Multiwavelength Light Curves of Two Remarkable Sagittarius A* Flares

Abstract

Sgr A*, the supermassive black hole (SMBH) at the center of our Milky Way Galaxy, is known to be a variable source of X-ray, near-infrared (NIR), and submillimeter radiation and therefore a prime candidate to study the electromagnetic radiation generated by mass accretion flow onto a black hole and/or a related jet. Disentangling the power source and emission mechanisms of this variability is a central challenge to our understanding of accretion flows around SMBHs. Simultaneous multiwavelength observations of the flux variations and their time correlations can play an important role in obtaining a better understanding of possible emission mechanisms and their origin. This paper presents observations of two flares that both apparently violate the previously established patterns in the relative timing of submillimeter/NIR/X-ray flares from Sgr A*. One of these events provides the first evidence of coeval structure between NIR and submillimeter flux increases, while the second event is the first example of the sequence of submillimeter/X-ray/NIR flux increases all occurring within ~1 hr. Each of these two events appears to upend assumptions that have been the basis of some analytic models of flaring in Sgr A*. However, it cannot be ruled out that these events, even though unusual, were just coincidental. These observations demonstrate that we do not fully understand the origin of the multiwavelength variability of Sgr A* and show that there is a continued and important need for long-term, coordinated, and precise multiwavelength observations of Sgr A* to characterize the full range of variability behavior.

Keywords: Galaxy: center; X-rays: individual (Sgr A*); accretion; accretion disks; black hole physics; infrared: general; submillimeter: general.

Figure 1.

Left: a total of ~58 hr of simultaneous observations with VLT/NACO at 2.18 μm and Keck/NIRC2 at 2.12 μm and Chandra X-ray Observatory/ACIS-I at 2–8 keV is displayed. The minimum observed flux density of 3.05 mJy was subtracted from the NIR data before plotting. Observations were taken in separate 3–7 hr intervals as shown by the colors but are merged here on a continuous time axis. Vertical lines mark NIR peaks. This graph was provided by Zhiyuan Li. Right: the X-ray (2–10 keV; XMM-Newton) and the L-band (3.8 μm; VLT/NACO) data from the very bright 2007 April 4 flare (Dodds-Eden et al. 2009). The data used to create this figure are available.

Figure 2.

2014 June 18 joint observations of a double-peaked flare from Sgr A* by Spitzer/IRAC at 4.5 μm (blue dots and green smoothed line, scale on the left ordinate) and the SMA at 875 μm (red dots, scale on the right ordinate). The SMA calibrator (NRAO 530) flux density (shown in light red at the bottom) is ~1 Jy (a constant of 2.2 Jy was added to place the data on the right ordinate scale). The blue smoothed line is the two-Gaussian curve fit to the 4.5 μm data. Dotted and dashed purple lines show two Gaussian curves fitting the SMA submillimeter data, and the solid purple line shows their sum. The data used to create this figure are available.

Figure 3.

2015 May 14 observations of a single-peaked flare from Sgr A*. Red squares show the Keck 2.12 μm data (scale on right ordinate), green triangles the 1.32 mm SMA data (scale is 500× the right ordinate, i.e., peak flux density is ~6 Jy), and blue points the Chandra 2–8 keV data (left ordinate). The X-ray light curve is summed over 300 s bins, and Poisson error bars on the X-ray count rate are shown by black error bars. The SMA calibrator (NRAO 530) flux density (~1.4 Jy), scaled the same as for Sgr A*, is shown by purple crosses. The data used to create this figure are available.