Microlensing constraints on primordial black holes with the Subaru/HSC Andromeda observation

TL;DR

This study leverages a dense-cadence, single-night Subaru/HSC observation of M31 to search for microlensing events caused by primordial black holes in the Milky Way and Andromeda halos, targeting the low-mass window $M_{ m PBH} \sim 10^{-11}$–$10^{-9}\,M_\odot$. Using pixel-lensing analysis with image subtraction, the authors identify one viable microlensing candidate among tens of millions of monitored stars, and they quantify the detection efficiency via Monte Carlo and injection tests. Consequently, they place a 95% confidence upper bound on the PBH dark-matter fraction in this mass range that closes part of the open PBH window and tightens constraints beyond prior Kepler data, while explicitly accounting for finite-source-size and wave-optics effects. The results demonstrate the power of high-cadence, wide-field, pixel-lensing surveys toward M31 for constraining light PBH DM, and outline a path for future refinement with additional nights and extended timescales.

Abstract

Primordial black holes (PBHs) have long been suggested as a viable candidate for the elusive dark matter (DM). The abundance of such PBHs has been constrained using a number of astrophysical observations, except for a hitherto unexplored mass window of $M_{\rm PBH}=[10^{-14},10^{-9}]M_\odot$. Here we carry out a dense-cadence (2~min sampling rate), 7 hour-long observation of the Andromeda galaxy (M31) with the Subaru Hyper Suprime-Cam to search for microlensing of stars in M31 by PBHs lying in the halo regions of the Milky Way (MW) and M31. Given our simultaneous monitoring of tens of millions of stars in M31, if such light PBHs make up a significant fraction of DM, we expect to find many microlensing events for the PBH DM scenario. However, we identify only a single candidate event, which translates into the most stringent upper bounds on the abundance of PBHs in the mass range $M_{\rm PBH}\simeq [10^{-11}, 10^{-6}]M_\odot$.

Figures (25)

• Figure 1: The background shows the HSC image of M31 as seen by the 104 CCD chips of the Subaru/HSC camera. The white-colored grid represents a predefined iso-latitude tessellation grid, called the HSC "patch" (approximately 12 arcmin on a side). Our data analysis including the image subtraction is performed on individual patches. We exclude those patches which are marked in dark-blue color from our analysis as the dense star fields in these patches result in a saturation of the CCDs.
• Figure 2: The expected differential number of PBH microlensing events per logarithmic interval of the full-width-at-half-maximum (FWHM) microlensing timescale ${t_{\rm FWHM}}$, for a single star in M31. Each solid line corresponds to a monochromatic PBH DM scenario and assumes that all the dark matter consists of such PBHs. We adopt DM halo models for the MW and M31 halos which reproduce their individual rotation curves. The event rate calculation includes distributions of impact parameters and velocities of PBHs relative to a source star. Given the cadence, our data has the highest sensitivity to measure lightcurves with $t_{\rm FWHM}\simeq [0.07,3]~{\rm hours}$ shown by the unshaded regions.
• Figure 3: An example of the image subtraction technique we use for the analysis in this paper. The left panel shows the reference image which was constructed by co-adding the images of 10 best-seeing epochs, with a typical seeing of $0.45^{\prime\prime}$. The size of the image is $222\times 356$ pixels (corresponding to about 0.63 sq. arcmin), and corresponds to the disk region in M31. The middle panel shows a target image (coadded image using 3 sequential exposures) with seeing size of $0.8^{\prime\prime}$. The right panel shows the difference image generated by our image subtraction pipeline properly accounting for the different seeing of the target and reference images even in such a densely populated stellar field. A variable star candidate shows up in the difference image at the center. In this case, the candidate object appears as a negative flux in the difference image, because the object was fainter in the target image than in the reference image.
• Figure 4: The single remaining candidate that passed all the criteria we impose to select microlensing events. The images in the upper panels show the postage-stamped images around the candidate: the reference image, the target image, the difference image and the residual image after subtracting the best-fit PSF image, respectively. The lower panel shows that the best-fit microlensing model (blue curve) gives an acceptable fit to the measured light curve. The error bars denote photometric errors in the brightness measurement in the different image at each epoch.
• Figure 5: The red shaded region corresponds to the 95% C.L. upper bound on the PBH mass fraction to DM in the halo regions of MW and M31, derived from our search for microlensing of M31 stars based on the "single-night" HSC/Subaru data and fills a large gap in the existing constraints by closing the PBH DM window around lunar mass scale. To derive this constraint, we took into account the effect of finite source size, assuming that all source stars in M31 have a solar radius, as well as the effect of wave optics in the HSC $r$-band filter on the microlensing event (see text for details). The effects weaken the upper bounds at $M\hbox{$\; \buildrel < \over \sim \;$} 10^{-7}M_\odot$, and give no constraint on PBH at $M\hbox{$\; \buildrel < \over \sim \;$} 10^{-11}M_\odot$. Our constraint can be compared with other observational constraints as shown by the gray shaded regions: extragalactic $\gamma$-rays from PBH evaporation Carretal:10, femtolensing of $\gamma$-ray burst ("Femto") Barnackaetal:12, microlensing search of stars from the satellite 2-years Kepler data ("Kepler") Griestetal:14, MACHO/EROS/OGLE microlensing of stars ("EROS/MACHO") EROS:07, and the accretion effects on the CMB observables ("CMB") Ali-HaimoudKamionkowski:17, updated from the earlier estimate Ricottietal:08.