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We report NuSTAR and Chandra observations of two X-ray transients, SWIFT J174540.7−290015 (T15) and SWIFT J174540.2−290037 (T37), which were discovered by the Neil Gehrels Swift Observatory in 2016 within r ∼ 1 pc of Sgr A*. NuSTAR detected bright X-ray outbursts from T15 and T37, likely in the soft and hard states, with 3–79 keV luminosities of 8 × 1036 and 3 × 1037 erg s−1, respectively. No X-ray outbursts have previously been detected from the two transients and our Chandra ACIS analysis puts an upper limit of LX ≲ 2 × 1031 erg s−1 on their quiescent 2–8 keV luminosities. No pulsations, significant quasi-periodic oscillations, or type I X-ray bursts were detected in the NuSTAR data. While T15 exhibited no significant red noise, the T37 power density spectra are well characterized by three Lorentzian components. The declining variability of T37 above ν ∼ 10 Hz is typical of black hole (BH) transients in the hard state. NuSTAR spectra of both transients exhibit a thermal disk blackbody, X-ray reflection with broadened Fe atomic features, and a continuum component well described by Comptonization models. Their X-ray reflection spectra are most consistent with high BH spin (a* ≳ 0.9) and large disk density (ne ∼ 1021 cm−3). Based on the best-fit ionization parameters and disk densities, we found that X-ray reflection occurred near the inner-disk radius, which was derived from the relativistic broadening and thermal disk component. These X-ray characteristics suggest the outbursting BH-low-mass X-ray binary scenario for both transients and yield the first BH spin measurements from X-ray transients in the central 100 pc region. [ABSTRACT FROM AUTHOR]

AX J1949.8+2534 is a candidate supergiant fast X-ray transient (SFXT) observed in outburst by the International Gamma-ray Astrophysics Laboratory (IGR J19498+2534). We report on the results of six Neil Gehrels Swift-XRT, one Chandra, and one Nuclear Spectroscopic Telescope Array observation of the source. We find evidence of rapid X-ray variability on a few kilosecond timescales. Fortunately, Chandra observed the source in a relatively bright state, allowing us to confidently identify the optical/NIR counterpart of the source. We also obtained an optical spectrum of this counterpart, which shows an Hα emission line and He i absorption features. The photometry and spectrum of the source allow us to constrain its distance, ∼7–8 kpc, and reddening, AV = 8.5–9.5. We find that the star is likely an early B-type Ia supergiant, confirming that AX J1949.8+2534 is indeed an SFXT. [ABSTRACT FROM AUTHOR]

Blazars constitute the most enigmatic class of extragalactic γ-ray sources, and their observational features have been ascribed to a relativistic jet closely aligned to the line of sight. They are generally divided in two main classes: the BL Lac objects (BL Lacs) and the flat-spectrum radio quasars (FSRQs). In the case of BL Lacs the double-bumped spectral energy distribution (SED) is generally described by the synchrotron self-Compton (SSC) emission, while for the FSRQs it is interpreted as due to external Compton (EC) emission. Recently, we showed that in the [3.4]-[4.6]-[12] μm color-color diagram the blazar population covers a distinct region (i.e., the WISE blazar Strip (WBS)) clearly separated from the other extragalactic sources that are dominated by thermal emission. In this paper, we investigate the relation between the infrared and γ-ray emission for a subset of confirmed blazars from the literature, associated with Fermi sources, for which WISE archival observations are available. This sample is a proper subset of the sample of sources used previously, and the availability of Fermi data is critical to constrain the models on the emission mechanisms for the blazars. We found that the selected blazars also lie on the WBS covering a narrower region of the infrared color-color planes than the overall blazar population. We then found an evident correlation between the IR and γ-ray spectral indices expected in the SSC and EC frameworks. Finally, we determined the ratio between their γ-ray and infrared fluxes, a surrogate of the ratio of powers between the inverse Compton and the synchrotron SED components, and used such parameter to test different blazar emitting scenarios. [ABSTRACT FROM AUTHOR]