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Register a new userV4641 Sgr: a candidate precessing microblazar
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The X-ray spectrum of the Galactic X-ray binary V4641 Sgr in outburst has been found to exhibit a remarkably broad emission feature above 4 keV, with inferred equivalent widths up to 2 keV. Such a feature was first detected during the X-ray flaring activity associated with the giant outburst that the source experienced in 1999 September. The extraordinarily large equivalent width line was then ascribed to reflection/reprocessing of fluorescent Fe emission within an extended optically thick outflow enshrouding the binary system as a result of a short-lived, super-Eddington accretion episode. Making use of new and archival X-ray observations, we show here that a similar feature persists over four orders of magnitude in luminosity, down to Eddington ratios as low as log(L_Edd) = -4.5, where the existence of an optically thick envelope appears at odds with any viable accretion flow model. Possible interpretations for this highly unusual X-ray spectrum include a blend of Doppler shifted/boosted Fe lines from unresolved X-ray jets (a la SS433), or, the first Galactic analog of a blazar spectrum, where the >4 keV emission would correspond to the onset of the Inverse Compton hump. Either requires a low inclination angle of the jet with respect to the line of sight, in agreement with the estimates for the 1999 superluminal jet (i_jet<10 deg). The fast variability of the feature, combined with the high orbital axis inclination (60 deg< i_orb<71 deg), argue for a rapidly precessing accretion flow around V4641 Sgr, possibly leading to a transient microblazar behavior.
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We present an analysis of three archival Chandra observations of the black hole V4641 Sgr, performed during a decline into quiescence. The last two observations in the sequence can be modeled with a simple power-law. The first spectrum, however, is remarkably similar to spectra observed in Seyfert-2 active galactic nuclei, which arise through a combination of obscuration and reflection from distant material. This spectrum of V4641 Sgr can be fit extremely well with a model including partial-covering absorption and distant reflection. This model recovers a Gamma = 2 power-law incident spectrum, typical of black holes at low Eddington fractions. The implied geometry is plausible in a high-mass X-ray binary like V4641 Sgr, and may be as compelling as explanations invoking Doppler-split line pairs in a jet, and/or unusual Comptonization. We discuss potential implications and means of testing these models.
In the microquasar V4641 Sgr the spin of the black hole is thought to be misaligned with the binary orbital axis. The accretion disc aligns with the black hole spin by the Lense-Thirring effect near to the black hole and further out becomes aligned with the binary orbital axis. The inclination of the radio jets and the Fe$Kalpha$ line profile have both been used to determine the inclination of the inner accretion disc but the measurements are inconsistent. Using a steady state analytical warped disc model for V4641 Sgr we find that the inner disc region is flat and aligned with the black hole up to about $900 R_{rm g}$. Thus if both the radio jet and fluorescent emission originates in the same inner region then the measurements of the inner disc inclination should be the same.
Microquasars are potential $gamma$-ray emitters. Indications of transient episodes of $gamma$-ray emission were recently reported in at least two systems: Cyg X-1 and Cyg X-3. The identification of additional $gamma$-ray-emitting microquasars is required to better understand how $gamma$-ray emission can be produced in these systems. Theoretical models have predicted very high-energy (VHE) $gamma$-ray emission from microquasars during periods of transient outburst. Observations reported herein were undertaken with the objective of observing a broadband flaring event in the $gamma$-ray and X-ray bands. Contemporaneous observations of three microquasars, GRS 1915+105, Circinus X-1, and V4641 Sgr, were obtained using the High Energy Spectroscopic System (H.E.S.S.) telescope array and the Rossi X-ray Timing Explorer (RXTE) satellite. X-ray analyses for each microquasar were performed and VHE $gamma$-ray upper limits from contemporaneous H.E.S.S. observations were derived. No significant $gamma$-ray signal has been detected in any of the three systems. The integral $gamma$-ray photon flux at the observational epochs is constrained to be $I(> 560 {rm GeV}) < 7.3times10^{-13}$ cm$^{-2}$ s$^{-1}$, $I(> 560 {rm GeV}) < 1.2times10^{-12}$ cm$^{-2}$ s$^{-1}$, and $I(> 240 {rm GeV}) < 4.5times10^{-12}$ cm$^{-2}$ s$^{-1}$ for GRS 1915+105, Circinus X-1, and V4641 Sgr, respectively. The $gamma$-ray upper limits obtained using H.E.S.S. are examined in the context of previous Cherenkov telescope observations of microquasars. The effect of intrinsic absorption is modelled for each target and found to have negligible impact on the flux of escaping $gamma$-rays. When combined with the X-ray behaviour observed using RXTE, the derived results indicate that if detectable VHE $gamma$-ray emission from microquasars is commonplace, then it is likely to be highly transient.
We report the unambiguous detection of non-thermal X-ray emission up to 30 keV from the Cannonball, a few-arcsecond long diffuse X-ray feature near the Galactic Center, using the NuSTAR X-ray observatory. The Cannonball is a high-velocity (vproj~500
km/s) pulsar candidate with a cometary pulsar wind nebula (PWN) located ~2 north-east from Sgr A*, just outside the radio shell of the supernova remnant Sagittarius A (Sgr A) East. Its non-thermal X-ray spectrum, measured up to 30 keV, is well characterized by a Gamma~1.6 power-law, typical of a PWN, and has an X-ray luminosity of L(3-30 keV)=1.3e34 erg/s. The spectral and spatial results derived from X-ray and radio data strongly suggest a runaway neutron star born in the Sgr A East supernova event. We do not find any pulsed signal from the Cannonball. The NuSTAR observations allow us to deduce the PWN magnetic field and show that it is consistent with the lower limit obtained from radio observations.
Periodic quasars have been suggested as candidates for hosting binary supermassive black holes (SMBHs), although alternative scenarios remain possible to explain the optical light curve periodicity. To test the alternative hypothesis of precessing radio jet, we present deep 6 GHz radio imaging conducted with NSFs Karl G. Jansky Very Large Array (VLA) in its C configuration for the three candidate periodic quasars, DES J024703.24$-$010032.0, DES J024944.66$-$000036.8, and DES J025214.67$-$002813.7. Our targets were selected based on their optical variability using 20-yr long multi-color light curves from the Dark Energy Survey (DES) and the Sloan Digital Sky Survey (SDSS). The new VLA observations show that all three periodic quasars are radio-quiet with the radio loudness parameters measured to be $Requiv f_{6,{rm cm}}/f_{{rm 2500}}$ of $lesssim$1.0$-$1.5 and the $k$-corrected luminosities $ u L_ u$[6 GHz] of $lesssim$5$-$21 $times$ 10$^{39}$ erg s$^{-1}$. They are in stark contrast to previously known periodic quasars proposed as binary SMBH candidates such as the blazar OJ 287 and PG1302$-$102. Our results rule out optical emission contributed from precessing radio jets as the origin of the optical periodicity in the three DES$-$SDSS-selected candidate periodic quasars. Future continued optical monitoring and complementary multi-wavelength observations are still needed to further test the binary SMBH hypothesis as well as other competing scenarios to explain the optical periodicity.
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