The X-ray Background Emission of the Galactic Center and Bulge with NuSTAR
Ekaterina Kuznetsova, Roman Krivonos, Kerstin Perez, Daniel R. Wik
aa r X i v : . [ a s t r o - ph . H E ] A ug The X-ray Background Emission of the Galactic Center and Bulgewith
NuSTAR
Ekaterina Kuznetsova, Roman Krivonos, Kerstin Perez , ∗ and Daniel R. Wik Space Research Institute of the Russian Academy of Sciences (IKI), Moscow,Russia; Department of Physics, Massachusetts Institute of Technology, Cambridge,USA; [email protected] Department of Physics and Astronomy, University of Utah, Salt Lake City,USA; ∗ Corresponding author
Abstract.
The Galactic di ff use X-ray emission (GDXE) is believed to arise from un-resolved populations of numerous low-luminosity X-ray binary systems that trace stel-lar mass distribution of the Milky Way. Many dedicated studies carried out over the lastdecade suggest that a dominant contributor to GDXE is a population of accreting whitedwarfs (WDs). The question arises about relative contribution of di ff erent subclassesof accreting WD population, namely non-magnetic WD binaries, magnetic interme-diate polars (IPs) and polars, in di ff erent regions of the Galaxy: the Galactic center,bulge, and ridge. Recent low-energy (E <
10 keV) studies indicate that non-magneticWD binaries, in particular quiescent dwarf novae, provide a major contribution to thedi ff use hard X-ray emission of the Galactic bulge. From the other side, previous highenergy (E >
10 keV) X-ray measurements of the bulge and ridge imply a dominant pop-ulation of magnetic CVs, in particular intermediate polars. In this work we use sideaperture of the
NuSTAR to probe the di ff use continuum of the inner ∼ − ◦ of theGalactic bulge, which allows us to constrain possible mixture of soft and hard popula-tions components of the spectrum. We found that GDXE spectrum is well-described bya single-temperature thermal plasma with kT ≈ NuSTAR measurements of the inner 10 pc and inner 100 pc of the Galactic center.
1. Galactic bulge di ff use X-ray emission with NuSTAR
The Galactic di ff use X-ray emission (GDXE) was discovered more than 30 years ago. Itextends along the Galactic plane over 100 ◦ and fills the Galactic center. It is convenientto consider following distinct regions of the GDXE: the Galactic center (GCXE) in theinner ∼
100 pc or | l | . . ◦
5, the Galactic bulge (GBXE) in the inner ∼ | l | . ◦ ,and the Galactic ridge (GRXE) | l | ≈ ◦ − ◦ .Origin of the GDXE emission is still not fully clear. Soon after discovery, themain question was whether GDXE is truly di ff use or consists of a large number ofunresolved sources. The broad-band spectral and large-scale morphology analysis ofthe GDXE revealed that its nature is mainly due to low-luminosity unresolved point1 Kuznetsova, Krivonos, Perez,and Wiksources (Revnivtsev et al. 2006, 2009). A truly di ff use origin was found unrealistic,because plasma must have too high temperature ∼ keV to be held in Galaxy’s gravi-tational potential (see, e.g. Tanaka 2002; Ebisawa et al. 2005).The studies of the GDXE in X-ray band are complicated due to several reasons.The instrument must have large field of view (FOV) to collect weak GDXE from thelarge regions of the Galaxy. The second reason is a di ffi culty of the separation ofphotons detected from di ff erent parts of the GDXE. For example, RXTE and
INTEGRAL can provide large enough FOV but their spatial resolution is not su ffi cient for detailanalysis of these regions. NuSTAR is a focusing telescope, which operates in the hard X-ray energy band3–79 keV.
