be reproduced by this one-zone modeling: the modeling underestimates the hard X-ray flux (upper left panel of Figure 13 in Baloković et al 2016).…”

“…Its X-ray and TeV emissions often show well-correlated variability, as do its optical and GeV emissions (e.g., Bartoli et al 2016;Kapanadze et al 2016;Ahnen et al 2016;Sinha et al 2016). Including γ-rays, its broadband SED can be well fitted by a one-zone synchrotron self-Compton (SSC) model (e.g., Cao & Wang 2013;Zheng et al 2014;Zhu et al 2016;Baloković et al 2016). Gorham et al (2000) estimated the central black hole mass of order ∼ 10 9 M ⊙ based on the imagery and broadline velocity dispersion.…”

“…The origin of the hard X-Ray excess Kataoka & Stawarz (2016) compared the spectra of the hard X-ray excess with the simultaneous 2 Fermi/LAT power-law γ-ray spectra (the photon index Γ = 1.78 ± 0.02; Baloković et al 2016). They found that this excess follows the extension of the γ-ray spectra well; therefore, they argued that this hard X-ray excess must be the low-energy part of the SSC emissions of the same electron population that is producing the Fermi/LAT γ-ray.…”

Automated Polarimetry with Smaller Aperture Telescopes: The ROVOR Observatory

“…Setup takes place during the day, the telescope performs all observations at night without operator assistance, and shuts down automatically at dawn. Over 30 students have been trained on it, while producing or contributing to approximately one refereed monitoring paper per year (e.g., [4][5][6][7][8][9][10]). …”