NuSTAR
FOV is about 13” for focusing optics, but it also allows to detectunfocused photons from a large side aperture (Perez et al. 2017). It provides a uniqueability to study the innermost Galactic center emission and the di ff use emission of thebulge with the same instrument.Observations of the Galactic center region with NuSTAR used in this work werecarried out during the period from July 2012 through October 2014 with a total exposure ∼ NuSTAR
FMPA focal plane detector not contaminated by stray-light from nearbybright X-ray sources. The collected spectrum was constructed from focused and un-focused aperture photons and instrumental background. Spectral analysis required acomplicated modeling of all spectral components, including GBXE, GCXE, cosmic X-ray background (CXB) and instrumental background (see details in Perez et al. 2019).The unfocused GBXE component was investigated with a single-temperature plasma(1T) model and an intermediate polars mass (IPM) model (Fig. 1). This approach al-lows to directly compare
NuSTAR measurements with a previous
NuSTAR observationsof the GC and
Suzaku , RXTE and
INTEGRAL investigations of the GBXE and GRXE.The best-fit plasma temperature kT and average white dwarf mass M WD was estimatedat kT ≈ keV and M WD = . − . M ⊙ for the 1T and IPM models, respectively. Thespectrum of the GBXE was found significantly softer than the Galactic center spectrumwith an average temperature kT > keV obtained with NuSTAR (Hailey et al. 2016;Perez et al. 2015) and the previously observed
Suzaku broad-band spectrum (Yuasa et al.2012). However, from the other side, Perez et al. (2019) result is well consistent with
Suzaku measurements at low-energies (see, e.g. Yamauchi et al. 2016; Nobukawa et al.2016; Koyama 2018). This soft spectrum indicates that the main source population ofthe Galactic bulge is probably dwarf novae, rather than intermediate polars.
2. Discussion and comparison with GCDE
Perez et al. (2015) discovered a faint di ff use extended emission in the inner part of theGC at energies above 20 keV with NuSTAR optics. This emission, referred as GCDE,was detected up to 40 keV with its peak located at the position of Sgr A* and extendedalong the Galactic plane (Fig. 2). The 2D image analysis of GCDE morphology showedthat the hard X-ray emission is significantly narrower in both longitude and latitude thanthe soft X-ray distribution studied by Heard & Warwick (2013) with
XMM-Newton .The hard emission component of the GCDE above 20 keV is well described byeither a power-law model with the photon index of ∼ . − . kT > keV .Perez et al. (2015) concluded that GCDE spectral properties are consistent with a pop-heGalactic Center and Bulge with NuSTAR Figure 1.
Left:
Data and folded best-fit model spectral components (solid unfo-cused GBXE, dashed focused GCXE, dash-dot unfocused CXB, focused CXB belowy-axis range, dotted detector background) for FPMA of 400032002001
NuSTAR ob-servation. The unfocused GBXE component is described by the 1T model.
Right:
The same as left, but the unfocused GBXE component described by the IPM model.Adopted from Perez et al. (2019). ulation of intermediate polars with a mean white dwarf mass M WD > . M ⊙ . Thispopulation is more massive than previously observed in the Galactic center and ridge( M WD ≈ . M ⊙ Muno et al. 2004; Krivonos et al. 2007; Heard & Warwick 2013) andin the Galactic bulge ( M WD ≈ . M ⊙ Yuasa et al. 2012).
Figure 2.
Left:
The image in the 20–40 keV energy band of the inner 12 pc × pc (5 ′ × ′ ) of the Galaxy. Solid ellipse demonstrates the FWHM of the best-fit2D Gaussian. Energy spectra were extracted from dashed polygons. Right:
The2–10 keV
XMM-Newton spectrum and 10–40 keV
NuSTAR spectrum. Dashed linesdemonstrate di ff erent model components. Adopted from Perez et al. (2015). The derived kT ≈ NuSTAR , is significantlylower than observed in the inner 10 pc and inner 100 pc of the Galactic center. It pointsout to that the di ff use hard X-ray emission of the Galactic center is dominated by IPsand emission of the Galactic bulge is dominated by DNe. The last is also consistent withrecent Suzaku studies of the Fe line properties and low-energy continuum of the bulge(Yamauchi et al. 2016) and updated luminosity distribution measurements of local DNe(Byckling et al. 2010; Reis et al. 2013).
Summary . NuSTAR provides possibility to study emission from the Galactic centerand bulge with the same instrument using both mirror’s response and side aperture. Thebroad-band continuum of the bulge measured by Perez et al. (2019) using
NuSTAR ’sside aperture is consistent with a dominant population of DNe, confirming the detailed Kuznetsova, Krivonos, Perez,and Wik
Suzaku studies of the Fe line properties and low-energy continuum of the bulge andupdated luminosity distribution measurements of local DNe.
Acknowledgments.
E.K. thanks for support RFBR, project number 19-32-90283.R.K. acknowledges support from the Russian Science Foundation (grant 19-12-00396).K.P. receives support from the Alfred P. Sloan Foundation.
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